FINAL TECHNICAL REPORT
Development of a Piezovibrocone for In-Situ Evaluation of Soil Liquefaction
Potential and Postcyclic Residual Undrained Strength of Silty and Sandy Soils:
Collaborative Research by Georgia Tech and Virginia Tech
September 1998 - March 1999
Award Number: 1434-HQ-97-GR-03128
Principal Investigator: Paul W. Mayne
School of Civil and Environmental Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332-0355
Phone: 404-894-6226
Fax: 404-894-0830
e-mail: pmayne@ce.gatech.edu
Program Element: Evaluate Urban Hazard and Risk
Key Words: Liquefaction, Earthquake Effects, Site Effects
Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under
USGS award number Georgia Tech Research Corporation # 1434-HQ-97-GR-03128. The views
and conclusions contained in this document are those of the authors and should not be interpreted
as necessarily representing the official policies, either expressed or implied, of the U.S.
Government.
Authors: Paul Mayne, James Schneider, & Tom Casey (Georgia Tech),
with James K. Mitchell, Tom Brandon, & John Bonita (Virginia Tech)
GTRC Project No. E20-M73
Report Dated March 12, 1999
SUMMARY
Current practice for assessing soil liquefaction susceptibility of sands and silts during
earthquakes and their post-cyclic undrained shear strength relies strongly on empirical
methodologies. Procedures for soil liquefaction evaluation include both in-situ and
laboratory test methods that require correction factors which are not always fullyunderstood nor well-defined. Consequently, much uncertainty still remains after a
routine analysis is conducted, particularly for natural soil deposits, reclaimed lands, and
geologies for which the empirical databases were not developed. Under funding from
both the USGS and NSF, the initial development and trial calibrations of an impulse-type
piezovibrocone test have begun as a joint study by Georgia Tech and Virginia Tech. The
piezovibrocone will serve as a specialized in-situ testing tool for the direct evaluation of
soil liquefaction potential and post-cyclic residual undrained shear strength on sitespecific projects. The data produced from preliminary field tests at historic liquefaction
sites in Charleston, SC will be evaluated qualitatively and ongoing research will be
reviewed to assess the potential for future quantitative analyses.
1 INTRODUCTION
Liquefaction evaluation of sandy and silty soils can include laboratory as well as in-situ
methods. Laboratory methods involve series of static and cyclic triaxial or cyclic simple
shear testing (e.g. Yamamuro & Lade, 1998), while in-situ tests may consist of the
standard penetration test (SPT; e.g. Seed et al., 1983), cone penetration test (CPT; e.g.
Stark and Olson, 1995), flat plate dilatometer test (DMT; e.g. Reyna and Chameau,
1991), or shear wave velocities (Vs; e.g. Andrus and Stokoe, 1997). Of additional
concern in seismic regions is the assessment of undrained residual strength. The
determination of this parameter has also been evaluated on the basis of empiricisms (e.g.
Seed & Harder, 1990). In order to provide a direct and more rational approach to sitespecific liquefaction susceptibility and post-cyclic residual strength analyses, a
piezovibrocone penetrometer has been under development in a collaborative effort
between Georgia Tech and Virginia Tech.
Conceptually, the vibrocone consists of a cone penetrometer coupled with a vibrating
shaker mechanism (Fig. 1a) that induces liquefaction locally in the vicinity of the probe
during penetration. Vertical penetration tests are conducted both statically and under
dynamic excitation in side-by-side soundings. Comparisons of cone tip resistance (qc),
penetration pore water pressures (um), and sleeve friction (fs), from adjacent paired
soundings are made to ascertain the liquefaction potential of subsurface soils. The
geometry and conduct of the vibrocone penetration test (VCPT) permit a rational
interpretation by analytical theories (bearing capacity, stress path, or cavity expansion) or
via numerical simulation techniques (finite difference, finite elements, discrete elements,
or strain path method) that can incorporate important soil behavioral aspects such as
effective stress, dynamic loading, cyclic pore pressure generation, soil fabric, and initial
stress state. It is hoped that the piezovibrocone will offer an improved and systematic
framework for evaluating liquefaction susceptibility and residual undrained strength of
loose and soft ground in seismically active regions.
A multi-element piezocone coupled with a vertical impulse pneumatic source has been
used to form the initial vibrocone unit. The device is currently being evaluated in
laboratory calibration chamber tests of saturated Light Castle quartzitic sand at Virginia
Tech. The sand is placed at relative densities of 25 and 65 percent, corresponding to very
liquefiable and borderline behavior, respectively (Mitchell et al., 1998). The effects of
confining stress level, vibration frequency, and vibration mode (transient vs. steady) are
under investigation. In all tests, continuous measurements of qc, fs, and pore water
pressure at two locations (u1 located mid-face and u2 located at the shoulder) are taken for
evaluation. Additional trial field testing to evaluate the robustness and initial
performance of the "Mark-I" vibrocone have been performed by Georgia Tech in Spring
Villa, AL, Atlanta, GA, and Charleston, SC. The analyses presented in this paper will
concentrate on the Charleston sites, since those soils have historically been shown to have
a high potential for liquefaction (Martin, 1990).
