In Situ Measurement of the Liquefaction Potential of Soils ... - CiteSeerX

S7-P04

In Situ Measurement of the Liquefaction Potential of Soils using a Shear-wave Vibrator Tomio INAZAKI Public Works Research Institute ABSTRACT An in-situ measurement method is proposed to evaluate the liquefaction potential of near-surface soils under the strong motion. The method utilizes a shear-wave vibrator as a dynamic loading source and electrical cones to monitor the response of pore water pressure at specific horizons and distances under the vibration. The shear wave vibrator has a power to oscillate the ground surface horizontally with 100 gal or more at 5 m to the vibrator. Results of field measurements showed generation of the excess pore water pressure during vibration accompanied with squeezing of groundwater onto the surface. The pore water pressure responses varied at depths and distances, which indicates the layered structure of near surfaces strongly controls liquefaction phenomena. It is very hard to take account of this stratification effect in the conventional laboratory test using small specimen. Thus the test results demonstrate the advantages of active monitoring or in-situ measurement for direct evaluation of the liquefaction resistance of near-surface soils. KEYWORDS: in-situ measurement, liquefaction, S-wave vibrator, dynamic oscillating source, pore water pressure. INTRODUCTION Liquefaction has commonly occurred during a number of earthquakes in soft grounds mainly composed of fine to medium grained sands. Soil liquefaction is a phenomenon deeply involving the property of constituting material but also the stratified structure of the ground. However, laboratory cyclic shear tests have been widespread for evaluating the liquefaction susceptibility by means of small specimen sampled from the ground with no regard to the influence of spatial anisotropy of ground. Accordingly, a dogmatic idea that liquefaction potential is mainly controlled by the grain size distribution characteristics of soils has been widely believed between geotechnical engineers. Namely, sands were considered to be the only type of soil susceptible to liquefaction, but liquefaction has also been observed in gravel and silt. Then they have been trying to obtain larger specimens including coarser grained materials without disturbing physical properties while sampling, but they have been still paying a little attention to the layered structure of actual ground. In current engineering practice, the liquefaction resistance of sandy soil is assessed usually based on N-value of in-situ Standard Penetration Test (SPT) and grain size distribution data [1, 2]. However, this empirical way is also inadequate as exemplified in Figure 1. Soil liquefaction and ground failure was broadly identified associated with the 1995 Hyogo-ken Nanbu earthquake. Hanshin Expressway, which ran through the disaster area of the earthquake, was also severely damaged by the liquefaction. Check drilling and sampling was carried out along the Hanshin Expressway to evaluate the influence of grain size distribution characteristics to the liquefaction. As shown in Figure 1, most of the test data for liquefied points (solid circles) are plotted in a shaded zone, which is empirically specified to have the high potentiality of liquefaction. However the data for unliquefied points (open circles) are also distributed in the shaded zone, which means the grain size characteristics are not effective to evaluate the liquefaction susceptibility. Furthermore, it seems geotechnical engineers do not know that particle grading of soils usually change into a finer sorted

one during liquefaction, which has been revealed by the detailed geological investigations of sand boils and liquefied grounds associated with large earthquakes [4, 5]. Namely, grain size characteristics for the liquefied point data drawn in Figure 1 are the result of grain differentiation caused by liquefaction and do not show the original state before liquefaction. Another important and fatal problem concerned in Figure 1 is in the discrimination of liquefaction. Soil liquefaction and ground failure are easily recognizable when surface events are clear as sand boils, cracking and ground flows, however, no evidence of such surface events does not mean the ground is unliquefied. Because liquefaction originates inherently in the ground under the control of spatial structure or horizontal layering of the ground. Actually, subsurface liquefaction without any surface evidences has been identified at many archeological sites [5]. Therefore, it is quite superficial to evaluate the liquefaction status based only on such the surface evidences. An in-situ method has long been requested to measure the response of actual ground, which is hard to reproduce in laboratory tests. Vibrocone [6] is one of such in-situ tools for evaluating soil liquefaction potential in a direct manner. The vibrocone is a family of Cone Penetration Test (CPT). Two cones are penetrated into ground under both static and dynamic excitation in paired side-byside soundings, and dynamic penetration with a vibrating shaker induces liquefaction locally in the vicinity of the probe. In comparison with both data, generation of cyclic excess pore pressure at any depth is measured, however, the measured pressure values are just transient. Time variant data at specific horizons are not obtainable in this method. Blasting can also induce liquefaction [7]. A large scaled field test of blast-induced liquefaction was successfully carried out at a harbor site, Hokkaido, Japan in 2001. Blasting is quite powerful to cause liquefaction in ground, however, it is hard to control the source energy. The author has proposed to utilize a vibrator, a mobile source used in the seismic reflection survey, for in-situ measurement of

