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ENCE 361
Soil Mechanics
Unconfined
Compression
Test
Overview
The unconfined compression test is
used to measure the unconfined
compressive strength of a cohesive
soil.
The unconfined compression test is
applicable only to coherent materials
such as saturated clays or cemented
soils that retain intrinsic strength after
removal of confining pressure; it is
not a substitute for the Q test.
Overview
In this test, a laterally unsupported
cylindrical specimen is subjected to a
gradually increased axial compression
load until failure occurs.
The unconfined compression test is a
form of triaxial test in which the
major principal stress is equal to the
applied axial stress, and the
intermediate and minor principal
stresses are equal to zero.
Overview
The unconfined compressive strength,
qu is defined as the maximum unit
axial compressive stress at failure or
at 15 percent strain, whichever occurs
first.
The undrained shear strength, su, is
assumed equal to one-half the
unconfined compressive strength.
Apparatus 1
Specimen Preparation Equipment
Similar to Triaxial Tests
Loading Device
Measuring equipment, such as dial
indicators and callipers.
Timing device, either a watch or clock
with second hand.
Balances, sensitive to 0.1 g.
Apparatus necessary to determine
water content and specific gravity.
Loading Device
Modern
Ancient
Preparation of Specimen
Similar to Triaxial Tests
Two Possibilities of Specimen
Preparation
Undisturbed Specimens
Disturbed Specimens
Testing of Disturbed Specimens
necessary to determine sensitivity of
soils
Undisturbed Specimens
Generally, undisturbed specimens are
prepared from undisturbed tube or
chunk samples of a larger size than
the test specimen.
Specimens must be handled carefully
to prevent remoulding, changes in
cross section, or loss of moisture. To
minimise disturbance caused by skin
friction between samples and metal
sampling tubes, the tubes should be
cut into short lengths before ejecting
the samples.
Undisturbed Specimens
Specimen preparation must be
dimensionally accurate.
As with triaxial specimens, a membrane
is placed over the specimen before
loading.
Sample ejection should be accomplished
with a smooth continuous and rapid
motion in the same direction that the
sample entered the tube.
All specimens shall be prepared in a
humid room to prevent evaporation of
moisture.
Undisturbed Specimens
If the specimen is not tested
immediately after preparation,
precautions must be taken to prevent
drying and consequent development
of capillary stresses.
When drying before or during the test
is anticipated, the specimen may be
covered with a thin coating of grease
such as petrolatum.
Part of the undisturbed soil should be
laid aside for a water content test.
Remoulded Specimens
Remoulded specimens usually are
prepared in conjunction with tests
made on undisturbed specimens after
the latter has been tested to failure.
The remoulded specimens are tested to
determine the effects of remoulding on
the shear strength of the soil.
The remoulded specimen should have
the same water content as the
undisturbed specimen in order to
permit a comparison of the results of
the tests on the two specimens.
Remoulded Specimens
Place the failed undisturbed specimen
in a rubber membrane and knead it
thoroughly with the fingers to assure
complete remoulding of the
specimen.
Remove the soil from the membrane
and compact it in a cylindrical mould
with inside dimensions identical with
those of the undisturbed specimen.
Remoulded Specimens
Carefully remove the specimen from
the mould, preferably by means of a
close fitting piston, and plane off the
top of the specimen. The specimen is
then ready for testing.
Weigh, measure and load the
specimen in the same manner as the
undisturbed specimens.
Procedure
Record all identifying
information for the sample such
as project, boring number, visual
classification, and other pertinent
data on the data sheet. The data
sheet is also used for recording
test observations described
below.
Procedure
Place the specimen in the loading
device so that it is centred on the
bottom platen; then adjust the loading
device carefully so that the loading
ram or upper platen barely is in
contact with the specimen.
Record the initial reading of the dial
indicator on the data sheet. Test the
specimen at an axial strain rate of
about 1 percent/minute.
Procedure
Observe and record the resulting load
corresponding to increments of 0.3
percent strain for the first 3 percent of
strain and in increments of 1 or 2
percent of strain thereafter. Stop the
test when the axial load remains
constant or when 20 percent axial
strain has been produced.
Procedure
Record the duration of the test, in
minutes, to peak strength (time to
failure), type of failure (shear or
bulge), and a sketch of specimen after
failure on the data sheet.