Friction Sleeve 150 cm2
7.3 cm
u1
3.57 cm
Vibratory Unit
Cone Tip - 10 cm2
a. Prototype piezovibrocone
Geophone
3.57 cm
u2
Friction Sleeve - 150 cm2
Cone Tip - 10 cm2
b. 10 cm2 seismic piezocone
c. 15 cm2 multi-element piezocone
Figure 1. Penetrometers used
60o
2 CURRENT PRACTICE
2.1 Liquefaction Potential from Field Data
Due to the difficulty and expense associated with obtaining undisturbed field samples of
sandy and silty soils, in-situ tests have become popular for evaluating how a soil deposit
will respond under earthquake loading. Data from post-earthquake field investigations
have been used to generate simplified curves related to surface phenomena associated
with subsurface liquefaction. Sites specifically showing evidence of sand boils, intrusive
dikes, lateral spreading, excessive settlement, and structural damage have been
extensively used. Databases of sites which have experienced obvious liquefaction, as
well as those where no apparent liquefaction occurred, have been evaluated for a number
of seismic events and related to the Standard Penetration Test (SPT), shear wave velocity
(Vs) tests, the Cone Penetration Test (CPT), and the flat plate Dilatometer Test (DMT).
Figure 2 shows the liquefaction curves for the four common in-situ tests that have been
related to liquefaction susceptibility of sandy soils.
Each curve relates a resistance parameter of the individual test [i.e., (N1)60, Vs1, qc1,
KD] to the soils resistance to cyclic loading (cyclic resistance ratio or cyclic stress ratio).
The cyclic resistance ratio, CRR, is the average cyclic shear stress (avg) normalized to the
effective overburden stress. It is a function of earthquake duration (magnitude),
maximum surface acceleration (amax), depth to soil element being analyzed, and total
(vo) and effective ('vo) vertical stress (Seed & Idriss, 1971). The normalized cyclic
shear stress was initially evaluated in terms of laboratory testing, but was later adapted
for field case histories by Seed & Idriss (1971). To distinguish field studies from
laboratory studies, Stark & Olson (1995) present their CPT data compared to the seismic
shear stress ratio (SSR), which is equal to the cyclic stress ratio (CSR). To maintain
consistency, the CPT data has been compared to the CSR. The CRS is generally
presented as:
CSR 
 avg
 ' vo
a
 0.65 max
 g
  vo

  ' vo

rd

(1)
where rd is a depth correction factor presented in Seed & Idriss (1971), and the other
parameters are as described above. To account for the duration of shaking, field
performance curves have been normalized to a magnitude 7.5 earthquake using
magnitude-scaling factors (MSF) as shown in Equation (2):
CRR 7.5 
CSR
MSF
(2)
Additional uncertainty is added to liquefaction curves by this normalization. Magnitudescaling factors were introduced by Seed et al. (1983), but current work by Youd and
Noble (1997) has shown discrepancies in these factors.
Normalization schemes have been incorporated into the resistance parameters for
each in-situ test. The SPT N-value has been corrected for rod energy and effective
overburden stress to get the (N1)60 parameter (Skempton, 1986). Additional corrections
are also recommended for borehole diameter, rod length, and sampling method
(Skempton, 1986). The stress normalized shear wave velocity, Vs1, is obtained by:
Vs1 = Vs (Pa/'vo)n
(3)
where Pa is a reference stress of 100 kPa and n is a stress ratio exponent. There is still
some debate on the approximate value of the exponent, but 0.25 is typically used (Andrus
& Stokoe, 1997). Normalization schemes for the CPT are also based on functions of the
ratio of an approximately 1- atmosphere reference pressure to the effective overburden
pressure. Some common normalization schemes are presented in Olsen (1994), Wroth
(1984), Kayen et al. (1992), and are reviewed by Wise (1998). Data used to generate the
Stark & Olson (1995) CPT-based liquefaction curves used the Kayen et al. (1992)
normalization. The DMT data are already normalized in terms of the index K D, which is
a dimensionless parameter (Marchetti, 1980).
0.6
FC (%) = 35
0.5
15
< 5
Liquefaction
Likely
0.4
Cyclic Resistance Ratio (CRR)
Cyclic Resistance Ratio (CRR)
0.6
Liquefaction
Not Likely
0.3
0.2
0.1
FC (%) = 35 15 < 5
0.5
0.4
0.3
0.2
Liquefaction
Likely
0.1
0.0
0.0
0
0.6
10
20
30
40
0.4
0.3
200
Vs1
a.
b.
300
0.6
Clean Sand
0.25 < D50 (mm) < 2.0
F.C. (%) < 5
Liquefaction
Not Likely
0.2
100
(N1)60
Cyclic Resistance Ratio (CRR)
0.5
0
50
Silty Sand-Sandy Silt
Silty Sand
D50 (mm) < 0.10
0.10 < D50 (mm) < 0.25
F.C. (%) > 35
5 < F.C. (%) < 35
Liquefaction
Cyclic Resistance Ratio (CRR)
Liquefaction
Not Likely
M=7.5
M=7.5
0.1
Marchetti, 1982
0.5
Liquefaction
Likely
0.4
Reyna &
Chameau, 1991
0.3
Liquefaction
Not Likely
0.2
Robertson &
Campanella,
1986
0.1
M=7.5
Clean Sands
M=7.5
0
0
0
5
10
15
20
qc1 (MPa)
c.
