HE LIQUEFACTION BEHAVIOR OF SANDS
SUBJECTED TO CYCLIC LOADING
by
KARL ROCKER,
JR.
B.S., Northeastern University
(1966)
Submitted in partial fulfillment
the requirements for the degree of
Master of Science
at the
Massachusetts Institute of Technology
(June 1968)
Signature of Author..
Department of Civil
Enqineeri nq,
May 23, 1968
Certified by.....
Thesis Supervisor
.............on Graduate
Students
Accepted by............
Chairman, Departmental Committee on Graduate Students
Archives
sS. INST. TEC4
JUN 24 1968
LIsRARtIEG
L
__
1
___V i- .rrr··--···--
ABSTRACT
THE LIQUEFACTION BEHAVIOR OF SANDS
SUBJECTED TO CYCLIC LOADING
by
Karl Rocker, Jr.
Submitted to the Department of Civil Engineering
on May 23, 1968 in partial fulfillment of the requirements
for the degree of Master of Science in Civil Engineering.
This thesis examines the liquefaction behavior of two medium to fine
sands subjected to cyclic reversing deviator stresses in the isotropically
consolidated triaxial state. Sand samples were tested over a range of
deviator stress and density, and at two effective confining stress levels.
Changes in pore pressure, axial strain, and axial stress were precisely
measured by electrical recorders.
Equipment for this experimental study was constructed or adapted and
proved satisfactory for these liquefaction tests. This equipment included: a modified triaxial cell; a load cell attached above the sample
top cap; a pore pressure transducer; a linearsyn differential transducer
(strain measurements); an air-pressure operated stress applicator; a
cyclic air-pressure application system; and the necessary recording
instrumentation.
These tests concluded: a) that there were significant differences
in the liquefaction behavior of the two sands tested; b) there are characteristic patterns of effective stress, pore pressure, and strain behavior
that may be identified; c) pre-cycling or a small percentage of air-filled
voids significantly increased deviator stress required to cause liquefaction; d) the two tested sands required considerably lower deviator stress
to cause failure at the same number of cycles as did previous tests on
similar sands under slightly different test conditions.
Thesis Supervisor:
Robert V. Whitman
Title:
Professor of Civil Engineering
-2-
ACKNOWLEDG EMENT
The author wishes to acknowledge his gratitude for the assistance of
the following persons and organizations:
Professor Robert V. Whitman, who provided guidance and constructive
criticism as thesis advisor of the author, and at whose suggestion this
area of research was studied.
Ford Foundation, for sponsorship of the M.I.T. Inter-American Program
in Civil Engineering.
Professor Anwar Wissa, for numerous suggestions and guidance on
equipment design and laboratory testing.
The Soil Mechanics staff, especially Mr. Car] Stahl and Mr. David
Driscoll, who provided considerable aid in equipment problems.
-3-
TABLE OF CONTENTS
PAGE
Title Page ......
Abstract ..................................
oeee
2
Acknowledgements ..........................
....
3
Table of Contents ..............
4
List of Figures ..........................
6
CHAPTER I Introduction ...................
9
A. Inter American Program .....
9
B. Sand Densification .........
9
C. Soil Liquefaction and Earthq uake
10
D. Scope of the Thesis ........
12
....
e....
....
....
....
CHAPTER II
Equipment ..................
14
....
CHAPTER III
17
Soils .....................
uake
17
A. Choice of Soils ............
B. Description of Soils .......
CHAPTER IV
17
....
C. Strength Characteristics ... ..n.
18
D. Relative Density ...........
19
Test Results ...............
21
A. Preliminary Testing Program
21
B. Tests on Denser Wing-Beach S
23
C. Tests on Denser Modified A3 Sand
23
D. Tests on Looser Modified A3 Sand
24
and
E. Tests on Looser Wing-Beach Si
F. Other Tests
.
..
.
.
..
...
0. ........
G. Presentation of Data ......
-4-
.
..
24
25
CHAPTER V Discussion of Test Results ..............
27
A. Effective Stress During Liquefaction
Strain During Liquefac -ti
nn
Pore Pressure Behavior
27
...........
29
...........
31
Liquefaction Potential of Tested Sands
33
ng
E. Effect of Saturation, Pre-c ycling, and Confi
re-0c C
...
CHAPTER VI
Comparison of Results with Previous Sttudies
CHAPTER VII
Conclusions and Recommendat ions ......
References
Tables ....
.
.
.
.
.
.
.
.
.
.
.
..
.
.
..
.
.
.
.
.
.
.
.
.
.
.
..............................
Figures ...
......
APPENDIX A Equipment ....................
......
APPENDIX B Testing Procedure ............
43
...
•
...
•
46
o
48
......
.o....o....
35
39
......
......
..
•
o
50
89
......
103
APPENDIX C Volume Measurements ..........
.. o...o
116
APPENDIX D Test Data ....................
......
119
APPENDIX E List of Symbols ..............
......
133
-5-
....
oo......
_____
__._Y1__6"
LIST OF FIGURES
Figure No.
1
Sources of Dynamic Loading in Soils
2
Stress-Strain Conditions During Liquefaction
3
Recorder Output Traces
4
Grain Size Distribution of Tested Sands
5
Microphotographs of Sand Grains
6
Summary of Drained Triaxial Shear Tests - Wing-Beach Sand
7
Summary of Drained Triaxial Shear Tests - Modified A3 Sand
8
Effect of Relative Density on Liquefaction - !1!ing-Beach Sand
9
Effect of Relative Density on Liquefaction - Modified A3 Sand
10
Stress Path During Liquefaction - Wing-Beach Sand at RD " 57%
11
Stress Path During Liquefaction - Wing-Beach Sand at RD
12
Stress Path During Liquefaction - Modified A3 Sand at RD ~ 65%
13
Stress Path During Liquefaction - Modified A3 Sand at RD z 83%
14
Strains During Stages of Liquefaction - Wing-Beach Sand
at RD
15
=
z
76%
57%
Strains During Stages of Liquefaction - Modified A3 Sand
at RD z 65%
16
Summary of Wing-Beach Sand Strain Data RD ~ 57%
17
Summary of Wing-Beach Sand Strain Data RD
18
Summary of Modified A3 Sand Strain Data RD
19
Summary of Modified A3 Sand Strain Data RD z 83%
20
Summary of Wing-Beach Sand Strain Data RD
series)
21
Cyclic Strain Following Initial Liquefaction - Wing-Beach Sand
at RD - 76%
-6__a
76%
65%
84% (preliminary
22
Cyclic Strain Following Initial Liquefaction - Modified A3
Sand at RD z 83%
23
Cyclic Strain Following Initial Liquefaction - Wing-Beach
Sand at RD " 84%
24
Summary of Pore Pressure During Compressive Cycle - WingBeach Sand at RD
-
76%
25
Influence of Number of Cycles to Liquefaction on Pore
Pressure Buildup
26
Dimensionless Behavior of Pore Pressure
2000 < Ni < 6000
27
Dimensionless Behavior of Pore Pressure
300 < N. < 700
28
Comparison of Wing-Beach and Modified A3 Sand:Initial
Liquefaction at RD = 60% and 80%
29
Comparison of Wing-Beach and Modified A3 Sand:Initial
Liquefaction at Yd = 95,1 and 99.6 PCF
30
Effect of Confining Stress on Initial Liquefaction
31
Effect of Imcomplete Saturation on Liquefaction
32
Influence of Saturation on Pore Pressure Buildup
33
Pore Pressure and Strain Behavior of Test SCS-RU20
34
Pore Pressure and Strain Behavior of Test SCS-RU17
35
Pore Pressure and Strain Behavior of Test SCS-RUl2
36
Change in Volume After Liquefaction
37
Comparison of Sacremento River Sand Liquefaction Data with
1
Tested Sands at RD = 78%
38
Comparison of Sacremento River Sand Liquefaction Data with
Wing-Beach Sand at RD = 78%, 60% and 38%
39
Grain Size Distribution Comparing Three Sands
-7-
Figures in Appendix
A-i
Cyclic Stress Test in Progress
A-2
Cyclic Triaxial Equipment
A-3
Dynamic Stress Control Device
A-4
Measuring Equipment
A-5
X-Y Recorder Record (Load Cell Output)
A-6
Sanborn Recorder Record (L.D.T. and Pressure Tranducer Output)
B-I
Preparation of Sample
B-2
Test Proceedure Prior to Cycling
B-3
Sample Calculation for SCS-RU21
B-4
Volume Change During Important Stages in Sample History
B-5
Initial Stress-Strain and Pore Pressure Data - Wing-Beach Sand
B-6
Initial Stress-Strain and Pore Pressure Data - Modified A3 Sand
C-I
Comparison of Density Computation Methods
D-1
Test SCS-RUl3
D-2
Test SCS-RUl4
D-3
Test SCS-RU15
D-4
Test SCS-RUl9
D-5
Test SCS-RU21
D-6
Test SCS-RU22
D-7
Test SCS-RU26 and RU30
D-8
Test SCS-RU29
D-9
Test SCS-RU32
D-10
Test SCS-RU33
D-11
Test SCS-RU37
D-12
Test SCS-RU38
-8-
CHAPTER I
INTRODUCTION
A. Inter American Program
This thesis is the third in a series of papers under sponsorship of
the M.I.T. Inter American Program investigating densification of sands
by vibrations.
The first paper, Repeated Load and Vibration Tests Upon Sand, Progress
Report No. 1 was published as M.I.T. Soils Publication No. 203 in August,
1967.
This covered initial procurement of equipment and reported on first
tests with dry, uniform, rounded sand.
Key aspects of soil behavior were
noted for further investigation.
The second paper, a thesis by Mr. Pedro Ortigosa De Pablo entitled
Densification of Sand by Vertical Vibrations with Almost Constant Stresses,
was published in February, 1968.
)
eration
sand
no
1
s
i
gni
fi
cant
levels
densification
sand
below
Conclusions from test results were:
densification
1.0
is
g;
occurs
2)
initiated
there
and
is
this
in
a
is
vibration
minimum
a
function
tests
acceleration
function
and nature of confining stress and initial relative density;
for
accel-
at
which
of
amount
3) a terminal
void ratio, dependent on confining stress, is achieved at accelerations
greater than a critical value.
B. Sand Densification
Several aspects of sand densification from dynamic loadings have now
been outlined.
This loading can occur from a number of sources under
-9-
--.
--
1
field conditions.
Figure 1* illustrates the three most important cases
of dynamically induced stress changes.
Vibration of heavy machinery foundations from eccentricity of moving
parts causes rapid cycling at low stress levels.
Acceleration levels
are usually low in these cases, confining stresses are high, and the number of cycles will be extremely large.
r
Vibratory compaction induces stress levels varying from a few g's
near the surface to almost insignificant levels more then several feet
below. Confining stress and the number of cyclic applications are relatively small.
Figure l~c illustrates shearing stresses in a horizontal soil mass
during an earthquake.
Acceleration levels of 0.3 g are not uncommon in
major earthquakes, although the vast majority of quakes are much lower.
Confining stress will vary from zero pressure at the surface to a high
level produced by overburden at several hundred feet beneath the surface.
Densification of sands during this phenomena can readily occur, but is
not often measured.
It is difficult to observe minor changes in ground
surface unless a building is located directly above the area.
Each of these three cases involve densification.
The last example
of dynamic loading, however, can produce catastrophic results not directly attributable to a density change problem.
This occurs through
soil liquefaction.
C. Soil Liquefaction and Earthquakes
A soil liquefies when forces normally supported by the structural soil
*
Figure 1 is a reproduction of Figure 1 from Ortigosa De Pablo Thesis,
1968.
-10-
r
skeleton are transfered to its pore water.
Pore pressure builds up until
the structure no longer takes any load, and behaves somewhat like a viscous
liquid.
