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International Conferences on Recent Advances 2001 - Fourth International Conference on in Geotechnical Engineering and Recent Advances in Geotechnical Earthquake Dynamics Engineering and Soil Dynamics

29 Mar 2001, 7:30 pm - 9:30 pm

Post-Liquefaction Pore Pressure Dissipation and Densification in Silty

S. Thevanayagam SUNY Buffalo, NY

G. R. Martin University of Southern California, Los Angeles, CA

T, Shenthan SUNY Buffalo, NY

J. Liang SUNY Buffalo, NY

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Recommended Citation Thevanayagam, S.; Martin, G. R.; Shenthan, T,; and Liang, J., "Post-Liquefaction Pore Pressure Dissipation and Densification in Silty Soils" (2001). International Conferences on Recent Advances in Geotechnical and Soil Dynamics. 23. https://scholarsmine.mst.edu/icrageesd/04icrageesd/session04/23

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. POST-LIQUEFACTION PORE PRESSURE DISSIPATION AND DENSIFICATION IN SILTY SOILS

S.Thevanayagam G.R.Martin Assoc. Prof., Dept. of Civil, Strut. and Env. Eng. Professor, Dept. of Civil and Env. Eng. 2 I2 Ketter Hall, SUNY Buffalo USC, Los Angeles, CA 90089-253 1 Buffalo, NY 14260 Email: [email protected] T.Shenthan, and J.Liang Tel: 7 16 645 2 I I4 Ext:2430 Grad. Students Fax: 716 645 3733 2 I2 Ketter Hall, SUNY Buffalo Buffalo, NY 14260

ABSTRACT

Pore pressure generation, and post-liquefaction dissipation and densification characteristics are data essential for detailed analysis of performance of sites containing liquefiable during and after . These characteristics are also necessary for the design, analysis and choice of appropriate ground modification systems to mitigate liquefaction-induced hazards. Past research has addressed such material characteristics for clean sands. However, there are many sites that comprise non-plastic silts or silty sands have experienced liquefaction-induced damage. This paper presents results from an experimental study on silts and silty sands. Pore pressure generation characteristics are evaluated and compared with that of sands. Pre- and post-liquefaction compressibility and coefficient of consolidation, and densification characteristics are determined from undrained cyclic tests data followed by dissipation. Implications of these findings on the earthquake performance of sites containing non-plastic silts and silty sands are discussed. Their impacts on the choice of ground improvement techniques are also discussed.

INTRODUCTION settlement hazards is mainly based on the extensive research work that has been conducted using clean sands (e.g. Seed et has been matter of great interest in al. 1976, Seed and Booker 1977). However, recent earthquake geotechnical engineering for more than three decades. Apart case histories indicate that sites containing a significant from its dramatic disasters such as , boils, percentage of finer grains, mostly non-plastic, also liquefy due cracks, failures, etc., excessive settlements can be to seismic loading. Only a limited amount of research hazardous too. Pore pressure builds up in saturated soils information is available for silty soils. research work due to cyclic shearing. At the same time, dissipation of this indicates that these soils behave differently from clean sands, shear induced pressure takes place at a rate depending on the and that the knowledge gained from past three decades of and volume compressibility of the soil research on clean sands does not directly translate to silty and available drainage paths. When the loading is such that the soils. Further modifications to the traditional ground rate of pore pressure generation is much faster than that of densification/drainage techniques are needed to mitigate dissipation, a non-plastic soil will temporarily loose a large liquefaction effects in silty soils. Installation of supplementary portion of its strength, which may lead to liquefaction, a stage, wick drains have been observed to help relieve pore pressures when soil looses almost all the strength and behaves like a developed during dynamic compaction and stone column liquid. Pore pressure dissipation will usually be accompanied installation in silty soils (Dise et al. 1994, and Luehring et al. by a reduction in volume of voids, hence settlement of ground 2000) and help densification. Therefore, not surprisingly, surface. This kind of settlements are lower for denser soils evaluation of pre- and post-liquefaction characteristics of silty than that for looser ones. This knowledge is the basic for soils, which are important in selecting the appropriate ground analysis of earthquake performance of sites and design of soil improvement technique and in designing such systems with improvement techniques such as dynamic densification, vibro- appropriate modifications, has recently attracted attention of stone columns, deep blasting, etc. researchers (Andrews 1998, Baez and Martin 1995).