25
30
0
2
4
6
8
10
Horizontal Stress Index, KD
d.
Figure 2. Observed boundaries for "Liquefy - No Liquefy" curves for in-situ tests:
a. SPT (NCEER, 1996)
b. Vs (Andrus & Stokoe, 1997)
c. CPT (Stark & Olson, 1995)
d. DMT (Reyna & Chameau, 1991)
Due to a combination of many factors, the amount of uncertainty inherent in
liquefaction curves is large. Most of the data have been accumulated from reports of
many different researchers, primarily in Japan, China, and western United States. The
data used to generate these curves are derived predominantly from post-earthquake field
investigations. During the field investigation, the soil has been altered from the state it
was in prior to the earthquake. Loose zones that have liquefied are potentially denser due
to settlement, and dense zones around layers that liquefied are potentially looser due to
flow of pore water from the liquefied zones (Youd, 1984). Frost et al. (1993) and
Chameau et al. (1998) examined data in fill soils of the San Francisco area before and
after the 1989 Loma Prieta earthquake. Their studies showed significant increases in Vs,
qc, and KD in the post earthquake soils when compared to pre-earthquake studies.
The effects of re-liquefaction need to be considered during susceptibility analyses.
After an earthquake, soils may have formed a more compressible structure that will
generate pore pressures more rapidly during cyclic loading, even if they are at a higher
relative density (Finn et al., 1970; Youd, 1977). Occurrence of liquefaction at the same
site has been discussed by Yasuda and Tohno (1984) for eleven Japanese sites over seven
different earthquakes, and for California earthquakes by Youd (1984). Analyses of the
effects of re-liquefaction on liquefaction databases can be studied using the Andrus and
Stokoe (1997) shear wave velocity database. Shear wave velocities obtained from recent
studies have been applied to four different southern Californian earthquakes (1979, 1981,
1987, and 1987), and two different San Francisco area earthquakes (1906, 1989) to verify
liquefaction curves (Andrus & Stokoe, 1997). It is not unexpected that a poor agreement
between the data and curves is achieved. Void ratio changes from liquefaction and
seepage into non-liquefied areas, and an increase in pre-straining from cyclic loading will
likely change shear wave velocities of soil deposits between earthquakes. Similar
changes will likely affect SPT N-value, CPT tip resistance, and DMT horizontal stress
index.
2.2 Post-cyclic residual undrained shear strength
Post-cyclic residual undrained strength of sands (Sus) is typically estimated by using a
combination of in-situ tests and laboratory tests or in-situ tests alone. A method of
combining laboratory evaluation of steady state strength and in-situ evaluation of void
ratio was described by Poulos et al. (1985). Fear and Robertson (1995) utilized a
combination of the state parameter for sands concept (Been & Jefferies, 1985), Critical
State Soil Mechanics (CSSM; Wood, 1990), and estimation of in-situ soil state from
shear wave velocity measurements (Cunning et al., 1995), to develop undrained shear
strength relationships for various sands. Correlations relying on in-situ tests alone have
compared Sus and SPT (N1)60 value (e.g. Seed & Harder, 1990), normalized Sus and SPT
(N1)60 value (Stark & Mesri, 1992), and normalized Sus and CPT qc1 value (Olson, 1997).
Currently, combined use of laboratory and field tests seems to be the most accurate
way of evaluating Sus. Two major problems that can arise from these methods are the
accuracy with which in-situ void ratio can be estimated and the additional costs
associated with laboratory testing. If undisturbed sampling is attempted without freezing,
loose sands will tend to densify, while dense sands will tend to loosen (Seed, 1971).
While freezing may provide a sample where in-situ void ratio can more accurately be
estimated, as well as a relatively undisturbed sample for testing, the additional cost and
difficulties in sampling will limit its use to large, critical projects. In-situ void ratio
determined by Vs measurements may not have suitable accuracy, and may not provide the
detail needed to identify weak layers needed for these analyses. Typically, shear wave
velocities are taken at one-meter intervals, which would result in an average void ratio for
the analyzed layer. An additional limitation of the method presented by Fear and
Robertson (1995) is that it is not unique for each soil type, and parameters in addition to
steady state parameters are needed for analyses. For larger projects, the issue of cost may
not be of much concern, but for smaller projects direct Sus correlations may be desirable.
The use of in-situ tests to directly determine Sus is based on relatively few case studies
(less than 30), different sands under different stress conditions, post-failure test results to
estimate pre-failure state, drained to partially undrained test results, and predominantly
western U.S., South American, and Japanese case studies. The scatter associated with the
curves to directly relate Sus to penetration resistance (Seed & Harder, 1990; Stark &
Mesri, 1992; Olson, 1997), combined with the relatively few case histories used to
generate the curves, leaves a great deal of judgement necessary during analyses.