Two major requirements for this to occur are saturation of the
soil and an undrained state in the zone being liquefied. Although liquefaction does not theoretically require water in the soil pores, the high
compressability of air-filled pores prevents major pressure buildun.
If
drainage occurs, this pressure buildup will also be lessened and liquefaction is not likely to take place.
Soil liquefaction can occur after a large number of cycles or from
the application of a single load.
hnth at
ljntahlV
lnOw
Point bar stream deposits and loess,
dpncit\_ ran linnipf\
a-ft-r a chnrk nf rlati\v•l\1
at low relative
", ASCE SM6, 1967)
11 not liquefy
ly hard to analyze.
e massive soilt and backfigure
nterest during
nted, and the
ingly random velomplified to a large
means that earthresponse in another
as recorded, are
-11-
1. Largest accelerations and velocities measured occur during the
first few seconds of an earthquake.
2. Minor accelerations and velocities are generated for up to several
minutes following initial motion.
3. Frequency of motion is in the order of 1/2 or 1 cycles per second.
4. Motion occurs in all three directions of a grid system.
Stresses induced by an earthquake cannot be realistically identified
but must instead be defined over a range of what was probably produced.
It is for this reason that direct correlation of laboratory testing to
earthquake behavior is not too far advanced an art.
Liquefaction may contribute to failure in a soil mass without total
liquefying of the mass itself.
The initial state of stress in an embank-
ment or sloping soil profile can exist near failure conditions. A large
mass of this soil may slide along a liquefied thin seam or strata of
cohesionless soil.
This liquefaction may be initiated by a blast-induced
shock as well as by an earthquake.
In order to determine liquefaction potential in a given problem, behavior of soils under cyclic loading must be analyzed and understood.
D. Scope of the Thesis
This thesis presents the results of a study into reversing cyclic
loading tests on cohesionless soils in the isotropic triaxial state of
stress.
Figure 2 shows applied principal stresses and induced shear
stress during testing. Deformation before and after liquefaction are
also presented in Figure 2. It is the purpose of the author to accomplish the following:
-12-
1
1. determine the behavior of sands during liquefaction in a triaxial
stress condition with respect to pore pressure buildup, axial
strain, and state of stress during testing.
2. determine the liquefaction potential of two cohesionless soils
tested; specifically to relate deviator stress, number of cycles,
relative density, and confining stress to liquefaction behavior
in each sand; then to compare the sands to one another in view
of different composition and grain characteristics.
-13-
--am
CHAPTER II
EQUIPMENT
Required equipment for reversing cyclic loading tests on saturated
sands can be categorized into three groups.
1. Loading equipment; stress application device, cyclic pressure
control, and pressure source
2. Testing equipment; triaxial cell and connections
3. Measuring equipment; stress, strain, and pore pressure measurement devices, power source and recording instrumentation.
This equipment was collected and assembled to provide the system shown
in Figure A.1 and detailed completely in Appendix A. Each piece was either
purchased outright, constructed from scratch, or modified from existing
equipment.
Availability, cost and time decided which method was used.
Requirements for the stress application device included ability for
rapid cycling, upward and downward force application, adaptability to
triaxial cells, and low friction or "frictionless" movement for several
inches of piston travel.
An experimental applicator designed by Dr.
Anwar Wissa of M.I.T. and built for Geomeasurements, Inc. of Cambridge,
Massachusetts was both suitable and readily available.
A few minor modi-
fications to the triaxial cell adaption members and to piston connections
were made before it could be used.
Performance of this device was satis-
factory overall, with some problem in piston alignment and resulting
friction from binding.
The extraordinary care required during assembly
is reflected by occasional poor matching of extension and compression
-14-
7
stresses in the preliminary test series (SCS-RUl to RU11).
1,
"F
Air pressure
pressure in
in two
two chambers
chambers or
or "pots"
"pots" controled
controled force
force applied
applied to
to aa
Air
sample.
sample.
Cycling speeds
speeds are
are theoretically
theoretically limited
limited only
by air
air flow
charflow charonly by
Cycling
acteristics.
acteristics.
indicates
cps, the
the fastet cycling
cycling speed used, indicates
Response at
at 11 cps,
Response
pp
of load
application.
"wave" of
nearly square
square "wave"
load application.
aa nearly
measured indirectly
indirectly
This isis measured
This
and shown
by strain
in Figure
Figure 3.c.
3.c.
measurements amplified
amplified in
and
shown by
strain measurements
(Force
measure(Force
measure-
reresponse inin the
the X-Y
X-Y rebecause of
of slower
slower response
not aa good
good indicator
indicator because
ment isis not
ment
k
t·
corder used.)
corder
two
connections for
for two
The stress control ~device
is aa series
series of
of tubing
tubing connections
device is
solenoid.
sources, and
two
pressure
pressure tanks,
and aaa solenoid.
solenoid.
two pressure
pressure sources,
pressure
tanks, two
sources, and
pressure tanks,
b4
An
electrical
An
electrical
An electrical
necessary for
the equipment
equipment necessary
complete the
counter complete
and cycle
cycle counter
divice and
timing divice
for
timing
to elecelecresponse to
solenoid response
Rapid solenoid
Rapid
applicator.
of the
the stress
stress applicator.
operation of
operation
in terms
terms
be measured
measured in
again to
to be
limitations again
cycling limitations
causes cycling
trical impulse
impulse causes
trical
the wave
wave
can change
change the
and diameter
diameter can
tubing length
length and
Varying tubing
Varying
flow.
of air
air flow.
of
square to
of an
puvse from
form of
square
shape. Only
Only square
a square
to quasi-sinusoidal
quasi-sinusoidal shape.
an air
air puase
from a
form
electrical timers used have
The two electrical
in these
these tests.
waves were used in
6
modisystem isis aa modiThis system
This
cpm.
cph to
to 55 cpm.
and 55 cph
to 66 cps
cps and
of 11 to
ranges of
cycling ranges
cps
cph
cycling
cycling
Program
American Program
Inter American
in earlier
earlier Inter
used in
box" used
the "stress
"stress box"
of the
fication of
fication
soils laboratory
investigations by
(1966-t967).
laboratory (1966-1967).
by the
the M.I.T.
M.I.T. soils
investigations
1
chamber
the
pressure
as
was
used as
cell
triaxial
(Genor)
A
Norwegian
pressure chamber
chamber
was used
used
as the
the pressure
A
triaxial cell
cell was
A Norwegian
Norwegian (Genor)
(Genor) triaxial
)"
tests.
for these tests.
prespore presincrease, pore
height increase,
for height
were made for
Adaptations were
Adaptations
of the
the
for lubrication
and for
connections, and
transducer and load cell connections,
lubrication of
sure transducer
o
o~~~~~~
triaxial
bushing.
l
aixairt
bushi
ng.
3.5 inches in
3.5
in
inches
stbnes at
stones
at
C~··
~~~L~~~V;m~+d~\·
r))f~,~Y~L·AI
~~ Clj~m~+A·CIC·
C-·~~~A C··~LAI~~~UI
Im+\
i·,~r l.~-U,
~~·.V
,~~A~U
~ ~~
Sample specimens 2 inches in diameter by approximately
Sampl
e
specimens
2
height were tested.
tested.
were
height
i
nches
i
n
diameter
y
approx
mate
Each sample was prepared with porous
sample
Each
was
with
prepared
porous
both ends to prevent pumping of soil into the pore pressure
ends
both
to
pumping
prevent
of
soil
into
the
pore
pressure
c
lines during testing.
lines
testing.
during
Poor pressures and volume measurments were conPoor
pressures
troled by a mercury pot and bur·ette
troled
by
a
mercury
pot
and
burette
and
system.
system.
-15__A ~
volume
measurments
were
con-
y
-1
Measuring equipment for pore pressure, strain, and axial stress was
b
testing.
to be
be electical
electical because
because of
of the
the dynamic
dynamic testing.
required to
required
Pore pressure
pressure
Pore
connected rigidly
rigidly to
to the
the triaxial
triaxial
was measured
measured by
by aa pressure
pressure transducer
transducer connected
was
base plate.
plate.
base
b
Axial strain
strain was
was recorded
recorded by
by an
an L.D.T,
L.D.T. (Linearsyn
(Linearsyn DifferenDifferenAxial
tial Transformer) connected to the triaxial piston and outside frame.
tial
Transformer)
to
connected
the
triaxial
piston
and
frame.
outside
Axial stress was measured by a load cell inside the triaxial cell conAxial
stress
was
by
measured
a
load
cell
inside
nected directly to the sample top cap.
nected
directly
to
the
sample
top
the
cell
triaxial
con-
Response time and accuracy in
cap.
Response
time
and
accuracy
in
all three instruments were well within tolerable limits.
three
all
were
instruments
well
limits.
tolerable
within
Output from each was monitored by amplifier-recorders.
from
Output
Lt
rf
each
monitored
was
amplifier-recorders.
by
and strain were traced on a Sanborn 321 dual channel
d
na
s
trai
n
were
traced
on
a
Sanborn
321
dual
channel
Pore pressure
Pore
recorder,
recorder
and 3,c,
3.c.
in Figure
Figure 3,b
3.b and
are shown
shown in
each instrument
instrument are
records from
from each
records
pressure
1
Amplified
Am
l\lllr
Response
ResPonse
limits.
well within
within tolerable
tolerable limits.
recorder isis also
also well
and accuracy
accuracy at
at this
this recorder
and
f
i
this testing.
testing.
for this
of equipment
equipment for
piece of
an excellent
excellent piece
is considered
considered an
is
vintage X-Y
X-Y recorder.
recorder.
traced on
on aa vintage
output was
was traced
cell output
X-Y
cell
in Figure
Figure 3.a.
3.a.
shown in
record isis shown
record
lified
ItIt
Load
Load
typical amplified
amplified
A typical
typical
amplified
AA
was
this recorder
recorder was
time of
of this
The response
response time
The
testing atat
record when
when testing
load-time record
of the
the load-time
distortion of
caused some
some distortion
slow, and
and caused
slow,
and
caused
some
distortion
of
the
load-time
record
when
testing
slow,
11 cps.
cps.
to
made
was
output
load
over
AA
prior to
A calibration
calibration over
over the
the load
load cell
cell output
output range
range was
was made
made prior
prior
to
drift.
scale factor
factor drift.
long-term scale
test to
to eliminate
each test
eliminate aa long-term
each
This instrument,
instrument,
This
was available,
available.
used because
satisfactory, was
than totally
while less
while
less than
totally satisfactory,
was used
because itit was
available.
through an
and L.D.T
to the
the pressure
pressure transducer
supplied to
Power was
was supplied
t'ransducer and
L.DT
an
Power
L.D.T through
recorder.
in the Sanborn recorder,
internal source in
internal
used.
D.C. were
were used.
5.0 volts
volts DC,
4.5 to 5,0
4,5
and 24
6, 20
20 and
by voltages
voltages of
excited by
was excited
The
load cell
The load
cell was
of 6,
24 volts
volts D.C.
D.C. depending
depending
the
of the
and on
on amplification
amplification limits
was available,
supply was
on
what power
on what
power supply
available, and
limits of
the
on
supply
and
recorder.
X-Y recorder.
X-Y
-16-
--.. M
CHAPTER III
SOILS
A. Choice of Soils
Two cohesionless sands were tested in the program. This choice of
soil was guided by several factors.
Among these are;
soils which have been known to liquify,
1) similarity to
2) similarity to soils which
have undergone previous liquefaction studies,
3) concern with lique-
faction potential of a well graded sand compared to a uniform sand,
4) similar concern with liquefaction porential of angular sands compared
to rounded sands, and
5) availability in uniform quantities.
B. Description of Sands
For these reasons a sub-rounded uniform quartz sand (Wing-Beach sand)
and an angular to sub-angular well graded quartz sand (Modified A3 sand)
were chosen.
Gradation curves and miscrospic photograohs of these sands
are shown in Figure 4 and Figure 5 respectively.
