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 EngineeringGeorgia Institute of Technology

Atlanta, Georgia 30332-0355Phone: 404-894-6226

Fax: 404-894-0830e-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 fully-understood 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 site-specific 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 site-specific 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).

a. Prototype piezovibrocone

b. 10 cm2 seismic piezocone

c. 15 cm2 multi-element piezocone

Figure 1. Penetrometers used

Vibratory Unit

7.3 cm3.57 cm

Friction Sleeve -150 cm2

Cone Tip - 10 cm2

u1

60oFriction Sleeve - 150 cm2

Geophone

3.57 cm

u2

Cone Tip - 10 cm2

60oFriction Sleeve - 225 cm24.37 cm

u2

Cone Tip - 15 cm2

u3 u1

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:

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):

Additional uncertainty is added to liquefaction curves by this normalization. Magnitude-scaling 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)

dvo

vo

vo

avg rg

aCSR

'65.0

'max

(1)

MSFCSRCRR 5.7 (2)

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 KD, which is a dimensionless parameter (Marchetti, 1980).

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)

c. d.

b.a.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10Horizontal Stress Index, KD

Cyc

lic R

esis

tanc

e R

atio

(CR

R)

Reyna & Chameau, 1991

Robertson & Campanella, 1986

Marchetti, 1982

Clean Sands M=7.5

Liquefaction Likely

Liquefaction Not Likely

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30qc1 (MPa)

Cyc

lic R

esis

tanc

e R

atio

(CR

R)

M=7.5

Silty Sand-Sandy SiltD50 (mm) < 0.10F.C. (%) > 35

Silty Sand0.10 < D50 (mm) < 0.25

5 < F.C. (%) < 35

Clean Sand0.25 < D50 (mm) < 2.0

F.C. (%) < 5

Liqu

efac

tion

Liquefaction Not Likely

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300

Vs1

Cyc

lic R

esis

tanc

e R

atio

(CR

R) FC (%) = 35 15 < 5

M=7.5

Liquefaction Likely

Liquefaction Not Likely

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50

(N1)60

Cyc

lic R

esis

tanc

e R

atio

(CR

R) FC (%) = 35 15 < 5

M=7.5

Liquefaction Likely

Liquefaction Not Likely

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 Sus, 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.

Table 1. Vibrocone Developmental ContributionsNation Details Results ReferenceJapan downhole vibration

32 kgf horizontal centrifugal force

200 Hz frequency

Potentially liquefiable zones showed a reduction in qc readings

Sasaki & Koga, 1982;Sasaki et al., 1984

Japan downhole vibration 80 kgf horizontal

centrifugal force 200 Hz frequency

Chamber tests and paired field soundings

Teparaksa, 1987

Canada uphole vibration vertical force with

unknown magnitude 75 Hz average

frequency

Reduction in qc, but no identification of cyclic pore water pressures at u2 position

Moore, 1987

Italy downhole vibration horizontal centrifugal

force with unknown magnitude

200 Hz frequency

Qualitative interpretation

Mitchell, 1988;Picoli, 1993

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.

a. Vibrocone results at a seismic site with no apparent settlement during cyclic loading

b. Vibrocone results at a site historically experiencing extensive cyclic-induced settlements

Figure 3. Results from original vibrocone(Modified after Sasaki et al., 1984)

Cone Tip Resistance (kg/cm2)0 40 80 120 160

0

2

4

6

Static, qcs

Dynamic, qcv

Depth(m)

1 m = 3.28 ft1 kgf/cm2 = 1 tsf

Japanese VibroconeCone Tip Resistance

0 40 80 120 160

0

2

4

6

Japanese VibroconeCone Tip Resistance

Static, qc

Dynamic, qcvDepth(m)

Cone Tip Resistance (kg/cm2)

1 m = 3.28 ft1 kgf/cm2 = 1 tsf

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.

a. Original Vibrocone (Sasaki & Koga, 1982)

b. Impact pneumatic piezovibrocone (Wise, 1998)

Figure 4. Original centrifugal vibrating piezocone and GT-VT vertically vibrating unit

VibratorComponent

Centrifugal Force = 32 kgfFrequency = 200 Hz

Length = 79.0 cmDiameter = 4.1 cm

CentrifugalMotion 4.1 cm qt

u2

fs

Vertical Motion

u1SolenoidAir Valve

AirCylinder

MassVibration

ImpactMass

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.