Figure 1. Grain size characteristics of sands sampled along Hanshin Expressway damaged by the 1995 Hyogo-ken Nanbu Earthquake. No significant difference is identified between liquefied and unliquefied samples. (drawn from the data listed in Matsuo & Ninomiya [3])

The Proceedings of IWAM04, Mizunami, Japan

S7-P04 liquefaction potential of the ground [8]. Because of its high mobility and high power of oscillating ground, it was expected to be a suitable loading source compared with explosives. The idea of utilizing vibrator for liquefaction measurement has been followed by the research group at the University of Texas at Austin (UTA) [9]. Field tests were performed at three sites in Japan from 1996 through 1998. Generation of the excess pore water pressure during vibration was measured accompanied with squeezing of groundwater onto the surface. The pore water pressure responses varied at depths and distances, which indicates the layered structure of near surfaces strongly controls liquefaction phenomena. OUTLINE OF MEASUREMENT SYSTEM The in-situ measurement technique is proposed to observe responses of pore water pressure at depths associated with the surface oscillation by means of a vibrator. The main instruments used for the measurement consist of an S-wave vibrator, a cone penetrator and measuring probes. The advantages of making use of S-wave vibrator are in the continuous oscillation which enables to reveal the time varying response of pore pressure and the predominance of SH-waves. The time varying response of pore water pressure is measured using pore pressure sensor incorporated in the measuring probe, which is previously penetrated into the target depth in a test site using cone penetrator. In contrast to the conventional Cone Penetration Test (CPT), the purpose of which is to obtain a log profile of each point, the measuring probes used for this technique are stayed at specific depth after usual log profiling. The measuring probes also have three component accelerometers within them so as to record the waveforms and amplitudes of ground vibration at stayed depths. S-wave vibrator We have introduced an S-wave type vibrator to generate strong SH-wave motion in the limited small area. A photograph of the Swave vibrator is shown in Figure 2, and its main specifications are listed in Table 1. Vibrators have been widely used in seismic reflection survey, however, the major is P-wave type and S-wave generation type has Table 1. Main specifications of the S-wave vibrator. ITEM

Specifications

Type: Maker: Hold Down Weight: Reaction Mass: Total Moving Mass: Baseplate: Peak Force: Frequency Range: Max. Stroke:

M18LF/V623 Mertz Inc. 26.7 ton 2.7 ton 5.4 ton 2.4×1.4m 133 kN 5-100 Hz 8 in

been rarely used as an exploration source. The S-wave vibrator we introduced was specially tuned to generate low frequency of 5 Hz, and characterized as buggy-mounted, hydraulically driven, and offroad type. It can generate both sinusoidal and sweep waves controlled under an electro-hydraulic system with feedback equipments composed of LVDTs and accelerometers mounted on a valve, reaction mass and a baseplate. The LVDT sensors detect every 1 msec the displacement of the valve and mass, accelerometers monitor the phase and force of mass and baseplate movements, which would vary momentarily due to the change in coupling condition between the baseplate and ground surface, then the electro-hydraulic system simultaneously controls hydraulic pump system to keep constant shape of waveform to be transmitted to the ground. The synthetic signal and ground force waveform can be outputted to a recorder. The maximum force output is 133 kN, about 50 % of the hold down weight. Measuring probe and recording system The measuring system for in-situ liquefaction test mainly consists of a cone penetrator, measuring probes, and a data acquisition system. The cone penetrator we have is featured as a crawler type, having the capability to work even on the soft ground, twin hydraulic pistons which reciprocally hold and push the rod down into the ground in order to keep constant and continuous penetration. Total weight of the penetrator is about 7.5 ton. Two types of probes were used for measuring vibration response in place; one is a common electrical cone of 36 mm diameter for tip resistance, sleeve friction, and pore water pressure measurement. The other is a seismic cone of 44 mm diameter in which three component accelerometers and a pore water pressure sensor are incorporated. Frequency response of the accelerometer is set to flat in range from 1 to 400 Hz. Data from several probes were recorded over several minutes continuously using digital data recorder with high sampling rate less than 1 millisecond. The stored data in digital tapes were re-sampled and transferred to a PC for the following analysis. FIELD LIQUEFACTION TESTS As mentioned above, liquefaction measurement tests were performed at three fields from 1996 through 1998. Due to some regulations, we had to set the field in riverbed terrace, and in the premises of our Institute, far from buildings. The outline of test and setting at each field is described below. Sawara TRR96L2 test The first liquefaction measurement test was conducted in 1996 on the river terrace at right bank side of Tone River, near Sawara City, about 30 km southeast of Tsukuba. Prior to the liquefaction measurement, we conducted dense