Complications are added to analyses by trying to normalize Sus to the effective vertical
stress. Uncertainty in the normalization scheme coupled with uncertainty in the initial
relationship compounds when a unique relationship is attempted. The work presented by
Olson (1997) combines CPT-based cases with SPT-based cases converted to equivalent
CPT qc1 values. These penetration resistance values are then compared to the undrained
strength ratio (Sus/'vo). In addition to the uncertainty mentioned previously, error is
induced by the SPT to CPT conversion. The large amount of scatter in these curves is
likely due to dealing with different sands under different in-situ stress conditions (Fear &
Robertson, 1995). A review of data presented in Thevanayagam et al. (1996) as well as
Fear & Robertson (1995) shows a great deal of scatter, but the scatter is predominantly
induced by analysis of different sands. Similarly to the uniqueness of the steady state line
(e.g. Been et al., 1991), each sand will have a unique relationship to Sus that may not be
able to be determined by in-situ tests alone (Fear & Robertson, 1995). In some cases,
post failure investigations were used to estimate pre-failure response. The analysis of the
failure of the lower San Fernando dam by Seed and Harder (1990) is a good example of
this, where they present typical post-earthquake (N1)60 values at various depths for the
downstream side of the dam (which did not fail). The SPT and CPT are commonly
believed to be indicators of drained parameters. To additionally be able to determine S us,
an undrained parameter, seems fundamentally unsound. Since the case studies are from
relatively few geologic areas, additional uncertainty can arise when trying to extrapolate
the results to unstudied areas.
3 VIBRATORY CONE PENETROMETERS
3.1 Previous Vibrocones
There have been a number of prior attempts to develop a specific tool for liquefaction
evaluation (Table 1). Liquefaction potential of a deposit was determined by comparing
tip resistance of a static sounding, qcs, to tip resistance of an adjacent dynamic sounding,
qcv. Figures 3a and 3b respectively show profiles of static and vibratory tip resistance
from Japanese sites where liquefaction typically did not occur, and where liquefaction
has occurred repeatedly. A drop in tip resistance is shown for both soundings, but it is
much more significant in the zone from 2 meters to 5 meters of the historically
liquefiable site. The original vibrocone (Fig. 4a; Sasaki & Koga, 1982) applied a
horizontal centrifugal force of 32 kgf and operated at a frequency of 200 Hz. Downhole
vibratory excitation came from an electric bar-type concrete vibrator coupled to the cone
penetrometer. However, the horizontal movement induced by the vibrator likely caused
gapping between the cone and the soil, thus questioning reliability of to the tip, sleeve,
and pore pressure readings.
The Italian vibrocone is similar to the Japanese vibrocone with a downhole centrifugal
force attached to a cone slightly larger than U.S. standards (Picolli, 1993; Mitchell,
1988). The Canadian vibrocone consists of an oscillating pair of eccentrically-loaded
counter weights attached above-hole to the actuator assembly in the University of British
Columbia (UBC) cone rig (Moore, 1987). Trial vibratory soundings did show a reduction
in tip resistance, however, the applied force and frequency of the system varied due to
energy fluctuations from the hydraulic pump, which powered the rig and vibrator. There
was also potential for additional energy loss with depth as more rods were added. A
single element cone with pore pressure measurement behind the tip (u2) was used, and no
excess pore pressures were recorded. This is likely due to fast dissipation of tip pore
pressures in sands before reaching the u2 element.
Nation
Japan
Japan
Canada
Italy
Table 1. Vibrocone Developmental Contributions
Details
Results
Reference
 downhole vibration
Potentially
Sasaki & Koga, 1982;
 32 kgf horizontal
liquefiable zones
Sasaki et al., 1984
centrifugal force
showed a reduction
 200 Hz frequency
in qc readings
 downhole vibration
Chamber tests and
Teparaksa, 1987
 80 kgf horizontal
paired field
centrifugal force
soundings
 200 Hz frequency
 uphole vibration
Reduction in qc, but Moore, 1987
 vertical force with
no identification of
unknown magnitude
cyclic pore water
 75 Hz average
pressures at u2
frequency
position
 downhole vibration
Qualitative
Mitchell, 1988;
 horizontal centrifugal
interpretation
Picoli, 1993
force with unknown
magnitude
 200 Hz frequency
3.2 Downhole Vertical-Pulse Piezovibrocone
The initial design of the current GT-VT piezovibrocone was aimed to address significant
criteria that were limiting the development of the vibrocone to be used in common
practice. Four main issues were addressed in the design including direction of
oscillation, frequency of operation, application of dynamic force, and economic
considerations.
0
Japanese Vibrocone
Cone Tip Resistance
Static, qcs
2
Dynamic, qcv
Depth
(m)
4
1 m = 3.28 ft
1 kgf/cm2 = 1 tsf
6
0
40
80
120
160
2
Cone Tip Resistance (kg/cm )
a. Vibrocone results at a seismic site with
no apparent settlement during cyclic loading
0
Japanese Vibrocone
Cone Tip Resistance
Static, qc
2
Dynamic, qcv
Depth
(m)
1 m = 3.28 ft
1 kgf/cm2 = 1 tsf
4
6
0
40
80
120
Cone Tip Resistance (kg/cm2)
160
b. Vibrocone results at a site historically experiencing
extensive cyclic-induced settlements
Figure 3. Results from original vibrocone
(Modified after Sasaki et al., 1984)
Many types of dynamic force generators were considered in the initial U.S. prototype
vibrocone device. A downhole system placed just above the penetrometer was desired
since an uphole unit would lose energy efficiencies with depth (similar to the SPT
problem). A closed-loop servo-controlled hydraulic system, similar to MTS or Instron,
would be an excellent source for uniform sine-wave cycles of loading. However, this
approach was discarded due to the difficult logistics of placing large hydraulic hoses
downhole, associated high costs of the system, and need to cool the uphole reservoir
against overheating. An electromechanical source, while easy to control, imparts forces
that are frequency-dependent and the vibrator cannot be made small enough to produce
sufficient forces (20 to 50 kg) at low frequencies to fit within a downhole module. Thus,
a pneumatic unit was chosen for economy, adequate forces, and practical size
considerations. The preliminary device applies impulse-type loading, however,
improvements and modifications are being made at this time.