The Wing-Beach sand was the most tested and was used in a prelininary
test series (tests SCS-RUl to SCS-RU11) to check equipment performance.
This sand was obtained from a dune deposit along the north-central
Massachusetts shoreline at Wingaershiek Beach.
Before testing, the sand
was washed to remove salts and traces of organic materials.
The sand
that was tested has D50 = 0.2mm, Cu = 1.3 and a measured S.G. = 2.65.
Less than 0.1 percent of the material passed the #200 U.S. Standard
Sieve (screen opening
Z
0.075 mm).
-17-
--
r
The Modified A3 sand is an artificial gradation of crushed quartz
aggregate obtained from the Ottawa Silica Company in Mystic, Connecticut.
The gradation was "designed" to produce a moderately well graded sand
with an average size partical similar to that of the Wing-Beach sand.
The composition is as follows:
22.7% ....
SAND #45
22.7% .... SAND #65
22.7% ....
SAND #100
22.7% ....
SAND #160
9.2%
....
SAND #20
This sand has D 50 = 0.21mm, Cu = 3.1 and S.G. = 2.65.
Approximately
5 percent of the material was finer than the #200 U.S. Standard Sieve.
A
good comparison of the two sands is possible through representative photographs (Figure 5) of two tested samples after oven drying.
The photographs
clearly show differences in angularity, gradation and uniformity.
C. Strength Characteristics
Several tests were run on these two sands for determination of significant strength properties.
Results of four stress-controled drained
triaxial compression tests are shown in Figure 6 and Figure 7. These
tests were run in the saturated state at a confining pressure of 14.2 psi.
The same equipment was used in shear and cyclic load tests to minimize
differences in testing conditions.
Two differences in shear behavior of these sands are noticeable.
The
Wing-Beach sand exhibits considerably larger initial stress-strain moduli
than does the Modified A3 sand.
Dilation tendencies are greater and con-
trol volume changes earlier in the Wing-Beach sand.
-18-
Further testing of
a sand nearly identical to Modified A3 has confirmed this behavior and the
computed friction angles.
(This data is as yet unpublished.
Other
testing on this sand currently underway at M.IT. includes direct shear,
high pressure triaxial undrained, plain strain and constant volume direct
shear tests.)
The use of strength and pore pressure data from the pre-
sented tests in direct comparison with rapid undrained cyclic loading
tests is limited.
It has been shown (Healy, Doctoral Thesis, 1963) that
strain rate differences alone can significantly alter these results.
D. Relative Density
Maximum and minimum relative densities for the two sands tested were
determined in the following manner.
Minimum densities for both sands
were obtained by loosely pouring each sand into a mold of known volume.
The sand was placed into a funnel held close to the surface as it was
brought up.
This method was found to be accurately repeatable as wit-
nessed by a maximum difference in unit weight of 0.6 PCF in four tests
on Modified A3 sand.
Maximum unit weight was determined by compacting the sands in several
ways, with an "extraneous" vibration mode producing highest densities in
both sands.
This highly non-repeatable method consisted of horizontal and
vertical excitation of a sand filled Harvard Miniature Mold by a mechanical vibration tool,
Modified and standard AASHO tests, accelerations
from 1.0 to 2.5 g's on a shaking table, and other methods were tried on
the Wind-Beach sand in an effort to achieve a higher density.
-19-
-0
Relative density is the most frequently referred to density parameter
in this report.
In contrast to void ratio and unit weight it is not a
completely definable term.
While void ratio and unit weight may be cal-
culated in terms of measureable values, relative density depends on what
method is used for determining the range of possible density. The "maximum" and "minimum" values are, then, only maximum and minimum for a particular method of their determination.
Once these densities are determined,
relative density has a specific one to one correspondance with void ratio
and unit weight.
The advantage of using relative density is in comparison
of one sand with another. Void ratio and unit weight may well lose significance as a basis of comparison, but relative density relates sands
in degree of possible compaction.
It is important to note that values of void ratio and relative density reported in this thesis refer to the condition just prior to application of reversing deviaitor stress - after consolidation under allround effective stress.
-20-
II
--
CHAPTER IV
TEST RESULTS
A. Preliminary Testing Program
A total of forty-one liquefaction tests were run in five basic series.
Each sample measured 2 inches in diameter by approximately 3.5 inches
high with little variation from one sample to another. The first eleven
tests were run to check out equipment and to determine ambient conditions
for future tests.
Wing-Beach sand was used and compacted at 80 to 86 of
relative density. A confining pressure of 54.2 psi and backpressure of
40.0 psi were applied to produce the initial test effective stress of
14.2 psi, or one kilogram per square centimeter.
The first four tests
were cycled at .13 cps and the remaining seven at 1 cps.
In each test, and in all subsequent tests run, the samples were isotropically consolidated to the initial state of stress.
A check of
saturation was made at this stage by increasing confining stress by 10.0
psi.
No drainage was allowed and the pore pressure response to this
increase noted.
The resulting increase in pore pressure for a saturated
sample should be close to 10.0 psi, depending on compressability of the
soil skelleton.
Measured response varied from consistantly near 100
percent (10.0 psi increase) in dense sands to between 88 and 95 percent
on most looser samples. A response above 88 percent was arbitrarily
considered to indicate complete saturation.
Several tests responded at
this level, with little increase after attempts at further saturation.
Cyclic stressing began after initial undrained stress-controled
loading to the first (compressive) cycle.
This step enables stress-strain
-21-
1
data to be collected and allowed uniformity comparison of "like" samples.
Data from this comparison is plotted in Figure B-5 and B-6.
Cycling was
begun and continued, in most cases, until liquefaction and necking of
the sample had occured.
Pore pressure of the undrained samples built up
to within 7 or 8 percent of cell pressure. Drainage of this excess pore
pressure was allowed, to return the sample to its initial backpressure.
Decrease in sample volume was measured.
The preliminary testing program led to the following conclusions:
1. Triaxial and measuring equipment performed very well during
cyclic testing.
2. The stress applicator performed satisfactorily although extreme
care would be necessary to accurately produce an equal and
opposite deviator stress.
Friction from piston binding was the
major problem.
3. A larger confining stress of 28.4 psi should be used in subsequent testing. This allows better definition in stress application from current pressure gages with 0.1 psi accuracy.
4. A cycling speed of 1 cps should be used.
This strikes an
"experimental" medium of being fast enough to prevent long duration tests and slow enough to allow measurement.
(This loading
is also more within the range of cyclic stress application during
an earthquake.)
5. The test proceedure, with a few minor changes, should be followed
for all subsequent tests.
The last 30 tests can be divided into four main groups or series.
Two series were run on Wing-Beach and Modified A3 sand samples.
In each
of the sands two relative density ranges (about 20 percent apart) in the
-22-
--.
9
medium to dense category were tested. The same range of investigation
was aimed for in both sands, but difficulty in being able to repeat test
density exactly caused some difference.
Several of these last 30 tests
were either not saturated, not liquefied, or not within the average density shown, and are considered separately.
B. Tests on Denser Wing-Beach Sand
The first of these test series was run on Wing-Beach sand with an
average relative density of 76%.
Eight tests (SCS-RUl2 through RUl9)
are included in the average covering a range of initial liquefaction
from 6 to 7,247 cycles.
Test RUl6 was cycled to 16,800 repetitions
with no liquefaction. Stress, stress path, and strain are summarized
in Figures 8, 11 and 17 respectively.
In addition, deviation stress
to cause initial liquefaction and to cause 5 and 10 percent double
amplitude strain is shown vs. number of cycles in Figure 21.
A
complete set of pore pressure data for these tests and for tests which
did not liquify is presented in Figure 24.
This test series is the second largest to the preliminary program.
In general the samples were consistantly prepared, varying between 73
to 81 percent relative density.
C. Tests on Denser Modified A3 Sand
The denser Modified A3 series had an average relative density of 83
percent with samples prepared from 79 to 86 percent. Six tests (SCS-RU21
through RU32) are included in this average covering a range of initial
liquefaction from 8 to 324 cycles.
Stress, stress path and strain are
summarized in Figures 9, 13, and 19 respectively.
-23-
II
--
Deviator stress to
cause initial liquefaction and to cause 5 andlO0 percent strain is shown
vs. number of cycles in Figure 22.
D.
Tests on Looser Modified A3 Sand
The looser Modified A3 series had an average relative density of 65
percent.
Five tests (SCS-RU26 through RU30) are included in this average
covering a range of initial liquefaction from 3 to 297 cycles.
Test RU31
liquefied at 934 cycles, but has RD = 55 percent and is used only as a
lower limit in several figures.
60 to 71 percent.
Other relative densities ranged from
Test RU27 had a pore pressure response of 77 percent,
somewhat below the assumed range for
complete
saturation.
Stress,
stress path and strain are summarized in Figures 9, 12, and 18 respectively.
In addition, strain during several phases of liquefaction is shown on a
p - q plot in Figure 15.
On this figure double amplitude strain at
initial, 5, 10, and 20 percent are contoured from stress paths on Figure
12.
E. Tests on Looser Wing-Beach Sand
The looser Wing-Beach series had an average relative density of 57
percent with samples prepared from 52 to 60 percent.
Six tests (SCS-RU33
and RU35 through RU39) are included in this average.
Initial Liquefaction
began from 0 to 3,724 cycles.
Test RU35 was cycled to 26,500 repetitions
before liquefaction occurred, the largest number in this testing program.
Stress, stress path and strain are summerized in Figures 8, 10, and 16.
Contours of double amplitude strain at initial, 5, 10 and 20 percent are
plotted on a p - q diagram in Figure 14.
-24-
II
---
F. Other Tests
Effects of saturation were studied with several tests (SCS-RUl8, 20,
27, 40, and 41) being run on partially saturated samples.
tests is presented in Figures 31, 32 and 33.
Data from these
Several tests were run at
pore pressure responses less than 80 percent, but were not intended to
investigate saturation.
The effect of pre-cycling Wing-Beach sand at a low stress level which
will not induce initial liquefaction was observed in two tests (SCS-RUl7
and RU40).
Data from test RU17 and a related test (RUl2) are presented
in Figures 34 and 35.
G. Presentation of Data
Figures presented in the main body of this thesis are primarily
summary plots.
Several individual tests are shown to examine satura-
tion and pre-cycling effects.
Individual presentation of tests is made
in Appendix D with plots of pore pressure and strain versus number of
cycles for thirteen tests.
Plotted tests are from the four major test
series and cover the range of cycling to cause initial liquefaction in
each series.
Data shown through figures in the main body is arranged in the following
order:
Figures 7 to 9
liquefaction summaries for each sand
Figures 10 to 13 stress paths for each series
Figures 14 to 20 strain summaries
Figures 21 to 23
strain data after liquefaction
-25-
Figures 24 to 27
pore pressure summaries
Figures 28 to 30 comparison of sands
Figures 31 to 36 saturation, pre-cycling, volume change
Figures 37 to 39 comparisons with previous tests
Significance of each figure will be discussed in the following two
chapters entitled Discussion of Test Results and Comparison with Previous
Tests.
Before proceeding into the analysis of presented data several fre-
quently mentioned parameters should be defined.
Initial Liquefaction is specifically defined as the first cycle at
which "appreciable" strains occur.
Appreciable usually meaning 0.2 to
0.4 percent axial strain, and indicates the first strain increase visible
on a recorded trace.
(Definition in terms of pore pressure would have
to be inconsistant as in no test did pore pressure increase above the
total confining stress.)
Relative density is based on experimental values of maximum and minimum density, and is used as the basis of comparison in many instances.
Section D of Chapter III discusses this reasoning.
Double amplitude strain is defined as the sum of strain in compression and in extension for the cycle mentioned.
Following initial lique-
faction use of the terms "extension" and "compression" strain becomes
somewhat less applicable, being based on the initial sample height.