Figure 3. Geologic section of Sawara TRR96L2 Site interpreted from dense CPTs. Solid triangles correspond to the horizons where probes were placed to measure in-situ pore pressure. Log curves besides each column represent tip resistance (qt) and pore pressure (ud), respectively. Figure 2. S-wave vibrator used for in-situ liquefaction measurement. The Proceedings of IWAM04, Mizunami, Japan

S7-P04 CPT at 10 m grid down to about 10 m depth as shown in Figure 4 in order to clarify the near surface geological structure. Next, we penetrated four probes of each pore pressure and seismic into the ground at the center of the square shaped site, and stayed them at different four depths, namely, -1.7, -3.0, -4.5, and -6.0 m for the pore pressure cones, and -3.0, -4.5, -6.0, and -16.0 m for the seismic cones. Figure 3 shows an interpreted geologic section on the diagonal along NE-SW of the site, reconstructed from CPT logs. Uppermost two meters of the surface is covered with flood origin sand which constitutes a river terrace. A thin clay bed occurs below the sand bed. Water table is found within or just beneath this clay bed. Another thin clayey bed, which is intercalated in fluvial sand layer, occurs about 4 m in depth. It is inferred that the clay bed at 2 m in depth is impermeable from the characteristic CPT pattern; very low tip resistance (qt) and high apparent pore pressure (ud) as drawn in Figure 3. The solid triangles in Figure 3 indicate the horizons where pore pressure probes were stranded. These four probes were closely located, as shown in Figure 4, so as to represent response of the same point. Penetration points of four seismic cones were also shown in Figure 4 as solid squares. The water level at the center of the test site was estimated about -2.0 m from the ground level (GL) within the clay bed, and the top probe was set just on this clay bed. Initial value of pore pressure at this horizon was measured as 1 kPa, slightly high to the static pore pressure calculated from the water level. It may be due to the impermeable clay bed. The probe at GL -3.0 m was located at the bottom of fluvial sand bed, and directly on an intercalated clayey bed. Static pore pressure at the level before shaking was 12 kPa, slightly higher than that calculated from the water level. These values are labeled on the left end of each curve in Figure 5. Probes at GL -4.5 m and -6.0 m were settled in underlying fluvial sand layer, but the CPT profile suggests that a thin silty bed was interbedded at about GL -4.5 m, just above the probe. Figure 4 shows the layout of liquefaction measurement test at Sawara TRR96L2 Site. Representative four vibration points are illustrated with notes indicating the locality of baseplate from the center of probe array. Because the baseplate shakes in a longitudinal direction, transverse component would dominate for Shake 42 and 43; radial component for Shake 41 and 46 at the cone array. We tuned the vibrator controller electronics (VCE) to generate waveform of constant frequency of 10 Hz (Shake 43, 41, and 46), or 10 to 40 Hz up sweep waves (Shake 42), both lasting 20 seconds, and repeated the shaking 10 to 15 times intermittently. Totally the ground was oscillated for 3 to 5 minutes. Figure 5 shows the time histories of pore water pressure

Figure 4. Layout of liquefaction measurement test at Sawara TRR96L2 Site. Prior to shaking, pore pressure and seismic cones were pushed in to the ground at the center of the site. After that the ground of the site was shaken using S-wave vibrator. Representative four vibration points are illustrated as the baseplate of the vibrator in the same scale. (revised from Inazaki [10])

Figure 5. Time histories of pore water pressure responses observed at different four depths during the shaking at different four vibration points. The ground was oscillated for 3 to 5 minutes intermittently with 10 Hz sinusoidal waves (Shake 43, 41, and 46) or 10 to 40 Hz up sweep waves (Shake 42), both lasting about 20 seconds. Remarkable change in the static pore pressure was observed as well as synchronized sinusoidal response to the surface vibration. Highest or lowest points of base line change are marked with arrows on the curves with their values, along with residual pore pressure if remained. Peak to peak values of dynamic response are also cited.