u2
Centrifugal
Motion
Vibrator
Component
Centrifugal Force = 32 kgf
Frequency = 200 Hz
4.1 cm
fs
qt
Length = 79.0 cm
Diameter = 4.1 cm
a. Original Vibrocone (Sasaki & Koga, 1982)
Vertical Motion
Solenoid
Air Valve
Air
Cylinder
Impact
Mass
Mass
Vibration
u1
b. Impact pneumatic piezovibrocone (Wise, 1998)
Figure 4. Original centrifugal vibrating piezocone and GT-VT vertically vibrating unit
After a review of various design alternatives (Wise, 1998), a downhole pneumatic
impulse generator was developed using compressed gas to generate vertical impacts at
frequencies of 1 to 30 Hz. The fabricated prototype consists of a housing, solenoid valve,
air cylinder, and impact mass directly coupled to a single element piezocone for field
studies (Fig. 4b), and a multi-element piezocone for calibration chamber tests. The
single-element, u1, field piezovibrocone with its necessary components is shown in
Figure 5. The piezovibrocone penetrometer is intended to provided a continuous log of
penetration pore water pressure and associated soil strength measured under partially
undrained and locally-liquefied states. Significant drop in tip resistance (Fig. 3b) will
likely be related to post-cyclic residual undrained shear strength. Multiple modes of pore
pressure generation were reviewed to analyze readings at the u1 and u2 positions.
2-Stage Regulator and
Size C Nitrogen Tank
Digital
Oscilloscope
Control Panel
Pneumatic
Impulse
Generator
Davey (u1)
Piezocone
Figure 5. Components of piezovibrocone (Wise, 1998)
3.3 Analysis of Quasi-Static-Cyclic Pore Pressures
Since liquefaction is a phenomenon resulting from the generation of excess positive pore
pressures, greater emphasis on the analysis pore pressure measurements has been
involved with this study than previously reported vibrocone studies. It is expected that
the measured pore pressures, umeas, will be equal to (Figure 6):
umeas = ushear + uoct
(4)
where ushear is a combination of the shear-induced pore pressure from the zone along the
face of the cone and the cyclic pore pressures induced by the vibratory module, and uoct
is the octahedral pore pressure induced by normal stress changes in the soil. Each of
these components will have different effects, depending upon where the pore pressure
element is located.
q = (v' -  h')
During quasi-static penetration in sands, additional measured pore water pressures at
the u1 position will reflect the octahedral component induced by the triaxial total stress
path under the cone tip. An evaluation of embedded rigid loading in an elastic medium
will give an estimated total stress path with a slope of 3/4 for a soil element in the area of
a mid-face filter (Schiffman & Aggarwala, 1961). The mode of failure at the u2 pore
pressure filter location most resembles direct simple shear (DSS) loading (Baligh, 1984).
At this location there would be no octahedral pore pressure component, and no shear
induced pore pressure component in sands, thus resulting in readings close to hydrostatic.
Impulse vibration is anticipated to add shear induced pore pressures to the u1 and u2
readings.
At the u2 position, a ninety-degree rotation of a DSS soil element will result from the
configuration of the cone, and the configuration of the vertical vibratory excitation. The
normal stress is equal to a function of effective overburden and the coefficient of lateral
earth pressure at rest, Ko. Since the cone penetrometer is coupled to the impulse vibrator,
a true cyclic motion is not induced into the ground. Cyclic pore pressures usually
increase on a positive stroke, and then decrease on a negative stroke of cyclic loading
(Seed & Lee, 1966). The Mark-I piezovibrocone is a pneumatic impulse generator with
only positive force excitation and relaxation to the in-situ stress state, so increasingly
positive excess pore pressures are expected in potentially liquefiable soils.
M
6 sin f ў
3 - sin f ў
q
1
ushear
uoct
(po', qo)
Total Stress
Path for an
element of soil
beneath the
cone tip, u1
Shear induced pore
pressures from
cyclic loading
p, p'
p' = (v' + 2h')/3
Figure 6. Stress path approach for obtaining pore pressure components
A magnitude 7.5 earthquake is expected to have about 15 equivalent stress cycles
(Seed et al., 1983) and have a frequency of about 0.3 to 5 Hz. During a quasi-static cone
penetration test, the penetrometer moves at 2 cm/sec. Input frequencies that are slightly
greater or at the high end of typical earthquake frequencies will likely be necessitated by
the continuous nature of the piezocone test and the excess pore pressures measured at the
u2 position from piezovibrocone tests in clean sands will solely be a result of cyclic shear
loading. While it is anticipated that the u2 pore pressures will dissipate rapidly, they are
necessary for correcting qc to qt (e.g. Lunne et al., 1986; 1997).
