PENETRATION AND LIQUEFACTION RESISTANCES: PRIOR SEISMIC HISTORY EFFECTS

By Raymond B. Seed,1 Associate Member, ASCE, Seung Rae Lee,2 and Hsing-Lian Jong3

INTRODUCTION

In situ penetration tests represent an important basis for evaluation of soil liquefaction potential. Both standard penetration tests (SPT) and cone penetration tests (CPT) are widely used to evaluate the liquefaction resistance of saturated soil deposits. Apart from factors involving testing equipment and procedures, and after correction (or normalization) of SPT or CPT results for overburden effects, it has been suggested (Seed 1979) that five main factors influence both the penetration and liquefaction resistances of saturated soil deposits. The first four of these factors (relative density, soil fabric, aging effects, and overconsolidation) have been shown to influence both penetration and liquefaction resistance in the same manner.

It has also been demonstrated that the fifth factor, prior cyclic strains (seismic history), can significantly increase the resistance of soils to liquefaction (e.g., Finn et al. 1970; Ishihara and Okada 1978; Seed et al. 1977; Singh et al. 1980). Low levels of prior cyclic strain history, such as might result from low levels of seismic excitation producing low levels of pore pressure generation, can significantly increase soil resistance to pore pressure generation during subsequent cyclic loading. Large strains, however, associated with large pore pressure generation and conditions of near or full liquefaction negate this effect and lower the resistance of the soil to pore pressure generation during subsequent cyclic loading. If penetration resistance is to be used as an index for establishing the liquefaction resistance of soils, it is thus important to determine the degree to which prior cyclic strains also affect penetration resistance.

This investigation consisted of two stages. In the first stage, undrained cyclic triaxial tests were performed on reconstituted samples of a uni­formly graded fine sand in order to establish the influence of prior cyclic strains on this test sand. The second stage of the investigation involved the performance of cone penetration tests in large-scale triaxial samples of the same sand in order to evaluate the influence of prior cyclic strains on cone penetration resistance.

TRIAXIAL TESTING PROGRAM

Undrained cyclic triaxial tests were performed on 7.1-cm- (2.8-in.-) diameter reconstituted samples of Sacramento River sand in order to

'Asst. Prof, of Civ. Engrg., Univ. of California, Berkeley, CA 94720. 2Grad. Res. Asst., Dept. of Civ. Engrg., Stanford Univ., Stanford, CA 94305. 3Grad. Res. Asst., Dept. of Civ. Engrg., Stanford Univ., Stanford, CA 94305. Note. Discussion open until November 1, 1988. To extend the closing date one

month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on August 3, 1987. This paper is part of the Journal of Geotechnical Engineering, Vol. 114, No. 6, June, 1988. ©ASCE, ISSN 0733-9410/88/0006-0691/S1.00 + $.15 per page. Paper No. 22516.

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100

EC 80

Z

O

UJ

40

20

G M H V t L SAND

Coarse to F j medium r m e

* < 6 z ; i

3 8 % 1 • v

-No.

40

-No.

100

-No.2

00—

\

\

1

\ ^ v V

! U.S. standard ) sieve sizes

G u n i

o 6

o o d

GRAIN DIAMETER, mm

FIG. 1. Sacramento River Sand Gradation Curve

establish the influence of prior seismic history in this material. Sacramento River sand, is a clean, fine, uniformly graded quartzitic sand, slightly micaceous, with subrounded to subangular particles. Fig. 1 shows the gradation of this material which has a maximum dry density of yd,max = 1,760 kg/cm3 (109.8 pcf), a minimum dry density of -y(/,min = 1,428 kg/cm3

(89.1 pcf), and a specific gravity of 2.74. All samples were formed by moist tamping.

A series of six samples were formed and consolidated isotropically to o-3iC. = 2.0 ksc at a relative density of £>,. = 56%. These samples were then subjected to uniform sinusoidal undrained cyclic triaxial loading until failure, which was defined as the occurrence of double amplitude cyclic axial strains greater than eA = ±5%. The results of this first test series established the cyclic strength curve for samples without prior cyclic-load history as represented by the solid line and solid points in Fig. 2.