Current design guidelines for practice of the above soil This paper presents results from an experimental study on improvement techniques to mitigate liquefaction and non-plastic sand-silt mixes having silt contents from 0% to

Paper No. 4.28 I 100% by weight, and a natural silt. Undrained cyclic triaxial This was done to make room for cyclic movement of the axial tests followed by dissipation were carried out in order to loading piston into and out of the triaxial cell during the cyclic determine pore pressure generation, pre- and post-liquefaction loading phase without adversely affecting the cell pressure. compressibility, coefficient of consolidation, and densification characteristics. Observations from this study are summarized. Cvclic Loading: Undrained cyclic loading was applied using a triaxial apparatus (GEOCOMP Inc., MA). The tests were EXPERIMENTAL PROGRAM conducted at a constant cyclic stress ratio (CSR=Ao,/2o,‘) of 0.2 at a frequency of 0.2 Hz. For safety purposes, the Laboratory tests were carried out using artificial soil mixes of maximum axial strain allowable was set at 8%. The axial a sand (Foundry sand g.55) and a non-plastic silt (Sil-co- displacement, cell pressure, and sample sil#40), which are commercially available from USSilica were monitored using a built-in data acquisition system. Once Company. The soils were mixed thoroughly until there was no the specimen liquefied, cyclic loading phase was terminated. obvious color difference. Table 1 summarizes the index properties of different . A limited number of tests Pore Pressure Dissipation. Post-liquefaction pore pressure were also conducted on a natural silt. dissipation tests were initiated immediately following the cyclic loading. The bottom end of the specimen was connected Specimen Preparation: Cylindrical specimens having typical to a pressure controlled volume measuring burette. The top dimensions of 155 mm in height and 75 mm in diameter were end of the specimen was connected to a pore pressure prepared using Moist Tamping Method. Each specimen was transducer with no drainage allowed from this end. This setup prepared at a different target . A known weight of simulated a one-way drainage condition. The dissipation tests dry solids required to reach the target void ratio was weighed were done in three stages. In the first stage the pressure in the and mixed thoroughly with water at a of about burette was set at a value such that the post-consolidation 5%. The soil was divided into four equal portions. Each in the specimen was 25kPa. In the second and portion was poured into a mold mounted on a triaxial cell, and third stages, the burette pressure was set at values such that the tamped gently using a wooden rod until the height effective stresses were, 50 and 100 kPa, respectively. corresponding to the target void ratio was achieved. The specimen was then percolated with CO2 and saturated with The pore pressure at the top of the specimen and outflow deaired water using back pressure saturation. The back volume of water from the bottom of the specimen versus pressure was increased gradually while maintaining the elapsed time were recorded in each stage. The duration of each effective confining pressure at I5 to 20 kPa. This process was stage varied from 16 set to more than 3 hours, depending on continued until the B (=Au/Ao,) factor exceeded 0.95. the silt content of the specimen. Following saturation, the specimens were consolidated to an effective isotropic consolidation stress of 100 kPa. ANALYSIS

Table I. Index Properties Pore Pressure Generation: Figure 1 shows pore pressure ratio, (r, = shear induced pore pressure due to cyclic loading/ cr,‘) versus normalized number of cycles to reach liquefaction for a few specimens, at different silt contents. The specimen notations are as follows: 0~25-408 = Ottawa sand/silt mix at 25% silt content and e=0.408. Also shown in this figure is the best-fit curve for sands proposed by Seed et al. (1976). The data for sand and silty sand up to about 25% silt content follow the pattern found by Seed at al. The pattern for silt and sandy silt at high silt contents appears to deviate from the above trend, and it needs further study.

Compressibilitv: Pre-liquefaction virgin consolidation lines e Ill,,, = minimum void ratios (ASTM Dl557), emax = maximum void and post-liquefaction consolidation lines (e vs. 03’) were ratios (ASTM D4254 method C). drawn from volume change data obtained during the tests. Figures 2a-b show example plots for sand and silt, In each stage the amount of water flowing into or out of respectively. The post-liquefaction consolidation line is nearly the specimen was recorded. Final void ratio at the end of parallel to the virgin consolidation line. This indicates that consolidation of the specimen was calculated using the during liquefaction the soil skeleton is completely remolded dry weight of the solids, specific gravity of solids, and net and behaves like a freshly deposited soil. The post- volume of water introduced into the specimen. liquefaction compression line follows a new virgin consolidation line differing from a typical recompression line. Following consolidation phase, a small amount of water was removed from the triaxial cell while the cell pressure was maintained the same as the value at the end of consolidation.