Due to the difficulty in separating octahedral and shear components of pore pressure
readings at the u1 position, pore pressure difference between dynamic and static
soundings will be evaluated. Difficulties may arise in analyses since it is currently
unknown whether localized liquefaction is fully occurring, partially occurring, or not
occurring at all. To determine the actual failure mechanisms occurring in-situ, a program
of laboratory tests, calibration chamber tests, and trial field tests is underway.
4 FIELD RESULTS
In 1886, the largest magnitude earthquake recorded in the Eastern United States occurred
near Charleston, South Carolina (Martin & Clough, 1994). A number of investigators
have evaluated paleoliquefaction evidence (e.g. Obermeier, 1996), standard penetration
test data (Martin, 1990), cone penetration test data (Martin, 1990), and laboratory test
data (Cullen, 1985) to evaluate the occurrence and extent of liquefaction during the 1886
Charleston earthquake. This study will evaluate trial field tests of the piezovibrocone
performed at historic earthquake sites, and assess its potential as a tool for predicting
liquefaction susceptibility of sandy soils. In addition to piezovibrocone tests, seismic
piezocone tests (SCPTu) have been performed at the selected test sites.
Preliminary testing of the Mark-I piezovibrocone was performed in February, 1998 at
previously studied (Clough & Martin, 1990; Martin, 1990; Martin & Clough, 1994)
historic liquefaction sites in Charleston, SC. Hollywood Ditch (HW) and Thompson
Industrial Services (TIS) were the two test sites selected. Thompson Industrial Services
was not a site studied by Clough and Martin, but was in between the studied sites of Ten
Mile Hill and Eleven Mile Post. Prior Figures 1a & b show schematics of the
piezovibrocone and the seismic piezocone used for field tests in Charleston, SC. Tables 2
and 3 display tests performed at Hollywood and TIS respectively. The soundings at each
site will be compared to evaluate the response of the piezovibrocone as a cone
penetrometer in general, and as a site-specific liquefaction index tool.
At Hollywood, tip resistance (qc) results for a commercially-pushed 10 cm2 Cone Tec
cone and results of the static 10 cm2 Davey cone attached directly in front of the vibrating
unit are displayed in Figure 7a. It should be noted that the two static piezovibrocone
soundings were separated by 3-meters. The agreement between the two soundings is
good considering the natural variation in the subsurface soils. Figure 8a displays a
comparison between static and dynamic tip resistance using the piezovibrocone.
Previous vibrocone data analysis (Sasaki & Koga, 1984; Moore, 1987) was based on
significant losses in tip resistance in potentially liquefiable layers (Fig. 3b). Even though
there was no significant variation in tip resistance between static and dynamic soundings
using the Mark-I piezovibrocone at Hollywood Ditch, an increase in u1 pore pressures
qc (MPa)
0
5
10
um (kPa)
0
15
20
-100
0
2
2
4
4
6
100
fs (kPa)
300
0
50
Vs (m/sec)
100 150
0
0
0
2
2
4
4
6
6
6
8
8
8
8
10
10
10
10
250
500
Depth BGS (m)
u2
u1
HW - 4; Gregg In-situ
HW - 4; u2
HW - 2; Static Vibrocone
HW - 2; u1
HW - 4; Seismic CPT
HW - 4; Gregg Insitu
SASW (Indridason,
1992)
Figure 7. Static piezovibrocone and seismic piezocone soundings, Hollywood Ditch, SC
a. tip resistance, qc; b. pore pressure, um; c. sleeve friction, fs; d. shear wave velocity, Vs
qc (MPa)
0
5
10
15
u1 (kPa)
20
0
0
0
2
2
4
4
100
200
300
Depth BGS (m)
Clean Sands
u1
Silty Sand to
Clayey Silt
6
6
Clean Sand to
Silty Sand
8
8
Silt to
Sandy Silt
10
10
HW - 1; Dynamic Vibrocone
HW - 1; Dynamic
HW - 2; Static Vibrocone
HW - 2; Static
Figure 8. Static and dynamic piezovibrocone soundings at Hollywood Ditch, SC
a. tip resistance, qc; b. mid-face pore pressure, u1; c. soil profile
qc (MPa)
0
5
10
15
um (kPa)
0
20
-100
0
2
2
4
100
fs (kPa)
300
0
50
Vs (m/sec)
100 150
0
0
0
2
2
4
4
4
6
6
6
6
8
8
8
8
10
10
10
10
250
500
Depth BGS (m)
u2
u1
TIS - 01; Gregg In-situ
TIS - 01; u2
TIS - 03; Static Vibrocone
TIS - 03; u1
TIS - 01; Gregg Insitu
TIS - 01; Seismic
CPT
Figure 9. Static piezovibrocone and seismic piezocone soundings at the TIS site, SC
a. tip resistance, qc; b. pore pressure, um; c. sleeve friction, fs; d. shear wave velocity, Vs
qc (MPa)
Depth BGS (m)
0
a.