Necking is the phenomena where a sample elongates and one crosssectional area of sample becomes considerably less than the average.
At this time common analysis assumptions are completely unreliable.
-26-
CHAPTER V
DISCUSSION OF TEST RESULTS
The stated purpose of this experimental thesis is to examine the
behavior of sands during liquefaction, and to determine the liquefaction
potential of the two tested sands.
Chapter V presents this behavior by
taking a close look at what is happening to stress, strain, and pore
pressure during testing.
A comparison of liquefaction potential in
Wing-Beach and Modified A3 sands is then made.
pre-cycling will also be discussed.
Influence of saturation and
A comparison of this test series
with liquefaction studies made primarily by Dr. H. Bolton Seed and
Dr. Kenneth L. Lee is included in Chapter VI.
A. Effective Stress During Liquefaction
In undrained reversing-stress cyclic loading tests, deviator stress
(al - G3 ) is applied alternately in compression and extension. If the
initial sample stress is isotropic, shearing forces are also equal in
magnitude and reverse with each cycle, as shown in Figure 2. Deviator
stress, by definition the difference between the two principal stresses,
is not affected by a change in pore pressure.
Effective confining stress
(F3), on the other hand, decreases with increasing pore pressure.
In un-
drained testing of cohesionless soils effective confining stress controls
sample strength.
Therefore if pore pressures during cyclic loading are
built up in excess of those associated with undrained shear, failure may
occur with reversing deviator stresses smaller than the usual undrained
compressive strength.
-27-
Undrained strength at liquefaction and stresses during testing can
be shown on a p- q diagram (where p
cr +cYH
=
2
ad
v
oH
Figures 10 through 13 are summary plots of this type for the four major
test series.
Each isotropic test originates at q
initial effective confining stress.
=
0, and p
=
&co, the
During initial compressive stress-
controled loading (first cycle) the sample behaves as if undergoing undrained shear.
When cycling begins each sample has a "stable" stress
state in both extension and compression which can be graphically shown.
A locus of points would trace horizontal lines, both above (compression)
and below (extension) the p-axis, progressing toward the q-axis.
Although
friction angle during liquefaction has yet to be defined, large strains
could be expected somewhere in the vicinity of the drained shear envelope.
Undrained strength is,of course, the applied deviator stress (providing
liquefaction occurs).
It is apparent that in each of the four series plotted Mohr-Coulomb
failure criteria (q = p tan ca)may be arpplied for developing a failure
envelope.
(This envelope can only apply over the range in which lique-
faction is possible.)
It is significant that failure envelopes in each
test appear nearly identical to respective criteria for undrained shear.
Cyclic stress application, while radically reducing undrained strength,
does little to alter mode of sample failure.
Small differences in failure envelope are noted between looser and
denser sands.
Looser sand envelopes (Figures 10 and 12) in extension
appear to be slightly above that from drained shear.
Quantitative use
of this data is limited, however, by scatter in individual plots and by
scarcity of test information.
It was impossible to present all tests
because axial stress data was not always recorded at the instant of
-28-
-i
liquefaction.
(Deviator stress changes slightly due to changes in binding
friction along the triaxial piston.)
B. Strain During Liquefaction
In comparison of magnitudes, strain during pore pressure buildup is
nearly insignificant in comparison to large post-liquefaction straining.
Sample deformation in response to cyclic stress changes before and after
liquefaction is shown in Figure 2. Axial strain, excluding non-sample
movements, between an extension and compression cycle varied from 0.4 to
Little change, if any, was
0.6 percent, depending on applied 1oads.
noted during cycling until a state near initial liquefaction was reached.
A very slight "drift" toward sample elongation was noted during most
tests.
Strain increased gradually, to merge at initial liquefaction with
large strains.
("Drift" meaning a net elongation, without increasing
strains - a decrease in compressive strain being equal to increase in
extension strain from cycle to cycle.)
Liquefaction was clearly defined
by a sudden strain increase in all cases.
Summaries of axial strain vs. number of cycles are plotted for each
test series and the preliminary test program in Figures 16 through 20.
Strain at initial liquefaction, and at 5 and 10 percent double amplitude
strain for three test series are shown in Figures 21 through 23.
In
Figures 14 and 15 contours of strain have been traced from stress path
information regarding the two looser sand test series.
Necking occured in nearly every sample tested.
In the denser sands
necking took place at or before 15 percent strain, while looser samples
consistantly reached 25 to 30 percent strain before necking.
Necking
is largely caused by reduction in area due to local stress differences
-29--.
A
during extension.
A larger number of cycles is quite often required to
reach 15% strain in looser samples than 30% strain in denser samples.
The number of cycles, after initial liquefaction, to reach any cycle
of double amplitude strain increased with increasing Ni , the cycle number
at initial liquefaction.
For denser samples of Wing-Beach sand which
required 7, 47, and 506 cycles to initiate liquefaction, 7, 10, and 20
additional cycles respectively were required to reach 5% double amplitude strain.
In looser Wing-Beach sand with 27, 82, and 683 cycles to
initial liquefaction, 1, 2, and 5 additional cycles respectively were
required to reach 5% double amplitude strain.
Modified A3 sand samples
displayed the same trend, but took more cycles to reach 5% strain in
both cases.
Large strains at initial liquefaction first occured during the extension phase of cycling in all tests.
Large compressive strains did
not occur until after several more cycles.
Figures 14 and 15 show that
the stress conditions at initial liquefaction fall somewhat inside the
effective stress Mohr envelope from shear tests.
Larger stresses and
"earlier" liquefaction correspond to data points furthest away from
this envelope.
In compression, as strain increases following initial
liquefaction, data points representing effective stress move closer to
the Mohr envelope.
For both sands in compression, the maximum friction angle corresponded
to the largest strains recorded, before necking developed.
The extension
envelopes sketched at various stages of double amplitude strain are not
as consistant.
For the same extension strain the liquefaction envelope
is slightly above the drained value with Modified A3 sand, and slightly
below that of Wing-Beach sand.
Such scatter is reason to believe break-30-
down of computational assumptions occurs before actual necking takes place.
Qualitatively, interest in number of cycles to cause varying percentages of strain is limited to tests where initial liquefaction occurs at
a low cycle number. It is important if large strains take 1, 3, or 10
cycles to develop when initial liquefaction begins after only several
cycles.
(Major earthquake induced forces do not generally continue for
many cycles.)
Large strains are occuring within 20 additional cycles
even at tests with initial liquefaction beginning at 800 to 1000 cycles.
This amounts to only 2 or 3% more cycling to reach large strains, compared with 100 or 200% more cycling required at a low Ni.
C. Pore Pressure Behavior
Pore pressure changes are controlled by amount of rearangement or
collapse of soil structure.
Changes in interparticle force occur twice
during every cycle, causing small relative movements between particles.
In cohesionless soils particle to particle contact is made over many
highly stressed small contact areas.
Repetitive
stressing of these
contact points causes particle rearangement from sliding and from
abrasive wear.
Both actions tend to move particle centers closer to-
gether - cyclic loading causes little translational straining with resulting possible dilation.
As particles become more closely packed they
move away from the sample boundary
(rubber membrane).
confining pressure is then shifted onto the pore water.
More total
Through this
mechanism pore pressure buildup will occur in all sands - even dense
sand which exhibit large dilation during undrained compressive shear.
Increase in pore pressure before liquefaction follows a definite
pattern which may be separated into three phases:
1) large initial
-31-
---..
d
increase during the first few cycles
in pressure per cycle, and then
2) more gradual but steady increase
3) rapid increase in buildup of pore
pressure per cycle until initial liquefaction occurs.
Peak pressure
during the compression phase of each cycle is plotted in Figure 24 for
9 tests on Wing-Beach sand of RD ~ 76%.
Test SCS-RU35 on Wing-Beach sand
at RD = 52% failed at N = 26,000, the longest test, and is also shown.
Figures 25 through 27 present pore pressure data for comparison
compression
of
vs. extension buildup, and for comparison of Wing-Beach vs.
Modified A3 sand.
Pore pressure during extension and compression for many
tests are also plotted in Appendix D.
Phases of pore pressure buildup are clearly shown by Figure 24.
initial buildup during phase 1 is partially shown by lines
values of initial pore pressure.
Large
connected to
It might be expected that dense dilatant
sands would decrease in pore pressure during the first application of
stress (by stress controled loading), but this is not the case.
Even in
sands showing a strong dilatant tendency during compressive shear, volume
decreases slightly until 0.3 or 0.4 percent axial strain. Applied deviator stress causes about 0.15 percent strain in most repeated loading
tests on Wing-Beach sand, showing a tendency for pore pressure to increase.
Pore pressure buildup during Phase 2 is remarkably linear on a pressure vs. log cycle plot.
The two curves that deviate slightly from this
behavior, SCS-RUl2 and RU41, have the lowest pore pressure response.
RU41 is partially saturated while RUl2 has a response of 90 percent and
is considered near fully saturated.
The beginning phase 3 is indicated
by a point of inflection at about 40 to 60% of the number of cycles at
initial liquefaction.
metic cycle scale.
Appendix D plots of pore pressure are to an arithe-
(The inflection point is less well defined on these
-32-
---
_A
figures.)
An increasing rate of pressure rise characterises phase 3. Pore pressure buildup over the last few cycles before liquefaction is larger than
in any other test period. Reduction of effective confining stress during
phase 2 significantly increases the deviator stress ratio
A( 1 - 03)
a3
Shearing force is now large in comparison to effective confining stress
and causes more particle slipping per cycle. As state of stress approaches
the failure criteria, large strains associated with liquefaction begin,
in turn causing more particle reorientation.
Following liquefaction,
pore pressure in compression and extension level off, respectively increasing and decreasing slightly.
Figures 24 and 27 are dimensionless graphs of pore pressure vs. number of cycle.
Excess pore pressure divided by initial effective confining
stress is plotted vs. number of cycles divided by the cycle at initial
liquefaction.
Figure 24 indicates that with longer tests pore pressure
shows significant increase proportionately later in the test.
Figure 27
plots pore pressure increase for samples from all four series in the
range of 300 to 700 cycles to cause initial liquefaction.
Figure 25
plots three tests, from the range of 2000 to 6000 cycles to cause initial
liquefaction, on a conventional scale of log number of cycles.
These
two figures (and others plotted but not included in the thesis) show that
there is no consistent pattern in pore pressure increase as a function
of density and nature of sand.
D. Liquefaction Potential of Tested Sands
Summary curves for Wing-Beach and Modified A3 sands are shown on
-33-
--e
Figures 8 and 9. All saturated tests from the four principal series are
shown.
Each test is represented by two points, the average deviator
stress, and the deviator stress during the extension part of the loading
cycle. Contours were drawn with respect to extension deviator stress if
a test involved a significant difference between those two values.
Con-
tours of 60 and 80 percent relative density for each sand are shown in
Figure 28.
Figure 29 presents a comparison of the same density sands
using Wing-Beach curves of Figure 28 and Modified A3 sand curves at the
same dry density.
Wing-Beach sand is considerably less succeptable to liquefaction
than is Modified A3 sand.
This is true over both density ranges shown,
and according to both relative density and dry unit weight comparisons.
The dry unit weight comparison indicates a smaller degree of difference,
especially at higher stress applications.
Relative density is consi-
dered by the author as a more meaningful method for comparison.
On this
basis 30 and 40 percent more deviator stress is required to liquefy
Wing-Beach sand in 100 cycles at densities of 80 and 60 percent. At 10
cycles the same ratio drops to 22 and 33 percent.
Considerably more resistance to liquefaciton is shown by the rounded
uniform sand than by a more angular and well graded sand of the same
density. Some aspects of stress-strain behavior, strength, dilatency
and grain characteristics will be discussed as contributing factors in
influencing this result.