S7-P04 responses observed at different four depths for the shaking at different four vibration points shown in Figure 4. It is remarkable that the hydrostatic pore pressure changed during the surface vibration associated with synchronized sinusoidal response. It is noticed that the probe at GL -3.0 m showed quite large responses both of the fluctuation in hydrostatic (long period) pores pressure and of the hydrodynamic response synchronic to the vibration compared with data of other three depths. The response of the top probe was small because it was located just on the original water table. Nevertheless, static shift amounted to 1.0 kPa concurrently with the shaking even at the top probe. The pore pressure responses at the lower two depths were also small, maximum increase or decrease in static pressure were +0.7 kPa or -0.4 kPa, 1.2 kPa (peak to peak) for synchronic dynamic pressure at most. Time response curves for the data at GL -3.0 m show irregular shapes. For instance, they suddenly descended concurrently with shaking, followed by rebound and gradual decrease, which indicates the generation and dissipation of excess pore pressure. Pre-eventual increase recognized in Shake 46 might be due to a residual pressure of previous shaking. The amounts of decline and buildup in hydrostatic pore pressure increased with bringing the vibrator close to the array. Vibration levels of each shaking test are listed in Table 2. Because the S-wave vibrator was set to shake in N-S direction at all shaking points, the N-S component should have the maximum values. However, the peak ground accelerations were recorded occasionally in other components. Especially in Shake 46, which was the last shake in the site, very large amplitude of acceleration was recorded in U-D component. It might be due to change in coupling of probes with ground or a local ground failure near the vibration point. The amplitude of hydrodynamic pore pressure was large when shaken in the radial direction (Shake 41 and 46) in contrast to transversal shaking (Shake 42 and 43). It was also identified the pore pressure response decreased with increasing shaking frequency (Shake 42).

Maximum buildup of hydrostatic pore pressure reached 4.4 kPa for Shake 43 at GL -3.0 m, but it was still low compared with the effective overburden pressure that was estimated to be about 30 kPa at that depth. However, we observed some surface evidences indicating liquefaction such as degassing from the ground, squeezing of significant amount of groundwater onto the surface, and sand boils as exemplified in Figure 6. The response of groundwater pressure to the surface vibration shown in Figure 5 indicates that the groundwater reacts in an open system in the field when the ground is shaken locally. Some constraints for lateral migration of groundwater are therefore requested to assess the behavior of groundwater in natural earthquake conditions. It is well known that the areas close to retaining structures and quay walls are susceptible to liquefaction. Actually, the Port Island, a huge reclaimed land entirely enclosed with seawall, suffered severe liquefaction damage by 1995 Hyogoken Nanbu Earthquake [11]. It was characteristic that liquefaction intensively occurred near the seawall surrounding the Port Island. This fact suggests such artificial structures play an important role as a barrier against the groundwater flow or dissipation of excess pore pressure in liquefaction. Sawara TRL97CA test The second liquefaction measurement test was conducted in 1997 on the river terrace at left bank side of Tone River, near Sawara City, about 30 km southeast of Tsukuba. Prior to the liquefaction measurement, we pushed into six seismic cones at three depths, next a number of steel sheet piles

Table 2. Vibration levels of each shaking test recorded at three depths. Bolds are the largest amplitudes among three components corresponding to the peak ground acceleration (PGA). Unit in gal. Shake No. Depth/Direction

41

42

43

46

-3.0m/E-W /N-S /U-D -4.5m/E-W /N-S /U-D -6.0m/E-W /N-S /U-D

12 64 56 56 41 38 12 26 31

11 29 15 6 24 7 10 22 10

33 145 31 11 69 23 7 48 20

30 26 77 15 19 49 12 14 54

Figure 6. Squeezing of groundwater and sands onto the surface caused by vibrator shaking.

Figure 7. Layout of liquefaction measurement test at Sawara TRL97CA Site. Three sides of seismic cone array were walled with steel sheet piles to constrain groundwater flow.