5
10
u1 (kPa)
15
20
0
0
0
2
2
4
4
6
6
8
8
10
10
250
500
u1
Sand to
Silty Sand
Silt to
Sandy Silt
Silty Sand
Clayey Silt
TIS - 02; Dynamic Vibrocone
TIS - 02; Dynamic
TIS - 03; Static Vibrocone
TIS - 03; Static
Figure 10. Static and dynamic piezovibrocone soundings at the TIS site, SC
tip resistance, qc;
b. mid-face pore pressure, u1;
c. soil profile
during dynamic excitation shows a potentially liquefiable layer from about 1.4 meters to
3 meters (Fig. 8b).
Table 2. Initial tests performed at Hollywood Ditch
Test Number
Description
HW - 1
Dynamic piezovibrocone
(with mid-face pore pressure element, u1)
Frequency: 5 Hz
Pressure: 650 kPa
HW - 2
Static piezovibrocone
(with mid-face pore pressure element, u1)
HW - 3
HW - 4
Dynamic piezovibrocone
(with mid-face pore pressure element, u1)
Frequency: 2.5 Hz
Pressure: 650 kPa
10 cm2 Seismic Piezocone
(with shoulder pore pressure element, u2)
A generalized soil profile was developed (Fig. 8c) based on prior knowledge of the site
(Martin, 1990) and empirical CPT classification schemes (Robertson et al., 1986;
Campanella & Robertson, 1988; Robertson, 1990; Lunne, Robertson, & Powell, 1997)
utilizing tip resistance, friction ratio, and pore pressures measured behind the tip, u2.
Dynamic soundings were performed 1.5 meters from the static sounding. There is a
general increase in pore pressure throughout the entire depth of the dynamic sounding,
with a substantial increase in the clean sand layer at the beginning of the water table (1.4
meters). This layer is considered to have partially liquefied under the impulse loading.
Future analysis will involve use of the timer input (5 Hz) and results of geophone or
accelerometer output to provide an estimated number of equivalent cycles at a specific
peak particle velocity (maximum local acceleration) to perform a cyclic stress or cyclic
strain based approach to liquefaction analysis. If local liquefaction had occurred, a drop
in tip resistance related to the post-cyclic undrained shear strength would have been
expected, however, tip resistance was similar in the static and dynamic soundings, and is
a topic for further study.
A similar analysis of the soundings at Thompson Industrial Services was performed.
Figure 9 shows qc, u1, u2, fs, and Vs results from the penetrometers used in this study. A
comparison of qc obtained by a commercially-pushed 10 cm2 Cone Tec cone with the
results of the static 10 cm2 Davey cone attached directly in front of the vibrating unit
shows good agreement between the two soundings, considering the natural variation of
the subsurface soils. The static piezovibrocone was about 3 meters away from the
commercial sounding, and dynamic soundings were about 1.2 meters from the static
piezovibrocone. There was no significant variation in tip resistance between static and
dynamic soundings using the Mark-I piezovibrocone at the TIS site (Fig. 10a), but an
increase in u1 pore pressures during dynamic excitation shows a potentially liquefiable
layer from about 4 meters to 5 meters (Fig. 10b).
The profile for the TIS site (Fig. 10c) shows considerably more fines than Hollywood
Ditch with no clean sands present. The water table was at approximately the same depth
(1.4 meters), and the large rise in pore pressures came in a silt to sandy silt layer at depths
of about 4 to 5 meters. This layer showed increases in static u2, as well as static u1 and
dynamic u1 pore pressures. Positive static u2 pore pressures are indicative of initial
undrained to partially undrained penetration. Therefore, the analysis of the dynamic u1
pore pressures must consider potential shear-induced pore pressures along the face in
addition to cyclically induced and octahedral pore pressures. The dynamic sounding at
TIS (Fig. 10b) produced much higher pore pressures than the dynamic sounding at
Hollywood (Fig. 8b), but still no reduction in qc was noticed (Fig. 10a). These increased
excess dynamic pore pressures are possibly from shear induced pore pressures on the face
of the cone from partially undrained behavior of the silt, and not a pure function of the
cyclic loading. It is not typically expected that a similar soil layer with a higher fines
content will be more susceptible to liquefaction, but the site specific nature of the
piezovibrocone has the potential to identify layers that will be strongly affected by cyclic
loading.
Table 3. Initial tests performed at the TIS site
Test Number
Description
TIS - 01
10 cm2 Seismic Piezocone
(with shoulder pore pressure element, u2)
TIS - 02
Dynamic piezovibrocone
(with mid-face pore pressure element, u1)
Frequency: 5 Hz
Pressure: 460 kPa
TIS - 03
Static piezovibrocone
(with mid-face pore pressure element, u1)
TIS - 04
Dynamic piezovibrocone
(with mid-face pore pressure element, u1)
Frequency: 5 Hz
Pressure: 650 kPa
5 CALIBRATION CHAMBER TESTS
Chamber testing has been progressing at the Prices Fork Laboratory at Virginia Tech
using a special mulit-element piezocone penetrometer donated by Fugro Geosciences.