"Reason" might first suggest that a soil with a higher friction angle
is likely to be less susseptible to liquefaciton.
There are a number of
reasons why this may not be the case in the two sands tested.
If lique-
faction is approached from the mechanism of pore pressure buildup as
-34-
presented in section C, effects on this buildup are of overiding importance.
This mechanism views pressure increase as being solely dependent
on how much particle slippage and wear occurs during cycling.
Drained shear tests (Figures 5 and 6) on Modified A3 sands indicate
an initial elasticity modulus only 25 percent that of Wing-Beach sand.
For the same amount of stress, strain
times greater.
in the "MA3" samples will be four
(This arguement is tempered by the fact that stress-
strain behavior is not the same in undrained and drained loading.)
From
these same figures a lower tendancy for dilation is noted in MA3. A
lower such tendancy means particle interference plays even less of a
role in strain than does sliding.
It is apparent that more particle
slippage is occuring in the MA3 samples.
ment is likely to take place.
Consequently, more rearange-
Sharp angular grains of MA3 sand may be
slightly more susceptible to wear or fracture than are the rounded water
worked beach sand grains.
It is suspected that these results and subsequent reasoning cannot
be extrapolated to all uniform or all angular sands.
At this time
sufficient data on susceptibility of various sands to liquefaction does
not exist to act as a basis for generalization.
Each sand in question
will have to be analyzed for liquefaction potential.
E. Effect of Saturation, Pre-cycling and Confining Stress
It was possible during this testing program to obtain considerable
data on the effects of saturation, and to a lesser degree on the effects
of pre-cycling and confining stress differences.
Partial saturation was found to have an extremely large effect on
resistance to liquefaction.
The deviator stress required to initiate
-35-
m
i
-1
liquefaction at a given number of cycles is increased with decreasing
saturation for otherwise identical samples.
cycles when S < 100% to number at S
Z
The ratio of number of
100% for initial liquefaction in-
creases logarithmicly with decreasing pore pressure response.
Figure
31 is a summary
of these effects for tested samples
at low
pore
presJ~IIIIIIHIJ
3Ulllr
1~3
iuvv
yvir;
rlr;3W
sure
sure response.
response.
Pore pressure
pressure response
response between
between 60
60 and
and 80
80 percent
percent continuously
continuously backbackPore
$1
calculates to
to aa saturation
saturation of
of only
only slightly
slightly less
less than
than 100
100 percent,
percent.
calculates
P
shows
shows that
that these
these tests
tests require
require 222 to
to 10
10 times
times the
the number
number of
of cycles
cycles for
for
shows
that
these
tests
require
to
10
times
the
number
of
cycles
for
liquefaction
than do
do samples
samples with
with aa higher
higher response.
response.
7iquefaction
than
response of
of 38%,
38%, has
has aa computed
computed SS >
99 percent,
percent, but
but took
took 5297
5297 cycles
cycles to
to
response
of
> 99
5297
cycles
b
fail.
fail.
RUl3,
RU13,a
RU13,a
a comparable
comparable test
test at
at 95%
95% response,
response, failed
failed after
after 47
47 cyclic
cyclic
cyclic
I
When the
the deviator
deviator stress
stress of
of
When
RU41 is plotted on the summary curve (Figure 8), it is 40 percent higher
RU41 is plotted on the summary curve (Figure
8),
it is 40 percent higher
than that··required
that-required to
to fail
fail an
"equivalent" saturated
sample at
saturated
at 5297
than
an "equivalent"
sample
5297
8),
it is 40 percent higher
RU41 is plotted on the summary curve (Figure
cycles.
cycles. · -
Test SCS-RU20
SCS-RU20 failed
failed in
in 605
605 cycles
cycles and
and has
has SS
Test
a
i
SCS-RU41 with
with aa
SCS-RU41
t
applications of
of thesame
the same deviator
deviator stress.
stress.
applications
i
Data
Data
Data
F
84 percent
percent with aa
84
pore
When
summary
pore pressure
pressure response
response of
of 5
5 percent.
percent.
When compared
compared to
to the
the summary
curves on Figure 7, deviator stress is about 2.9 times
curves on Figure 7, deviator stress is
about
2.9
times
failure
failure at
at the
the same
same number
number of
of cycles
cycles of
of a
a saturated
saturated sample.
sample.
that for
that for
For
For the
the
3
i
same stress
stress applied
applied to
to aa saturated
saturated sample,
sample, failure
failure should
should occur
occur during
during
same
the
first few
few cycles.
cycles. The
summary data
data on
on Figure
Figure 31
was compiled
with
the first
The summary
31 was
compiled with
no reference
reference to
to relative
relative density
density or
or type
of sand,
sand.
no
type of
Not enough
enough data
data isis
Not
available
available to
to form
form a
a basis
basis of
of comparison
comparison in
in either
either case.
case.
Figure
pressure buildup
buildup
Figure 32
32 compares,
compares, on
on a
a dimensionless
dimensionless basis,
basis, pore
pore pressure
for
tests SCS-RU41
with tests
tests upon
upon saturated
saturated
for partially
partially saturated
saturated tests
SCS-RU41 and
and RU20
RU20 with
relative densities.
specimens of
of the
the same
same relative
specimens
densities.
Large
pressure increase
Large pore
pore pressure
increase
-36-
..A
occurs later in the partially saturated tests when compared to saturated
samples which liquified at the same number of cycles.
(Section B found
that difference in pore pressure buildup was consistantly dependent only
on cycle range.)
In partially saturated samples a buildup of pore
pressure causes air to go into solution.
well as in sample density results.
An increase in saturation as
Test RU41 had a pore pressure re-
sponse before testing of 36 percent and after testing of 88 percent.
Considerably more rearangement of soil grains is required to build up
pore pressure in the unsaturated state.
Pore pressure in compression and extension for test RU20 is plotted
in Figure 33.
When pressure increase per cycle begins to rise shortly
before liquefaction, difference in the extension and compression pore
pressure value also increases.
This is not noticeable in saturated
tests, and is a good indicator
of increasing saturation in RU20.
Pore
pressure responds more to stress changes when at a higher state of
saturation,
If a sample is pre-cycled at a lower deviator stress and not liquefied, it will be more resistant to liquefaction than a non pre-cycled
sample. Two tests run after pre-cycling and reconsolidation provided
the basis for this conclusion. Test SCS-RU17 was cycled 16,800 times
at a deviator stress of 9.0 psi with no liquefaction (this test called
RUl6).
(Reconsolidation changed the relative density by only 2%).
Test
RUl7 is plotted in Figure 34 and pore pressure data for RUl6 is shown
in Figure 24.
Test RUl2 is comparable to RU17 and failed at 46 cycles,
as shown in Figure 35.
The pre-cycled test RU17 failed after 2446
cycles, or over 5 times as many.
Test RU40 was pre-cycled in several stages for 500 cycles per stage.
-37-
Finally deviator stress was increasedduring cycling and the sample liquefied at an extension stress of 24.5 psi,
This is 40 to 50 percent higher
than that stress required to fail a similar sample on the first cycle,
Initial pore pressure response of this test was only 38 percent, somewhat
complicating any use of its data, and the test is not shown plotted in
this thesis.
It is clear from these two tests that to some degree resistance to
liquefaction is increased by pre-cycling.
Similar strength increase
from pre-cycling was noted in cyclic (non-reversing) drained tests run
on Ottawa sand for the Inter-American Program during 1967
(Reported
in Progress Report No.1)
Volume change measurements were made after liquefaction, when drainage
was allowed as samples were returned to the pre-test isotropic state.
A summary of these volume decreases is shown in Figure 35.
Although
considerable data scatter is evident, curves could be sketched for each
test series but the looser Wing-Beach sand.
Scatter can well be expected
because of many influencial factors not considered, such as number of
cycles after liquefaction, amount of necking, and slight density variations, Even so, two definite patterns of behavior emerge from this plot:
1I
Induced volume decrease is larger with increasing number of cycles
to initial liquefaction in every tested series,
2. Looser sands undergo more volume change than do denser sands
after failure at the same number of cycles.
It is noted that
strains reached before necking in loose sands are about twice
as large as dense sample strains.
3.
Modified A3 sands tend to decrease in volume more than WingBeach sands.
-38-
CHAPTER VI
COMPARISON OF RESULTS WITH PREVIOUS STUDIES
Comparison of soil liquefaction behavior will be made to similar
testing of Sacremento River sands at the University of California.
This
data has been published primarily by Dr. Kenneth L. Lee and Dr. H. Bolton
Seed in several A.S.CE. Soil Mechanic Journals from November, 1966 (see
reference list).
Similarities in behavior, test results, and possible
differences in these and in test conditions will be noted,
Triaxial isotropic cyclic tests were reported by Dr.'s Lee and Seed
primarily in Journals SM6, November 1966, and in SMI, January 1967.
Fractionated samples of Sacremento River sand were tested under a number
of confining stress values at cyclic loadings of 2 cycles per second and
6 seconds per cycle.
Sacremento River sand is quite similar in gradation
and void ratio range to Wing-Beach and Modified A3 sands.
Figure 39
presents the gradation curve of all three tested sands and respective
maximum and minimum void ratios,
Grain size at D50 is virtually the
same with uniformity of Sacremento grains about midway between Wing-Beach
and Modified A3.
Microphotographs of untested Sacremento River sand in-
dicate a composistion of sub-rounded or subangular grains.
Performance of the three sands during liquefaction is quite similar.
Both testing programs conclude the following general behavior:
1. Cyclic stress applications will induce liquefaction over the
range of density tested in this thesis.
2. Number of cycles required to induce liquefaction increases as
deviator stress is reduced.
-39-
3. Increasing test confining pressure causes a similar increase in
cyclic deviator stress necessary to fail a sample in the same
number of cycles.
4,
Deformations after liquefaction in looser sands become large at
fewer cycles than do deformations in denser sands.
5o
Cyclic deviator stresses required to fail a sample are considerably lower than the static stress required under similar sample
conditions,
It is evident that deviator stress magnitude, number of cycles of
stress application, confining pressure, and void ratio at time of test
are major factors governing liquefaction behavior,
Comparison of re-
corder traces for pore pressure and strain are quite similar.
(In fact
both pore pressure traces record a curious impulse of pressure following
load changes after liquefaction )
Comparison of initial liquefaction potential of the three sands
mentioned is shown in Figure 37.
Curves for a relative density of 78
percent in Wing-Beach sand and Modified A3 sand are compared to a
reported curve for Sacremento River Sand (as referenced in Figure 37)
The California tests were run at I and 5 kilograms per square centimeter
but not at 2 kg, per sq. cm., the confining pressure of most tests in
this thesis,
Both Sacremento curves are shown and indicate considerable
difference in required deviator stress for liquefaction even after "normalization".
It is evident from these curves that both sands at M.I T.
appear more susceptable to liquefaction than does the Sacremento River
sand,
stress.
1.
That is, they will fail after fewer cycles under the same deviator
Three possible reasons for this difference are now suggested:
Wing-Beach sand and Modified A3 sand are more susceptable to
-40-
d
liquefaction than is the Sacremento River sand.
2. The sands tested are not being compared correctly on a basis of
relative density, due to differences in definition,
3,
Test conditions are not similar enough to keep test-associated
differences at an insignificant level.
Figure 38 compares Wing-Beach sand at RD = 78% and 60% to Sacremento
River sand at RD = 78%, 60% and 38%.
Tests at both 1 and 5 kg, per sq.
cm. are shown for Sacremento sands of RD = 78% and 38%, but were not
available at 5 kg. per sq. cm. for RD = 60%.
(It is interesting to note
that at RD = 38% both confining stress levels yield similar normalized
curves, but not at RD = 78%.)
Figure 38 indicates that the second pro-
posed reason for difference is unlikely to be of great significance-. For
this to explain differences, a relative density of 38 percent for Sacremento
River sand would have to be equivilant to RD = 60 percent in Wing-Beach
sand.