A second series of five samples were formed and consolidated by identical procedures and to the same initial isotropic confining stress. These five samples were then subjected to undrained uniform cyclic loading at a cyclic stress ratio of CSR = 0.26 for 20 cycles, resulting in generated pore pressure ratios of approximately 40-55%, as listed in Table 1. Cyclic loading was then stopped, and the samples were allowed to drain and reconsolidate isotropically to aj = 2.0 ksc. These samples, now with prior cyclic-load histories, were then cyclically loaded to failure under

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0.8

-

-

-

—

-

V

a 31 = 2 ksc Dr »56%

^ - PRIOR CYCLIC LOAD HISTORY: CSR = 0.26 for 20 cycles

SAMPLES WITHOUT PRIOR / C YCLIC LOAD H ISTORY —

°--_ - - O —

-

-

-

—

-

1 2 5 10 20 50 100 200

No. of Cycles to ±5% £A

FIG. 2. Cyclic-Strength Curves for Sacramento River Sand with and without Prior Cyclic Load History

TABLE 1. Undrained Cyclic Triaxial Tests of Sacramento River Sand with and without Prior Cyclic Load History

Test number

0) l 2 3 4 5 6 l-A 2-A 3-A 4-A 5-A

Initial

id (pcf)

(2)

99.49 99.67 99.58 99.72 99.67 99.48 99.57 99.57 99.58 99.67 99.54

CSR

(3)

_ — — _ — _

0.26 0.26 0.26 0.26 0.26

Preloading

Number of cycles

(4)

_ — — — — — 20 20 20 20 20

l'u

(%) (5)

— — — — — — 44 53 41 49 54

yj a"er preloading

(pcf)

(6)

99.49 99.67 99.58 99.72 99.67 99.48 99.85 99.68 99.72 99.86 99.70

D,

(%) (7)

55.4 56.2 55.8 56.4 56.2 55.4 57.0 56.2 56.4 57.0 56.3

CSR

(8)

0.524 0.395 0.332 0.317 0.290 0.258 0.463 0.416 0.376 0.343 0.316

cycles to

± 5% e,,

(9)

3 6 8

13 20 29 6 8

13 30 70

Notes: (1) CSR = ffi/(7(2o-^.); and (2) /•„ - pore pressure ratio achieved during cyclic preloading. (;•„ = Ait/trjj).

undrained conditions. This established the cyclic strength curve repre­sented by the dashed line and open circles in Fig. 2. As shown in this figure, samples with prior cyclic load history exhibited a cyclic strength increase of 25-35% (in terms of CSR) relative to the samples with no prior cyclic-load history.

CONE PENETRATION TESTING PROGRAM

Two series of large-scale triaxial samples of Sacramento River sand, 30 cm (12 in.) in diameter and 76 cm (30 in.) high, were prepared by moist tamping at relative densities of approximately 40, 49, 54, and 65%. One

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TABLE 2. Results of Static Cone Penetration Tests in Samples with and without Prior Cyclic Loading

Test number

(1)

I 2 3 4 5 6 7 8

Initial DR

(%) (2)

40.3 49.5 53.5 65.1 40.6 48.9 53.9 64.7

Preloading Applied

CSR (3)

— _ _ —

0.14 0.21 0.33 0.43

Number of cycles

(4)

— — — — 20 22 20 42

>'tt

(%) (5)

— — — — 46 50 50 40

Penetration testing DR

(%) (6)

40.3 49.5 53.5 65.1 41.2 49.3 54.1 65.1

«<• (ksc) (7)

60.8 66.4 74.1 82.9 60.1 68.6 76.1 83.3

f.< (ksc) (8)

0.45 0.50 0.62 0.69 0.52 0.64 0.71 0.76

series of samples were penetrated with a cone penetrometer inserted through their top caps immediately after being formed; the other series of samples were cyclically loaded prior to penetration testing in order to induce in them a seismic history similar to that given to the small-scale triaxial samples described previously. A comparison was then made between the penetration resistances of the samples with and without prior cyclic-strain histories.

The top cap of the triaxial samples and the triaxial cell were modified so that a cone penetrometer could be inserted through the top cap and used to penetrate the sample while maintaining the desired confining stresses on the sample. The cone penetrometer used in thse studies was a conventional electronic friction penetrometer conforming to the specifications of ASTM D3441-79, and was advanced at a rate of 2 cm/s during penetration.

During actual penetration testing, all samples were maintained under undrained isotropic confining stress conditions. It was recognized that this did not ideally simulate most field conditions. It was also recognized that the ratio of sample diameter to cone diameter was less than that recom­mended by Parkin (1971) and Veismanis (1974) and that this would also affect the measured penetration resistance. However, since the purpose of these tests was simply to compare the penetration resistance of otherwise identical samples with and without prior cyclic-strain histories, and not to measure absolute penetration resistances, it was considered that reason­able comparative results would be obtained if all tests were performed under identical conditions except for the absence or presence of prior cyclic-stress history.