The initial static CPTu results by J. Bonita showed that the porewater pressure
transducers of the penetrometer were affected by thermal drifts associated with frictional
heat during the CPT in sand. Thus, the penetrometer was shipped to Fugro/Netherlands
where Mr. Denis Lawson outfitted the penetrometer with thermally-compensated
transducers with higher accuracy and lower range, specifically for the series of chamber
tests. The modified penetrometer arrived in Blacksburg in September, 1998, and a series
of static and dynamic tests has been started. Figure 11 shows the Fugro multi-element
piezocone attached to the modified vibrating unit. Figure 12 displays quasi-static and
vibratory pore pressure measurements taken during a calibration chamber test on loose
light castle sand. To compare static and dynamic results in a uniform deposit, the first
half of the sounding was pushed without vibration, and vibration was turned on for the
second half.
2-Stage
Regulator
Control
Panel
Size C
Nitrogen Tank
Fugro
Multi-element
Piezocone
Digital
Signal Analyzer
AW Rod
Split
Adapter
Pneumatic
Impulse
Generator
Figure 11. Fugro Multielement Piezocone Attached to Modified Vibrocone and
Associated Equipment (tape measure is 2.05 m (6 feet) in length)
Figure 12. Quasi-static and Vibratory Pore Pressures Induced During Calibration
Chamber Tests
6 FUTURE WORK
Data provided by a vertically-vibrating penetrometer could ideally be evaluated using
theoretical effective stress models developed for representing the behavior of sands and
silts. In particular, the concepts of static liquefaction, instability, steady state, and
compressibility, as well as the aspects of contractive vs. dilative behavior could be
incorporated. As such, the research program will include a full suite of laboratory
characterization tests on the Light Castle sand that is being used currently in the CPT
chamber test series. Lab testing will consist of static and cyclic triaxial tests using CK
Chan apparatuses and companion sets of direct simple shear tests. Resonant column
testing will provide the fundamental small-strain stiffness measurements at varying
relative densities. Complementary series of undrained and drained triaxial testing will
provide the necessary ingredients to permit the characterization of the steady-state lines
(e.g., Been et al., 1991; Yamamuro and Lade, 1998).
For the next generation of vibrocone, a linear displacement unit is being developed to
more accurately address cyclic straining of a soil element in the vicinity of the cone tip
and pore pressure filter elements. Impulse forces are ideally replaced with a form of
uniform sinusoidal loading. Alternative devices for producing cyclic or repetitive loading
are also under consideration, including electronic solenoids, mechanical cams, as well as
the use of a programmable air regulating (PAR) valve. Other important facets currently
under investigation include the thermal effects on transducer measurements (Lunne, et.
al. 1986), porous filter material (Campanella & Robertson, 1988), backpressurization of
the hydrostatic water in saturated chamber tests, and technical details related to the
accuracy of measurements by the cone sensors. Of specific mention herein are the
difficulties in choice of filter face elements (e.g., ceramic, plastic, carborundum, sintered
brass, stainless steel) due to effects of smearing, heating, abrasion, compressibility, and
wear. Of additional mention is the baseline drift of transducers caused by frictional
heating as the penetrometer is pushed in sands. These technical aspects will be shared in
later reports issued by GT & VT.
7 CONCLUSIONS
The initial design and field testing of an impulse piezovibrocone has been completed as
an improved means for evaluating soil liquefaction susceptibility and post-liquefaction
residual undrained shear strength of sandy and silty soils. The initial design of the GTVT piezovibrocone has isolated vertical motion into a downhole pneumatic unit coupled
with conventional electric piezocone penetrometers.
Initial field trials at historic liquefaction sites in Charleston, SC have shown an
increase in mid-face pore pressures in potentially liquefiable layers, but no corresponding
change in tip resistance. A series attempted in the New Madrid seismic region by GT
resulted in final burial and loss of the prototype unit. Controlled CPT chamber tests are
just now beginning at VT with a second pneumatic vibro-unit. Ongoing improvements to
the vibrocone design are underway to provide a completely electrical device with better
control of force generation, frequency, and consistency of the measured responses. The
research will continue with calibration chamber tests, associated laboratory tests, and
field tests in liquefiable and non-liquefiable deposits.
8 REPORTS AND PUBLICATIONS
Preliminary design and operations of the unit have been described in a thesis (Wise,
1998), as well as two technical papers (Schneider, et al., 1999; Wise et al., 1999).
Presentations on the vibrocone have been given at two opportunities: (1) The ASCE
Geotechnical Earthquake Engineering & Soil Dynamics Conference, Session 1, Seattle,
August 3-6, 1998; and (2) The NSF Workshop on Physics & Mechanics of Soil
Liquefaction, Baltimore - Johns Hopkins University, September 10-11, 1998 which is
now published by A.A. Balkema, Rotterdam.
ACKNOWLEDGEMENTS
The authors appreciate the support of Dr. John Unger of the U. S. Geological Survey and
Dr. Cliff Astill of the National Science Foundation. Craig Wise of B&V designed the
first pneumatic device and Tracy Hendren of H&H constructed the second unit. Recep
Yilmaz and Rick Klopp of Fugro Geosciences are thanked for the use of the multielement piezocone and assistance during field tests, as well as to Brad Pemberton of
Gregg In-Situ for help with initial field trials in Charleston, S.C.
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