S7-P04 pore water pressure is estimated to be 6.5 kPa. This value was roughly equal to the effective overburden pressure. Hydrodynamic responses ranged from 0.5 kPa peak to peak for cone 6, to 2.7 kPa for cone 1, the deepest one. The peak ground accelerations ranged from 230 gal at the farthest cone 2, to 1150 gal at the nearest cone 4. Concurrent pressure drop at the starting stage identified in the TRR96L2 test was not measured in this test, which indicates this decline phenomenon was due to the characteristic response of groundwater in the open system. CONCLUSIONS An in-situ measurement method is proposed to evaluate the

Figure 8. Hydrodynamic pore pressure response of Cone 6 in relation to the peak ground acceleration for a liquefaction test at Sawara TRL97CA Site. were hammered down to 6 m in depth around the cone array to form a barrier to the near surface groundwater. After that we operated the S-wave vibrator at the open side as shown in Figure 7. In contrast to the test results at TRR96L2 Site, buildups in hydrostatic pore pressure associated with vibration were not clear, whereas the concurrent hydrodynamic responses were obvious at the site. This was likely to be caused by the constraint of pore pressure dissipation by means of the sheet pile barrier. Figure 8 shows positive correlation between the amplitude of hydrodynamic pore pressure and the peak ground acceleration that was controlled by changing vibration point as shown in Figure 7. It is to be noted that the data for initial shaking, marked by pale color, deviated from the main trend. Either way, maximum value of hydrodynamic pore pressure amplitude was only 2.0 kPa under the 10 Hz sinusoidal vibration of 200 gal. Tsukuba PWRI98TP test The last liquefaction measurement test at the first research phase was conducted in 1998 in the premises of Public Works Research Institute (PWRI) using an artificial ground model. First, we dug a 3.3×3.3×3 m test pit within an experimental facility in PWRI. The test pit was lined with PVC sheets to be sealed from the surrounding ground, and backfilled with loose sand. After manual compaction, seismic cones were inserted to 1.0 m or 1.25 m in depth to form a cross array with 1.0 m spacing each other as shown in Figure 9. Water level in the model ground at initial state was set to –25 cm to the surface by recharging water into filling pipes. Before the vibration, we measured the initial value of S-wave velocity by plank hammering at 3.0 m east to the center cone 6. We then operated the vibrator at 3.2 m north and 1.5 m east to the center of the array and observed the responses of water pressure. Loading pattern was as same as those of previous field tests. Figure 10 shows the time histories of the excess pore pressure buildups. Especially at cone 4, the nearest one to the vibrator, peak values of the water pressure reached to a plateau and saturated, which indicates the excess pore pressure built up equal to the effective overburden pressure. Actually, squeezing of the water was observed at 7th cycle (about 150 seconds) onto the surface near the filling pipe at the corner close to the vibrator. Liquefaction within the reconstituted sands was inferred to begin at 120 seconds in elapsed time, when a surge in pore pressure curve took place due to a sudden settlement of the cone. At the end of shaking, water level rose to +01 cm and +07 cm in the filling pipes and it overflowed onto the test pit surface. For cone 4, maximum value of the excess

Figure 9. Layout of liquefaction measurement test at Tsukuba PWRI98TP Site. A 3×3×3 m test pit was dug and filled with reconstituted sands. After manual compaction, seismic cones were pushed to 1.0 m deep. Vibration point was set as close as it can to the test pit. (revised from Inazaki [10])

Figure 10. Time histories of pore water pressure responses observed at PWRI98TP Site. The ground was oscillated for about 6 minutes intermittently with 10 Hz sinusoidal waves lasting about 20 seconds. Liquefaction occurred about at 120 sec in elapsed time after starting vibration. Surges marked with arrows were due to sudden settlement of cones. The excess pore pressure buildup reached a plateau at 200 sec for the nearest cone (C4).