A total of eight tests were performed, as listed in Table 2. The first four tests, those penetrated without prior cyclic-strain history, were performed on samples which were prepared by moist tamping, saturated, and then isotropically consolidated to 1-ksc confining pressure at relative densities of 40.3, 49.5, 53.5, and 65.1%. The second series of four tests were performed on samples given cyclic-strain histories prior to penetration. These samples were formed by moist tamping, saturated, and then isotropically consolidated to 1 ksc at roughly the same four relative densities as the first series of samples. After consolidation, however, each sample was subjected to undrained cyclic loading designed to be similar to that used to induce prior cyclic strain histories in the small-scale triaxial samples described previously. Predetermination of the uniform cyclic load

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T 7 ^ / /

/ / ^ y / ® ® NO PRE-LOAD _

/ O CYCLICALLY W PRE-LOADED

.. J 1 I I 30 40 SO 60 70 80 30 40 50 60 70 80

RELATIVE DENSITY (%) RELATIVE DENSITY (%)

FIG. 3. Cone-Tip Penetration Resistance and Friction Sleeve Resistance versus Density for Samples with and without Prior Cyclic-Strain Histories

histories necessary to give these samples "similar" cyclic-strain histories to those of the small-scale triaxial samples, which were tested at different density and confining stress levels, was accomplished in the following manner:

1. The 20 cycles of loading at a cyclic-stress ratio of CSR = 0.26 and an initial confining pressure of 0-3 = 2 ksc was considered equivalent to 20 cycles at CSR = 0.28 for the large samples under 1-ksc confining pressure, based on the correlation between cyclic stress ratio (CSR) and confining stress (0-3) proposed by Seed (1979).

2. The CSR needed to generate similar pore pressures (r„ approximately equal to 50%) in the large-scale samples at relative densities of other than Dr = 55% were estimated based on the approximate relationship between relative density and standard penetration resistance proposed by Marcu-son and Bieganousky (1977) together with the relationship between stan­dard penetration resistance and CSR necessary to cause liquefaction in a given number of cycles proposed by Seed (1979).

3. Steps 1 and 2 were recognized to be very approximate. Accordingly, it was determined that 20 cycles of the "equivalent" CSR estimated by these procedures would be applied, but that loading would be stopped at less than 20 cycles if a pore pressure ratio greater than r„ = 50% developed, and would be continued for more than 20 cycles if this was necessary to develop a pore pressure ratio of at least r„ = 40%.

By this process, it was considered that the cyclic-load histories (pre­loading) applied to the large-scale samples would be similar to those applied in the small-scale triaxial testing program, where 20 cycles of loading at CSR = 0.26 produced pores pressure ratios of approximately ru = 40-55%. The success of this three-step estimation process is reflected in the fact that three of the four samples achieved the desired pore pressure ratio range at approximately 20 cycles of loading (see Table 2). Following this undrained loading to impart prior cyclic-strain histories, the samples were allowed to reconsolidate under an isotropic confining stress of 1 ksc and the cone penetration tests were performed.

The resulting average cone-tip resistances (qc) are shown, as a function of sample density, in Fig. 3a, In this figure, the solid circles represent test

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results for samples having no prior cyclic strain history and the open circles represent similar results for the samples with prior cyclic strain history. The dashed line in this figure represents predicted penetration resistances estimated based on the equation of Durgunoglu and Mitchell (1975) using DeBeer's (1970) shape factor. Agreement between this pre­diction by Durgunoglu and Mitchell's equation, which is based on average data for numerous soils, and the measured cone-tip resistance values is good.

There is some scatter in the measured tip resistances, both with and without prior cyclic-strain histories. Nonetheless, it can be seen in Fig. 3a that the samples with prior strain histories did not exhibit significantly different tip penetration resistances than those without prior strain histo­ries. This is so, even though "similar" prior cyclic-strain histories causing similar pore pressure generation under similar numbers of preloading cycles resulted in cyclic strength curves showing 25-35% more resistance to liquefaction (in terms of CSR) in small-scale cyclic triaxial testing than samples without prior cyclic-strain histories.