After the test, place the entire
specimen or a representative portion
thereof in a container of known
weight and determine the water
content of the specimen.
Computations
Index
Properties
Stress-Strain
Properties
Water Content
Axial Strain
Volume of
Solids
Corrected Area of
Specimen
Void Ratio
Compressive Stress
Degree of
Saturation
Dry Density
All of these quantities
should be computed at
each load increment
during the test
Presentation of Results
Stress-strain curve(s) should be plotted
for each
Unconfined compressive strength qu is
the compressive stress at 15% strain
When remoulded specimens are tested,
the sensitivity ratio is
S t
qu
undisturbed
qu
remoulded
Possible Errors
Test not appropriate to type of soil.
Specimen disturbed while trimming.
Loss of initial water content. A small
change in water content can cause a larger
change in the strength of clay, so it is
essential that every care be taken to
protect the specimen against evaporation
while trimming and measuring, during the
test, and when remoulding a specimen to
determine the sensitivity.
Rate of strain or rate of loading too fast.
Final Exam Schedule
Last Lecture Session 26 November
2001
Final Review Session 28 November
2001
Final in Two Parts
Take Home Part - 50%
In-class Part = 50%
Given out 28 November – Due 3 December
Done In Class 3 December
Last Homework Day 3 December 2001
Stability of Slopes
Slope Stability
One of the most important topics in
the development of geotechnical
engineering
Also an important topic in actual
practice
Unsupported slopes are found in a
wide variety of projects
Highways
Elevation changes on sites
Development of Theory
The need for angled slopes has been
understood since antiquity
The theory necessary to analyse these
slopes dates from the years during and
after World War I
The subject was especially important in
Scandinavia, where sensitive clays were
very subject to catastrophic failure
Gothenburg Harbour Failure
5 March 1916
Gothenburg Harbour Failure
5 March 1916
Soft clay deposit, 150'
deep
50' was dredged out
and replaced by sand
fill; piles were driven
to stabilise the quay
Several hundred feet
of wall slid seaward as
shown
Appointment of Swedish
Commission
In 1913, the Swedish State Railroad
Administration appointed a special
Commission – the first so titled – to
study these types of failures and to
recommend a solution
Its chairman, Wolmar Fellenius,
developed the basic methods for
analysing rotational failures of slopes
which, with improvement, we use today
Types of Slope Failure
Falls
Topples
Slides
Spreads
Flows
Falls
Slope failures
consisting of soil or
rock fragments that
drop rapidly down a
slope
Most often occur in
steep rock slopes
Usually triggered by
water pressure or
seismic activity
Topples and Slides
Topples
Similar to a fall,
except that it
begins with a
mass of rock of
stiff clay rotating
away from a
vertical joint
Slides
Slope failures that
involve one or more
blocks of earth that
move downslope by
shearing along welldefined surfaces or thin
shear zones
Types of Slides
Rotational slides
Most often occur in
homogenous materials,
such as fills or soft
clays
Translational slides
Move along planar
shear surfaces
Compound slides
Complex and
composite slides
Spreads and Flows
Spreads
Similar to
translational slides,
except that the
blocks separate and
move apart as they
also move outward
Can be very
destructive
Flows
Downslope
movement of earth
where the earth
resembles a viscous
fluid
Mudflow can start
with a snow
avalanche, or be in
conjunction with
flooding
Homework Set 7
Textbook Readings
Chapter 15 (pp. 612668)
No Laboratory Soils
Testing experiments
Problems
15-8: Use Fellenius
Method (hand or
spreadsheet)
Problems (cont'd)
15-10 (use Simplified
Bishop or Slope-W)
15-14 ((a) and (b) only;
use Taylor charts or
Slope-W)
15-29 (use
Morganstern's rapid
drawdown curves)
Borrow volume
problem
Borrow Volume Problem
A 3.0 ft deep cut is to be made across an
entire 2.5 acre site. The average unit
weight of the soil is 118 pcf, and the
average moisture content is 9.6%. It also
has a Proctor maximum dry unit weight of
122 pcf and an optimum moisture content
of 11.1%. The excavated soil will be
placed on a nearby site and compacted to
an average relative compaction of 93%.
Compute the volume of fill that will be
produced, and express your answer in cubic
yards.
Due Date: 3 December 2001
Questions?