S7-P04 liquefaction potential of near-surface soils under the strong motion directly in the field. Because the conventional laboratory testing for liquefaction evaluation cannot take account of spatial inhomogeneity of the ground, which is inherent and essential feature subjecting liquefaction, an in-situ method has long been requested to measure an actual ground response. The proposed method utilizes an S-wave vibrator as a dynamic loading source and electrical and seismic cones to monitor the response of pore water pressure at specific horizons and distances under the vibration. Liquefaction measurement tests were performed at three fields from 1996 through 1998. Generation of the excess pore water pressure during vibration was successfully measured accompanied with squeezing of groundwater onto the surface. The major results of the field measurements are as follows; first, pore water pressure response varied at depths and distances, especially it was large at a sand bed bounded by impermeable clay or silt layer. This indicates the layered structure of near surfaces strongly controls liquefaction phenomena. Second, decline concurrent with shaking followed by rebound in pore pressure was characteristic during vibration, which means the groundwater reacts in an open system in the field when the ground is shaken locally. Some constraints for lateral migration of groundwater are required to simulate the behavior of groundwater in natural earthquake conditions. One of such constraints is to make a barrier at a target site, analogous to retaining structures, because it is well known that liquefaction has been prone to occur near underground walls. Another constraint is to make use of two or more vibrators with synchronized operation. Walling with steel sheet pile was adopted to make a barrier in the field. It worked successfully to suppress dissipation of pore pressure, however, maximum buildup was still low to the effective overburden pressure. Third, despite of deficit of excess pore pressure, we observed some surface evidences indicating liquefaction such as degassing from the ground, squeezing of significant amount of groundwater onto the surface, and sand boils. This indicates partial liquefaction occurred in the ground. These results demonstrate the availability of S-wave vibrator for in-situ measurement of liquefaction potential.

[7] Charlie, W.A., Jacobs, P.J., Doehring, D.O., Blast induced liquefaction of an alluvial sand deposit, Geotechnical Testing Journal, vol.15, no.1, 14-23, 1992. [8] Inazaki, T., In-situ testing of the liquefaction potential of soft ground using an S-wave vibrator, Proc. 94th SEGJ Conference, 7982, 1996, in Japanese. [9] Stokoe,II, K. H., Rathje, E. M., Chang, W. J., and Cox, B. R., Development of an in-situ test for direct evaluation of the liquefaction resistance of soils, Proc. U.S.-Taimwan Workshop on Soil Liquefaction, 22p, 2003. [10]Inazaki, T., Field measurement of nonlinear property of the soft ground using a shear wave vibrator, Proc. 2nd Intl. Symp. on the Effect of Surface Geology on Seismic Motions, 809-814, 1998. [11] Hamada, M., Wakamatsu, K., and Ando, T., LiquefactionInduced Ground Deformation and Its Caused Damage, Proceedings of Sixth Japan-US Workshop on Earthquake Resistant Design of Lifeline Facilities and Countermeasures Against Soil Liquefaction, Technical Report NCEER- NCEER-96-0012, National Center for Earthquake Engineering Research, Buffalo, 137-152, 1996.

ACKNOWLEDGMENT The author wishes to thank T. Tanaka of Sofih Corporation, Tokyo, and A. Pettigrew of formerly Mertz Inc., Ponca City for their help of operating the S-wave Vibrator during field tests. Additional thanks are expressed to T. Kurahashi and T. Ohtani of PWRI for their supports. Research works using a vibrator was started in 1996 at PWRI until 1998, and regretfully hang up for years. Recently, it has been revived associated with institutional reform of PWRI.

REFERENCES [1] Tokimatsu, K, and Yoshimi, Y., Empirical correlation of soil liquefaction based on SPT N-value and fines content, Soils and Foundations, vol. 23, no. 4, 56-74, 1983. [2] Seed, H.B., and De Alba, P., Use of SPT and CPT tests for evaluating the liquefaction resistance of sands, Proc. of In-Situ '86, ASCE Geotechnical Special Publication, no. 6, 281-302, 1986. [3] Matsuo, O. and Ninomiya, Y, Soil liquefaction and ground flow, Jour. Res. PWRI, vol. 33, 107-133, 1997. [4] Tohno, I, and Shamoto, Y., Liquefaction damage to the ground during the 1983 Nihonkai-Chubu (Japan Sea) Earthquake in Aomori Prefecture, Tohoku, Japan, Jour. Natural Disaster Science, vol.8, no.1, 85-116, 1986. [5] Sangawa, A., Paleoliquefaction features at archaeological sites in Japan, Jour. Geography, vo.l.108, no.2, 391-398, 1999, in Japanese. [6] Sasaki, Y., and Koga, Y., Vibratory cone penetrometer to assess the liquefaction potential of the ground, Proceedings, 14th U.S.Japan Panel on Wind and Seismic Effect, NBS Special Pub. 651, 541-555, 1982.

Tomio INAZAKI Public Works Research Institute, Minami-hara 1-6, Tsukuba, Ibaraki, 305-8516 JAPAN, E-mail: [email protected]