The author considers reasons 1 and 3 to be plausable influences on
reported behavior,
Both "true" and testing-related differences are likely
to be of influence in comparison of results.
Although the three sands
are very similar on an indirect and outward comparison they may not behave similarly when subjected to cyclic loadings,
Differences in test conditions have been known to cause varying test
results in even the most standardized of tests.
It may well be that
enough difference exists in equipment to accentuate or even reverse true
potential for liquefaction.
The author is not familiar enough with
testing equipment and precedures used at the University of California
to offer an explanation on this basis,
play
a
significant
role
in
Several considerations which may
test-oriented
factors
are
now
suggeste
d:
-41-L_
-i
~~1
1. Sample size effects (2 inch q by 3.5 inch high at M.I.T. and (?)
1,4 inch ý by 3.5 ± inch at California)
2. Sample uniformity, stress history or disturbance differences
3. Stress application effect (slight play in top cap connection at
M.I.T. may cause different mode of application than a locked
1
5
4.
connection)
Satpeuraindiffeiysrenesoyordsubne
ifrne
Stress measurement difference (load cell located inside the
triaxial cell at M.I.T. and outside at California)
5.
I
Saturation difference
6.
6. Rate of cycling and shape of stress wave
wave
In conclusion,
conclusion, no direct comparison of liquefaction potential for two
In
sands can be made at this time if
equipment,
if testing was on different equipment,
This is
is aa "temporary"
"temporary" conclusion
conclusion which could be clarified by comparative
comparative
tests.
tests.
Such tests may be available in
in the future on Wing-Beach sand. Dr.
Lee, at the University of California at Los
Los Angeles,
Angeles, has been sent a
a
quantity of Wing-Beach sand and plans to run several cyclic
on
cyclic tests on
similar density samples as those reported in
in this thesis,
thesisl
-42-
--I
IHPER\I
CHAPTER VII
CONCLUSIONS AND RECOMMENDATIONS
Conclusions are outlined for the two main areas under study in this
thisis:
1) behavior of isotropically consolidated triaxial samples under
reversing cyclic load changes, and
2) comparison of liquefaction poten-
tial of Wing-Beach sand and Modified A3 sand.
This study into behavior of triaxial samples during a liquefaction
test results in the following conclusions:
1. Although strength of sand samples is much lower under cyclic
loading than under static testing, failure criteria appears to
follow the effective stress Mohr-Coulomb failure envelope from
drained static shear tests.
2. Stress application by reversing cyclic loads of equal magnitude
will result in failure occuring initially during the extension
phase of cycling.
3. Pore pressure buildup before liquefaction follows a definite
pattern with three distinct stages:
a. large initial increase on first few cycles,
b,
smaller but steady increase in pressure over intermediate
cycles
c. rapid increase in new pore pressure generated per cycle near
liquefaction,
4. Ultimate pore pressure buildup during liquefaction is a function
of effective stress principles, Mohr-Coulomb failure envelope,
and applied deviator stress
-43-
5. Deviator stress magnitude, number of cycles of stress application,
confining pressure, and void ratio at time of testing are major
factors governing liquefaction behavior.
6. Partial saturation will have a very large effect on liquefaction,
even at levels of saturation near 99 percent. The effect will be
to increase the number of cycles required for failure at a given
cyclic deviator stress.
7. Triaxial samples undergoing pre-cycling of deviator stress (with
no liquefaction) will require more cycles for failure to occur
than will non-precycled samples,
Conclusions from the study into liquefaction potential of Wing-Beach
sand compared to Modified A3 sand can be summarized as outlined below:
1. Modified A3 sand is more susceptable to liquefaction than is
Wing-beach sand - at all comparable values ot relative density
tested.
2. It is not unreasonable for a rounded uniform sand (Wing-Beach)
with a lower friction angle to be more resistant to liquefaction
than an angular, better graded sand (Modified A3).
3. Volume changes, when liquefied samples drain to initial conditions, increase with increasing void ratio of samples.
LV also
appears to increase with decreasing applied deviator stresses.
Conclusions from comparison of test run at M.IoT. with test by Dr.'s
Lee and Seed at the University of California are as follows:
1. Basic liquefaction behavior is the same in both test series
reported.
2. Both Wing-Beach sand and Modified A3 sands appear considerably
more susceptable to liquefaction than does Sacremento River sand.
-44-
m
kC~---
3. A large possibility exists that potential for liquefaction is
much influenced by test and sample conditions which may vary from
one laboratory to another.
4. No direct comparison of liquefaciton potential for sands tested
in different equipment can be made until a comparison of equipment is made.
Recommendations for future research in cyclic loading effects are
virtually unlimited.
Several suggestions for areas particularly of
interest to the author are:
1. Isolate and investigate the effect of gradation on liquefacti on
potential of sands.
2. Isolate and investigate the effect of grain shape on liquefac ti on
potential of sands.
3.
Investigate the effects of partial saturation, especially nea r
the threshold of complete saturation.
4. Investigate the effects of pre-cycling loads on samples in a
systematic program.
5. Investigate and define effects of testing equipment and of pr
o0-
ceedure on liquefaction.
Compare results from several testin!g
apparatus in use.
-45-
r
REFERENCES
Healy, Kent A., (1963).
"The Dependence of Dilation in Sand on Rate of
Shear Strain", Ph.D. Thesis, Massachusetts Institute of Technology.
Lambe, T. W. and Whitman, R. V., (1964).
"An Introduction to Soil
Mechanics", notes from a forthcoming book, published by Massachusetts
Institute of Technology.
"Drained Strength Characteristics
of Sand", Proceedings, ASCE Journal of the Soils Mechanics and
Foundations Division, Vol. 93, No. SM6, pp 117-142.
Lee, K. L., and Seed, H. B.,
(1967).
Lee, K. L., and Seed, H. B. (1967).
"Cyclic Stress Conditions Causing
Liquefaction of Sand", Proceedings, ASCE, Journal of the Soil Mechanics
and Foundations Divison , Vol. 93, No. SMI, pp 47-70.
Lee, K. L., and Seed, H. B., (1967). "Dynamic Strength of Anisotropically
Consolidated Sand", Proceedings, ASCE, Journal of the Soil Mechanics
and Foundations Divison, Vol. 93, No. SM5, pp 169-190.
Luscher, U., Ortigosa, P., Rocker, K., and Whitmen, R. V,, (1967).
"Repeated Load and Vibration Tests upon Sand, Progress Report No. 1",
Research Report R67-29, Soils Publication No. 203, Massachusetts
Institute of Technology.
"Densification of Sand by Vertical Vibrations with Almost Constant Stresses", Masters Thesis, Massachusetts Institute of Technology.
Ortigosa De Pablo, P. (1968).
Seed, H. B., and Idriss, I. M., (1967).
"Analysis of Soil Liquefaction:
Niigata Earthquake", Proceedings, ASCE Journal of the Soil Mechanics
and Foundations Division, Vol. 93, No. SM3, pp 83-108.
Seed, H. B., and Lee, K. L., (1966). "Liquefaction of Saturated Sand
During Cyclic Loadings", Proceedings, ASCE, Journal of the Soil
Mechanics and Foundations Engineering, Vol. 92, No. SM6, pp 105-134.
-46-
~~1
Seed, H. G., and Lee, K. L., (1967).
"Undrained Strength Characteristics
of
Soils",
ASCE,
Ot Cohesionless
COneSlOnleSS
30% IS", Proceedings,
rrOceedlnSs,
AZICt, Journal of
ot the Soil
~oi I
Mechanics and
and Foundations
Engineering, Vol,
Vol. 93,
93, SM6,
SM6, pp
pp 333-362.
333-362.
Mechanics
Foundations Engineering,
-47-
d
~-.-4
~Lr~--'~L~
·
·...-~i~
-. i;'-·CII
Idl
------
-
I·I.~··~~...
_
TABLE I SUMMARY OF CYCLIC LOADING TESTS
-~-~-7
....
II
TEST
SCS-
SAND
RU1
RU2
RU3
RU4
RU5
RU6
RU7
RU8
RU9
RU10
RUI11
RU12
RUl 3
RUI14
RU15
RUI 6
*RU17
RUl 8
RU19
RU20
RU21
RU22
RU23
RU24
RU25
RU26
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
WBS
MOD
MOD
MOD
MOD
MOD
MOD
MOD
Yd - PCF
238
657
1
1
21
330
1,341
1,067
1
5
1
46
47
6
510
16,800N/L
2,446
5,350N/L
7,247
605
8
30
81
23
167
24
•
.......... ..
i ..
3
·
100.6 t
100.1"
100.8
100.4
99.6
100.5
100.0
100.4 t
100.8
99.8
99.0
.657
.676
.653
.660
.673
.658
.667
.660
.663
.658
.672
.682
.690
.679
.671
.693
.678
.781
.696
.669
.673
.681
.666
.736
t
98.4
97.9
98.6
99.0
97.7
98.6
92.9~ t
97.5t
99.1
98.8
98.4
99.3
95.3
IY..
eo
I--
RD%
Ce - psi
54.2
54.2
54.2
54.2
54.2
54.2
54.2
54.2
54.2
54.2
54.2
58.4
58.4
58.4
57.4
56.4
56.4
56.4
56.4
56.4
56.4
55.4
55.4
55.4
55.4
55.4
uo - psi
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
30.0
30.0
30.0
29.0
28.0
28.0
28.0
28.0
28.0
28.0
27.0
27.0
27.0
27.0
27.0
) - psi
PORE
PRESSURE
RESPONSE
COMP.
EXT.
%
3.8
11.2
15.1
9.8
6.7
4.6
6.1
6.6
8.1
4.1
4.1
11.4
10.3
9.7
9.8
7.3
11.8
8.6
8.9
14.5
16.9
11.9
11.0
11.8
8.1
8.1
4.8
6.4
13.4
10.8
6.9
5.6
5.9
5.6
7.7
4.8
4.8
10.2
10.7
14.5
9.1
7.5
10.0
7.4
7.7
14.4
15.0
9.7
10.8
10.1
8.7
8.6
A(c 1 - a
3
CPM
CPM
CPM
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
-
100
99
95
100
100
95
98
100
90
95
98
95
A H2 0
AFTER
TEST
CC
0.70
3.55
~ 2.30
1.92
90
75
6
68
5
63
87
65
96
80
87
5.04
1.48
5.48
5.32
4.99
~2.82
4.38
4.06
4.35
5.87
t
-
-iC·L
RU27
RU28
RU29
RU30
RU31
RU32
RU33
RU34
RU35
RU36
RU37
RU38
RU39
*RU40
RU41
i
-
MOD
MOD
MOD
MOD
MOD
MOD
WBS
MOD
WBS
MOD
WBS
MOD
WBS
MOD
WBS
A3
A3
A3
A3
A3
A3
24
51
297
3
934
324
683
A3
1
A3
A3
A3
26,500
3,724
82
27
1
500 N/L
5,297
93.8
93.3
94.8
92.4
91.1
98.1
95.1
90.6
93.5
94.8
94.1
95.2
94.9
97.9"
98.4
.765
.774
.745
.791
.816
.686
.740
.827
.770
.744
.758
.739
.761
.689
.682
55.4
54.4
54.4
54.4
54.4
54.4
54.4
54.4
54.4
53.4
53.4
53.4
53.4
53.4
53.4
27.0
26.0
26.0
26.0
26.0
26.0
26.0
26.0
26.0
25.0
25.0
25.0
25.0
25.0
25.0
9.5
6.9
5.4
11.4
4.6
6.5
5.6
8.0
5.2
6.7
8.0
11.1
11.5
10.9
12.1
9.5
7.0
5.7
10.3
3.8
7.9
7.3
8.5
5.4
6.7
8.9
11.1
11.9
12.9
10.7
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
CPS
8.27
4.83
5.82
4.20
8.37
4.72
4.79
4.69
9.00
5.62
7.25
1.05
2.65
4.10
5.70
Values shown for yd, eo, RD are computed from physical sample measurements, all other values are from water contents.
t A small void was visible near the top cap after sample preparation.