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.

Pneumatic Impulse

Generator

Davey (u1) Piezocone

2-Stage Regulator and Size C Nitrogen Tank

Control Panel

Digital Oscilloscope

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.

Figure 6. Stress path approach for obtaining pore pressure components

p, p'

q

p' = (v' + 2h')/3

(po', qo)

q =

(v'

- h')

1

sin3

sin6M

ushearuoct Total Stress

Path for anelement of soilbeneath thecone tip, u1

Shear induced porepressures fromcyclic loading

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

Figure 7. Static piezovibrocone and seismic piezocone soundings, Hollywood Ditch, SCa. tip resistance, qc; b. pore pressure, um; c. sleeve friction, fs; d. shear wave velocity, Vs

Figure 8. Static and dynamic piezovibrocone soundings at Hollywood Ditch, SCa. tip resistance, qc; b. mid-face pore pressure, u1; c. soil profile

0

2

4

6

8

10

0 5 10 15 20qc (MPa)

Dep

th B

GS

(m)

HW - 4; Gregg In-situ

HW - 2; Static Vibrocone

0

2

4

6

8

10

0 50 100 150fs (kPa)

HW - 4; Gregg In-situ

0

2

4

6

8

10

-100 100 300um (kPa)

HW - 4; u2

HW - 2; u1

u2

u1

0

2

4

6

8

10

0 250 500Vs (m/sec)

HW - 4; Seismic CPT

SASW (Indridason,1992)

Clean Sands

Silty Sand toClayey Silt

Clean Sand to Silty Sand

Silt toSandy Silt

0

2

4

6

8

10

0 100 200 300

u1 (kPa)

HW - 1; Dynamic

HW - 2; Static

u1

0

2

4

6

8

10

0 5 10 15 20

qc (MPa)

Dep

th B

GS

(m)

HW - 1; Dynamic Vibrocone

HW - 2; Static Vibrocone

Figure 9. Static piezovibrocone and seismic piezocone soundings at the TIS site, SCa. tip resistance, qc; b. pore pressure, um; c. sleeve friction, fs; d. shear wave velocity, Vs

Figure 10. Static and dynamic piezovibrocone soundings at the TIS site, SCa. tip resistance, qc; b. mid-face pore pressure, u1; c. soil profile

0

2

4

6

8

10

0 5 10 15 20qc (MPa)

Dep

th B

GS

(m)

TIS - 01; Gregg In-situ

TIS - 03; Static Vibrocone

0

2

4

6

8

10

0 50 100 150fs (kPa)

TIS - 01; Gregg In-situ

0

2

4

6

8

10

-100 100 300um (kPa)

TIS - 01; u2

TIS - 03; u1

u2

u1

0

2

4

6

8

10

0 250 500Vs (m/sec)

TIS - 01; SeismicCPT

Sand toSilty Sand

Silt to Sandy Silt

Silty Sand

Clayey Silt

0

2

4

6

8

10

0 250 500

u1 (kPa)

TIS - 02; Dynamic

TIS - 03; Static

u1

0

2

4

6

8

10

0 5 10 15 20

qc (MPa)

Dep

th B

GS

(m)

TIS - 02; Dynamic Vibrocone

TIS - 03; Static Vibrocone

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 DitchTest 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 Dynamic piezovibrocone(with mid-face pore pressure element, u1)Frequency: 2.5 Hz Pressure: 650 kPa

HW - 4 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 siteTest 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.

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

Pneumatic Impulse

GeneratorFugro

Multi-element Piezocone

Size C Nitrogen Tank

Control Panel

Digital Signal Analyzer2-Stage

Regulator

Split Adapter

AW Rod

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 GT-VT 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 multi-element 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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