Fig. 3b shows the measured cone friction sleeve resistances (fs) in samples with and without prior strain histories. Once again there is some scatter in the measured results. Nevertheless, it may be seen that the samples penetrated with prior cyclic-strain histories exhibit somewhat higher sleeve friction resistance. The increase in fs due to the prior cyclic-strain histories applied is on the order of 15-20%,

SUMMARY AND CONCLUSIONS

Cyclic triaxial tests and cone penetration tests performed in large-scale triaxial samples were used to evaluate the effects of prior cyclic-strain history on the liquefaction and penetration resistances of sandy soils. Prior cyclic-strain history was found to increase both liquefaction and cone-sleeve friction penetration resistance (fs), but had no significant effect on cone-tip penetration resistance (qc). This, in turn, suggests that cone-tip penetration resistance represents a somewhat conservative basis for evaluating the liquefaction resistance of sandy soils with beneficial prior cyclic-strain histories (prior seismic histories), and also has ramifications with regard to the reliability of soil classifications based in part on the ratio qjfs in soils with prior cyclic strain histories.

ACKNOWLEDGMENTS

Financial support for these studies was provided by the U.S. National Bureau of Standards, Grant No. 60NANB5D0531, and this support is gratefully acknowledged. The writers also extend their thanks to Mr. Clarence Chan of the University of California at Berkeley for his help and advice with respect to large-scale triaxial and penetration testing.

APPENDIX. REFERENCES

DeBeer, E. E. (1970). "Experimental determination of the shape factors and the bearing capacity of sand." Geotechniqae, 20(4), 387-411.

Durgunoglu, H. T., and Mitchell, J. M. (1975). "Static penetration resistance of soils: I—analysis." Proc. ASCE Specialty Conf. on In-Situ Measurement of Soil

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Properties, ASCE, vol. 1, 151-168. Finn, W. D. L., Bransby, P. L., and Pickering, D. J. (1970). "Effect of strain

history on liquefaction of sand." J. Geotech. Engrg. Div., ASCE, 96(6), 1917-1933.

Ishihara, K., and Okada, S. (1978). "Effects of stress history on cyclic behavior of sands." Soils and Found., 18(4), 31-45.

Marcuson, W. F., and Bieganousky, W. (1977). "Laboratory standard penetration tests of fine sands." J. Geotech. Engrg. Div., ASCE, 103(6), 565-587.

Parkin, A. K. (1971). "The friction penetrometer—laboratory calibration for the prediction of sand properties." Internal Report 52108-5, Norwegian Geotechnical Institute.

Seed, H. B. (1979). "Soil liquefaction and cyclic mobility evaluation for level ground during earthquakes." J. Geotech. Engrg. Div., ASCE, 107(4), 501-518.

Seed, H. B., Mori, K., and Chan, C. K. (1977). "Influence of seismic history on liquefaction of sands." / . Geotech. Engrg. Div., ASCE, 103(4), 257-270.

Singh, S., Donovan, N. C , and Park, F. (1980). "A re-examination of the effect of prior loadings on the liquefaction resistance of sands." Proc. 7th World Conf. on Earthquake Engrg., Sep. 8-13, Istanbul, Turkey, vol. 3, 321-325.

Veismanis, A. (1974). "Laboratory investigation of electrical friction penetrome­ters in sand." Proc. European Symp. on Penetration Testing, European Sympo­sium on Penetration Testing (ESOPT I), Stockholm, Sweden, vol. 2.2, 407-419.

MODIFIED CALCULATION OF PILE-GROUP SETTLEMENT INTERACTION

By Harry G. Poulos,' Fellow, ASCE

INTRODUCTION

The analysis of pile groups using elastic interaction factors has provided a practical means of estimating the group settlement and load distribution (Poulos and Davis 1980; Banerjee and Davies 1977; Randolph and Wroth 1979). Generally reasonable agreement has been found between the theory and laboratory and field measurements, although there has been a ten­dency for the theory to overpredict the group settlement and to predict a more nonuniform pile load distribution than is actually observed. O'Neill et al. (1977) have pointed out that, when considering interaction between piles, it is appropriate to use the modulus of the soil for the low strain levels which exist between the piles, rather than the soil modulus near each pile, where the strain level is considerably greater. While the latter is appropriate for computing the settlement of a single pile, the former should be used for computing the interaction between piles.

'Prof, of Civ. Engrg., School of Civ. and Mining Engrg., Univ. of Sydney, Sydney, N.S.W., Australia, 2006.

Note. Discussion open until November 1, 1988. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on April 5, 1987. This paper is part of the Journal of Geotechnical Engineering, Vol. 114, No. 6, June, 1988. ©ASCE, ISSN 0733-9410/88/0006-0697/S1.00 + $.15 per page. Paper No. 22516.

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