* Stage test, where more than one level of deviator stress was applied all data shown is
for stage reported under Ni..
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-5"0-
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FIG.•R6 2
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APPENDIX A
EQUIPMENT
The equipment used in the cyclic loading tests is shown in Figure A-1
with a test in progress.
Appendix A will describe individual characteris-
tics under sub-headings of loading, testing and measurement equipment.
A.1
Loading Equipment
The stress application device is shown in Figures A-2 and B-2.c.
This apparatus consists of two pressure chambers or "pots" with rubber
diaphram seals at both ends of each pot. A movable piston passes through
the center of each pot and is guided by bushings at either end of the
device.
Air pressure in the top or bottom pot forces the piston upward
or downward over a total distance of travel equal to 2 inches.
The
stress applicator is positioned over a triaxial cell and is rigidly
connected by three legs, adjustable for allignment. Attachment to the
piston of the terminal cell is made through a threaded connector into
which both pistons are clamped.
The lower half of this connection ap-
pears in Figure B-2.b with the slotted piston grove and split washer
covered by the connection nut.
Each pot can withstand inside pressures
greater then 100 psi, although not over 40 psi was used during testing.
An axial piston force of approximately 40 pounds is created by a change
in pot pressure of 10 psi.
The dynamic stress control device used for cycling air pressure to
the stress applicator is shown schematically in Figure A-3 and is the
wooden box in the center of Figure A-1.
-89-
This device consists basically
of two large capacity pressure tanks,
a solenoid valve, an electric
counter, and tubing with required pressure regulators.
Cyclic loads are
applied by maintaining a constant pressure in the bottom pot of the stress
applicator while cycling pressure between 0 psi and the pressure required
to produce an equal but opposite stress.
The "bottom pot pressure tank"
is connected to the bottom stress pot and a compressive deviator stress
is built up to the first cycle slowly with air pressure.
The top stress
applicator pot is connected to the "top pot pressure tank" through a
solenoid initially open to atmospheric pressure. Activation of the
solenoid opens the flow path between the pot and tank, thus applying tank
pressure to the top stress pot.
When the maximum pressure in the top pot produces twice the axial
force as does the constant pressure in the bottom pot, equal but opposite deviator stresses are applied to the triaxial cell piston. Actual
pressures are not simply X psi and 2X psi because of cell pressure acting over the triaxial piston area, adding an extra force in the extension direction.
The pressures must be adjusted for this force dependent
on cell pressures,and for piston friction.
Cell pressure is maintained
by another connection to a regulated high pressure source.
Pore pressure and volume flow measurements are controled by a system
of mercury pots, burettes,
and valves shown in Figure A-1.
The control
board is set up in such a way as to permit easy measurement of volume
changes and control of top and bottom pore pressure lines.
Two timer systems were used for this testing program. All tests run
after SCS-RU5 were cycled at 1.0 cycles per second.
Tests SCS-RUI to
RU5 were run at 8.0 cycles per minute, and are part of the equipment
testing series.
In the later tests, two Intermatic industrial control
-90-
timers (solid state SS-10222-Bl) were connected so to provide continuous
ON-OFF timing controls, and theoretically has a timing range of .05 to
0.5 seconds for each half cycle.
Calibration of the device shows that
cycling periods between 1.0 and 6.0 cycles per second are possible. The
other timer system (described in IAP Progress Report I, 1967) has a
range of 5 cycles per hour to 5 cycles per minute.
A.2
Testing Equipment
This section contains a description of the modified Norwegian
(Geonor) triaxial cell shown in Figures A-2 and B-loa. This cell was
previously used for test series reported in two previous M.I.T. InterAmerican publications.
The latest tests were run in the saturated state
and under reversing cyclic loads, requiring considerable change.
An additional lift or shim was added to the base plate to allow room
for large strains in extension, A pore pressure transducer connection
was machined to provide for measurement of pressure at the base of
tested samples (Figure A-4.a).
High rigidity is required to prevent
expansion during pore pressure increases in this and all other connections open to the sample pores.
A new top cap was designed to allow for
addition of a top drainage line and for positive connection to the piston
during the extension phase of cycling.
Several methods of top cap to piston connection were studied.
Consi-
deration was given to limited space available, sequence of assembly,
degree of freedom, and degree of "positive" grip.
The design chosen is
shown connected in Figure A-4.b and unconnected in Figure B-Lc. This
is the most simple method considered, but because of its pin connection
allows some relative movement between the top cap and piston. The
-91-
connection, visible in the two figures mentioned, is actually made between
the sample top cap and the load cell attached to the piston.
Constant volume valves were installed in pore pressure line connections.
Due to allignment problems it was necessary to provide constant lubrication of piston area in contact with the triaxial cell bushing.
Standard
methods could not be used because the cell stays only two thirds full of
water, and bushing-piston clearance was great enough to cause some oil
flow toward the atmospheric end.
A continuous oil bath was fed at 0.2
psi above cell pressure to a small ring chamber directly beneath the
bushing.
This allowed significantly better calibration of the stress
applicator and prevented most air leakage from the triaxial cell.
A.3
Measurement Equipment
In rapid cyclic testing it is virtually a necessity to monitor de-
sired quantities electrically.
measured during testing.
This was done with the three parameters
A load cell, pressure transducer and L.D.T.
(Linearsyn Differential Transformer) were used for axial force, pore
pressure, and strain measurements.
Electircal output from each device
was then recorded on either a dual channel continuous recorder or on
an X-Y recorder.
The load cell used was placed above the sample top cap and measured
force transmitted directly on the sample as shown in Figure A-4.b.
This
cell, manufactured by Strainsert of Bryn Mawr, Pennsylvania, is unaffected
by chamber pressure and has its connections sealed against water contact.
The seal had to be replaced several times due to cracking.
Calibration
checks (over a dozen) were run after every few tests and revealed a
-92-
drift in zero-load output but no change in slope of the calibration curve.
A zero-load reading was taken before each test for accurate force calculation,
Specifications on the load cell are as follows:
(Strainsert Flat Load Cell-Universal)
Output signal = 3 mv/volt
Force range = 0 to 1000 pounds
(also linear over - 50 to 0 pounds)
Single bridge
Accuracy = 0.2 pounds (under conditions used)
Excitation voltages used were 6.00, 20.00 and 24.00 volts D.C. depending on magnitude of output desired and on available power sources.
Load cell output was traced by an X-Y recorder described later in this
section. A sample test record of X-Y trace for SCS-RUl2 is shown in
Figure A-5.
Because of poor recorder response, tracings such as that
in Figure A-5 serve only to monitor the magnitude of pulsating deviator
stress.
Special tests (at short tracing-arm movements) were used to
establish the load-time history form as shown in Figure 3.a.
The pore pressure transducer used in testing is shown in Figure A-4.a.
It is connected to the base plate at the triaxial cell and has a one
sixteenth of an inch channel open to a porous stone at the base of the
sand sample,
This transducer is built by Dynisco of Cambridge, Massa-
chusetts, and has the following characteristics:
Output signal = 3 mv/volt
Pressure range = 0 to 100 PSIA
Excitation voltage - 6 volts
Transducer output was traced continuously on one channel of the dual
track recorder described later in this section,
-93-
Performance of this
transdu
peratur
for ini
The
Measurei
to the
the pin
other "
larger
in cycl
percent
maintaii
and was
Packard
charact
Exc
4.5 vol
traced i
(SCS-RU;
by the
tion.
The
pressur(
-94-
channel are located on either side of the control panel.
The recorder
has built in D.C. power at 4.5 volts for both channels, if desired. A
test recording is shown full size in Figure A-6.
Important specifications
and characteristics are as follows:
(Sanborn 321 Dual Channel
Carrier Amplifier - Recorder)
Power Requirements = 120 v, 60 cycle, single phase
Recording speeds = 1, 5, 20, 100 mm/second
Sensitivity = 0.01 mv/paper division
Attenuation factors = 1, 2, 5, 10, 20, 50, 100, 200
Response time = 5 milleseconds (10% to 90% deflection)
Overshoot = 4% maximum
Excitation = 4.5 to 5.0 volts at 2400 cycles per second
Recordings were made on the dual 10 cm. (50 division) track paper shown
in Figure A-6.
Timer markings are traced on the extreme right of the
paper at one second intervals for speed-time correlation.
The paper it-
self is heat sensitive and is marked by heated "points" on galvanometer
arms.
Positioning and balancing for initial output are accomplished by
a relatively simple prodeedure of resistance bridge adjustment.
Perfor-
mance of the recorder was excellent for this work although some channel
interaction was discovered under specific conditions.
This interaction
was linear and could be corrected with calibration curves.
From the
listed specifications above, it can be seen that limitations on these
cyclic tests are not imposed by the Sanborn recorder.
The X-Y recorder used for load cell output recordings is shown in
the lower right of Figure A-1.
The recorded trace (Figure A-5) used as
an example shows the time (cycles) versus load cell output (inmillivolts)
-95-
I
mode of operation.
Tracings are made by mechanical movement in an arm
weighing several grams.
Controls enable amplification and horizontal
travel speed to be varied over any range desired in the tests.
Use of this recorder in dynamic tests is limited by relatively slow
following ability of the recording arm and point. Most tests were run
at higher attenuation (less arm movement) than is shown in Figure A-5
because of this following difficulty.
The trace in Figure A-5 yields
stress data, but no information on "form" of load application .
With
carefully set controls it would be possible to work at a cycling speed
of 2 to 3 cps.
Calibration checks were made before each test over the
scale of output to be used in the test. No data on specifications is
presented for this recorder because the device is really not suitable
for future use.
-96-
~----
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r
FIGURE A.1:
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for Strain Measurement
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APPENDIX B
TEST PROCEDURES
This appendix will outline testing procedure used for running reversing stress liquefaction tests and drained shear tests.
Figure B-1
and B-2 present six sequential photographs of sample preparation.
B.1
Cyclic Loading Tests
B.l.a
Sample preparation
All cyclic tests reported were run on unused samples of the two soils
described in Chapter III.
the only exceptions.
Tests SCS-RU40 and RU41 on Wing-Beach sand are
(Microphotographs show no increase in partical de-
gredation after these tests.)
on the two sands.
Different methods of preparation were used
Wing-Beach sand was placed in the sample mold in the
air dry state while Modified A3 sand was placed into a water filled mold.
These two methods were necessary because of different saturation problems
at low testing backpressures.
Pouring Wing-Beach sand into a water filled
mold and placing Modified A3 sand in the dry state both resulted in less
than saturated conditions.
For all tests sand was placed in six to eight layers with various degrees of compaction depending on desired density.
Figure B-l.a shows a
picture of mold, membrane and suction connection at time of sample preparation.
The membrane is sealed against the base plate with an 0-ring.
A saturated porous stone is then placed in the bottom-prior to fitting of
the three-piece mold.
It was necessary to apply a suction pressure be-
-103-
tween mold and membrane in order to achieve uniformity of sample shape.
Wing-Beach samples were "saturated" by raising the water table up through
the sand-filled mold under a slight backpressure. Modified A3 samples
were saturating while being prepared and required only matching the
water table and sand surface before placement of the top cap.
The top
cap was put into place at this time and sealed against the membrane by
another 0-ring.
Sample collapse after removal of the sand mold was avoided by applying a small negative backpressure of 2 to 3 osi.
This creates a confining
pressure of 2 to 3 psi, preventing the sample from collapsing under its
own weight.
B.l.b
Sample consolidation and measurement
The triaxial cell top was put into place and the sample consolidated
to the effective pressure at which it was to be tested.
shown in Figure B-l.c.
This stage is
Note that consolidation is isotronic and there
is no contact between top cap and load cell.
Primary consolidation took
place within seconds after effective stress application.
Drainage was
allowed for 5 to 10 minutes under secondary compression with very small
volumes of drainage observed.
Pore pressure lines were closed off at this time and the cell pressure was removed.
A backpressure of -3 to -6 psi was observed in most
nearly-saturated tests.
Air comes out of solution when pore pressure
drops much below atmospheric pressure, preventing a large negative backpressure.
Initial volume was now computed after this sample rebound by
making three height and five circumference measurements.
Corrections
for measured membrane and paper thickness are made. Most samples had
-104-
-M
uniform cross-sectional areas with some slight tapering near the top cap.
Occasionally a small void would be noticeable against the top cap due to
uneveness at time of fitting.
meter by 3.5 inches high.
Typical sample size is 1.98 inches is dia-
Diameter varied only from 1.97 to 2.00 inches
in all tests, while height varied from 3.3 to 3.6 inches.
(Several early
tests had measured heights of 3.1 or 3.2 inches.)
Sample volume was also measured at the end of testing, after liquefaction.
This method depended on determining the total volume of water
in a sample.
To do this the water content of each sample was taken after
the breakdown of equipment. To this was added the volume of water removed
with drainage following liquefaction.
Volume taken by the soil phase of
a sample is computed from known soil weight and specific gravity.
If 100
per cent saturation is assumed, Yd' e, and relative density can be calculated with this data. A sample calculation is shown in Figure B-3 for
test SCS-RU21.
B.l.c
Reconsolidation and saturation
Steps followed in reassembly of the triaxial cell are attachment of
the load cell pin connection, placement of cell top (without bushing),
and positioning of bushing and loading equipment. The critical stage in
this operation is the triaxial cell lowering over the attached piston
and load cell.
Weight of the connected piston applies a compressive
stress of 0.4 psi on the sample.
Care must be taken to prevent binding
caused by missallignment that will exert additional stress on the sample.
The effective confining stress at this stage is still 3 to 6 psi.
When the triaxial cell is in place, it is bolted together and the
bushing screwed to the cell.
The cell is then filled with deaired water
-105-
to a level somewhat below the load cell electrical connections.
After al-
lignment of pistons, the stress application device is firmly attached to
the cell and the pistons screwed together.
A height adjustment is made
to allow piston movement of equal distance in the application device,
Reconsolidation takes place when cell pressure is brought up to the level
of previous consolidation.
During this and subsequent changes of cell pressure, forces on the
piston are being changed
The magnitude is easily computed by multiplyThe problem of preventing
ing cell pressure changes by the piston area
anisotropic
pression.
loading is compounded by movement of the top cap with recomThe best method of preventing unwanted deviator stress was
found to be through use of a wedge (shown in Figure B-2 b) to fix the
piston so no force will be applied to the top cap
Sample re-consolida-
tion strains were allowed because of the loosness in the top cap extension pin connection
Monitoring of force transmitted through the load
cell reveals proof of the ability to limit deviator stress to less than
1.0 psi.
Maximum undesirable stresses occur only after the sample is at
an effective confining stress at the test-ing leve
(28 4 psi for all
cyclic tests after SCS-RUII).
The typical sample will require some Iime to bring its saturation
level up to 100 percent,
Saturation was achieved at the effective stress
of the test, and at a backpressure 10 0 psi higher than the test condition.
Pore pressure response was measured after continued saturation
until response was greater than 88 percent
Seven tests (noted) were run
at responses lower than 88 percent, when this value could not be reached
despite flushing the sample with de-aired water
were purposely run in a partially saturated state
-106-
Several other tests
Pore pressure response
for saturations of 69 and 84 percent were 6 and 5 percent respectively.
B.l.d
Testing
All cyclic tests were loaded to the first compressive cycle in undrained stress controlled loading,
The extension half of the first cycle
marked the beginning of rapid cycling. All subsequent stress changes
took place at the frequency of test (1,0 cps or
12 cps -8 cpm).
apparatus at time of testing is shown assembled in Figure B-2.c.
The
Stress
controlled loading for the initial compressive cycle was desirable for
two reasons.
Stress paths for this loading can be compared to yield data
on uniformity of samples and typical behavior
Pore pressure and stress-
strain data are plotted in Figures B-5 and B-6 for the four major test
series.
The second reason is to establish a zero strain position. This
is necessary because of ambiguity from movement in the pin connection at
zero stress.
During this loading axial force is measured by a voltmeter, The last
force reading is approximately the compressive stress during cycling, and
is used as a known reference voltage for calibration of the X-Y recorder.
Before cycling is begun the attenuation scales on each recording device
are set so to record high pore pressures and strains should the sample
fail on the first few load applications.
better resolution if desirable.
These are then adjusted for
(Data on one or two early tests were
lost when this procedure was not followed,)
Cycling was allowed to continue until necking of the sample.
This
generally occured after less than 15 percent double amplitude strain in
denser sands.
Strains up to 30 percent were reached before necking, in
the looser sands tested. The point at which "normal" strain increases
-107-
with cycling ended and necking-induced strains began was easily identified
in most tests.
Visual observation and characteristicly large recorded
extension strains identify the cycle.
Necking usually occured about one
third down from the sample top cap.
When cycling was stopped a compressive force remained applied to the
sample.
In every case observed removal of this force caused an increase
in pore pressure. After an isotropic state was again reached, pore pressure was decreased to pre-test conditions. The loss of water in this
step was measured and is plotted in Figure 36.
In the final step of
dis-assembly, water content of each sample was measured and care taken
to backfigure the volume of water at the end of each test.
B.l.e Volume changes
Volume changes occur during several phases of the test procedure.
Figure B-4 shows these changes, determined by burette measurement, for
loose samples of both sands.
each sand.
These samples were the loosest tested for
Measured volume changes before saturation may indicate sample
cavitation and not actual volume changeMost pre-test volume changes occur during consolidation of the sample
with negative backpressuring and by pre-consolidation to test stress
levels.
Disturbance when the piston is attached and the triaxial cell
top brought down over it caused significant changes in both tests.
They
are considered to have been particularly disturbed during this phase.
Volume change is then noted when the samples are saturated, and take on
up to 0.6 cc of water.
Significant volume changes do not occur following
this until after liquefaction has taken place.
-108-
B.2
Drained Shear Tests
B.2.a
Sample conditions
Sample preparation for drained shear tests follows exactly the same
procedure as used in cyclic tests, until after physical measurements are
made. At this stage the pin-connection to the top cap is not made since
only compressive stresses are applied. Standard procedure is still followed but without fear of applying
B.2.b
anisotropic forces to the sample.
Testing
Drained shear tests were run under stress controled loading.
The
stress increments were small, requiring several dozen to reach failure.
Volume measurements were made through one or more burettes, visible in
Figure A-1.
As large strains were occuring, volume change was continu-
ous with no definable change for each stress increment.
A standard
reading pause of 25 seconds after stress application was set up for
uniformity of reading times.
of from 24 to 46 minutes.
Each test failed over a total time period
Results of these tests are plotted in Figures
6 and 7.
-109M
FIGUREBl
(a)
(b)
Viwoprprdsml
PRPRTO
fsml
odfrpearto
an
OFSML
(c) Consolidation of sample
-110-
(a)
TEST PR(
a)
Phys
-3
FIGURE B.2
DURE PRIOR TO CYCLING
Df sample
it
-5 c
b) Posi
exter
durir
)revent
de~ýl opi ng
confining
stres
c)
Tri a
equiI
(c)
-111-
-
T-H
r
f~I
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IL
APPENDIX C
VOLUME MEASUREMENTS
me (density) is of critical concern in these tests because
Two methods of
s are made on a basis of relative density.
ume were used in this program and have been described in
1) physical measurements after initial consolida-
ey are
Both methods
mputation of water volume after testing.
possible as a check against "losing" the relative density
ithout knowing this relative density, a test is only useracteristic liquefaction behavior.
this study into volume measurements are plotted in Figure
igure "difference in measured density" is plotted against
Complete agreement of the two methods is repre-
esponse.
a point falling on the dashed zero-difference line.
eresting results can be drawn from this statistical reprefirst is that there is very good agreement between the
econdly, neither method is consistantly higher of lower
a good indication that the same thing is being measured.
atterns are also apparent, although not drawn from so
1 data as the first two conclusions.
ples where small voids were noted after preparation, all
_ .1 •. 1. ..
. .. .
... ...
1
Tour
end
naa
of
pnysically
testing.
ignores
surement
more
surprising
measured
This
the
result
of
is,
and
void
is
that
..
volumes
larger
course,
reasonable
. .
.,. • •
..-
than
those
volume
with
-116-
• • -., ,.,.-.-I
measured
a
because
overestimates
even
.I.I..
. --
the
assumption
by
,,I -I,
•h "
at
of
100
e
mea-
physical
A
amount.
that
t
percent
aturation there is fairly good agreement at pore pressure responses of
5 to 80 percent.
It would be expected that if actual saturation was
ess than 100 percent its overestimation would indicate an end-of-testolume less than the measured volume.
Put another way, an overestimation
f S in the equation Gw = Se would indicate smaller void ratios than
ctually exist.
There should be a trend toward points falling above the
xis as pore pressure resonse decreases, an indication of partial saturaTests RUl8 (S 69%, response = 6%) and RUl9 (S 84%, response = 5%)
ion.
onfirm this by overpredicting density by 14.5 and 7.1 PCF respectively.
Four conclusions are then drawn from this study into sample measureents.
1. Density determination on consolidated saturated sand samples by
careful physical measurement and by water volume measurement can
agree very closely.
2. Neither method consistently predicts a density too high or too
low when compared with the other method.
3. Small voids in a sample can easily result in a physically measured density of 1.0 to 2.5 PCF lower than that value determined
by volume measurement.
4. A "low" pore pressure response of 75 to 80 percent in nearly
saturated sands does not produce significant disagreement between the two sample volume measurement methods.
However,
samples saturated at 84 percent with a pore pressure response
of 5 percent may cause overprediction of density by 7.1 PCF
if complete saturation is assumed.
-117I
.1.
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Figure
in terms o
informatio
SCS-RUl3,
in these f
Figure D-7
SCS-RUl2,
summarized
The te
test serie
not liquef,
tests.
P1
and high N
pore press
Each t
phases of
of these v
pressure s
"Number of
mically in
Vertic
in extensi
until neck
-119-
ressive values.
Significant values of initial lique-
15%, 20% and 25% strain are marked.
It is noted that
sistantly reached 30% double amplitude strain before
the denser sands had not reached 15% strain when
-120I
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-132-
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APPENDIX E
LIST OF SYMBOLS
TEST SCS-RU6
refers to triaxial cyclic shear test, undrained with stress
reversal, number six
TEST SD-3
refers to triaxial compressive shear test, drained, number 3
C
coefficient of uniformity
D5 0
diameter of average (by weight) soil grain
e
void ratio
emax
maximum void ratio for a given sand
e mi.
minimum void ratio for a given sand
n
immediately prior to testing
Ea
ga
axial strain (+ is compression)
N
number of cycles
N.1
number of cycles to initial liquefaction
p
average of vertical and horizontal confining stress,
acceleration of gravity
equal to Cv
+
Gh
2
q
average difference of vertical and horizontal confining
stress, equal to Gv
H
2
RD
relative density
SG
specific gravity
u
pore pressure
sample diameter
H
sample Height
C
sample circumference
-133-
dry unit weight
maximum angle drawn to peak of stress path
tan a = sin (angle of internal friction)
a1
major principal stress
c0
3
minor principal stress
GV
vertical stress
cH
horizontal stress
A(a1 - a3 )
deviatoric stress
initial effective confining stress
coa
a1,
Cf3'
aV
aH
effective stress condition (i.e.,
-134-
r3 = 3 - u)