Paper No. 4.28 2

. 0.8 0.8 a, 0.6 A 0~15.622 0.8 2 0.4

-. . 1 10 100 1000

o,'(kPa) 1+Seed et al. 0 0.2 0.4 0.6 0.8 1

1.0 Normalized Cycles, N/N, (b) n --a

0.8

0.6 1 IO 100 1000

L' 03, 0.4

Fig. 2. Pre and post-liquefaction consolidation

0 0.2 0.4 0.6 0.8 1 lE-02 Normalized Cycles, N/N,

lE-03 2 Fig. I. Pore pressure gerreration: (a) sands (silt<25%) and (b) + z silts (silt>25%) i lE-04 Figures 3a-b show pre- and post-liquefaction volume compressibility data (m,) for sand, silty sand, and sandy silt lE-05 specimens. Compressibility values of silt and silty sand are of 1 10 100 the same order of magnitude as that of sands at the same effective stress. Also pre- and post-liquefaction o,'(kPa) I compressibility values do not differ significantly from one another. lE-02

Coefficient of Consolidation: Pre-liquefaction coefficient of consolidation (c,) values were calculated based on hydraulic 1 E-03 4 conductivity and volume compressibility data for virgin c loading. Post-liquefaction c, values were back calculated z using pore pressure and volume change measurements i lE-04 obtained during post-liquefaction dissipation tests. Back calculations were done by fitting the measured pore pressure Past-Liquelaction vs. time data at the closed-end of the specimen to the lE-05 I theoretical solution for pore pressure at that end based on 1 10 100 Terzaghi’s one dimensional consolidation (Coduto 1999), o,'(kPa) given by: 1 Fig. 3 Pre and post-liquefaction volume compressibility

Paper No. 4.28 None of the specimens was subjected to dissipation tests before liquefaction occurred. Hence no direct data exist to determine the applicable c,, value during generation of pore pressure up to liquefaction (viz. before remolding of soil where Hdr = length of longest drainage path, T, = time factor, structure). It is thought that the c, values for such u = excess pore pressure, &, = nearest distance to the drainage generation/dissipation stages would be similar to the value end, and Ao = change in total stress. during unloading/reloading stage of the soil along the recompression line. However, this needs to be verified. Similarly, volume change data was also used to back calculate c, using Terzaghi’s one dimensional consolidation theory. Further, for the same soil, c, is lower by more than one order of magnitude at an effective confining stress of IO kPa Figures 4a-b show typical plots for each case, respectively, compared to its value at 100 kPa (FigsSa-b). This indicates for a 100% silt specimen (0~100-838) at void ratio of 0.838. the need for use of confining stress dependent c, values for The back calculated values in each case are in close agreement post-liquefaction dissipation analyses. except for minor differences. Coupled analysis of pore pressure generation and dissipation r requires use of appropriate values adjusted to the status of the 1 soil.

OSlOO-636 The c, values are smaller for silty soils by more than one order ~-cv=O.2 cm’/sec- of magnitude compared to sand. It is predominantly affected by permeability of the soil which is affected by silt content. $r. OX Cod. = 50 kPa 3 0.4 Hydraulic conductivity of the soil specimens ranged from 0.6 to 1.3x10-’ cm/s for the sand, 9x10-’ cm/s for 15% silt, 0.6 to 0.2 I .2x10m5cm/s for 25% silt, and 3 to 5x I Om6cm/s for 60% and 100% silt soils. The c, values are affected in the same order. i o-cU I 0 1000 2000 3000 I Time (set) 100 J

- 0.6 $ Conf. = 50 kPa m ; 0.4 1 10 100 u,’ (kPa) 0.2

0 0 1000 2000 3000 Time (set)

Fig. 4. Back calculation of c,. using (a) pressure measurements, md (b) volume measurements (U = degree of consolidation)

Figures 5a-b show the c, values as a function of effective confining stress. For the same specimen, at the same effective 10 confining stress, the c, values for pre-liquefaction Q’ (kPa) consolidation along virgin loading and post-liquefaction c, values are nearly the same for the same soil. The reason for this is the same as that identified for the prior observation for Fig. 5. Coefficient of consolidation: (a) pre-liquefaction and m,. The soil is completely remolded following liquefaction (b) post-liquefaction and it behaves as a freshly deposited soil. Its consolidation line parallels that of the pre-liquefaction virgin loading.

Paper No. 4.28 4 The c, values for the natural silt in FigsSa-b are somewhat Figures 6a-c show the post-liquefaction densification data for higher than those for artificial silt (Sil-co-silW0). But the m, the soils tested. There is no single relationship for volumetric values (Fig.3) are nearly the same. The difference in c, is due compression against void ratio for all soils (Fig.6a). When the to the difference in and permeability for these two data are split into two parts, one for sands and silty sands up to silts. Grain size (d& for the natural silt is about 38pm versus 25% silt content, and the other for sandy silts, and plotted 1Oym for the artificial silt. Hydraulic conductivity values are against the equivalent intergranular and interfine void ratios of the order of 2x 10m4cm/s for the natural silt versus 3 to 5x10. (e,),, (= [e+( I-b)fc]/[ I-( I-b)fc], e = global void ratio, fc = ’ cm/s for the artificial silt. The difference in hydraulic FC/lOO, FC = (silt) fine content in percentage, and b = a conductivity is reflected in the c, values for these two silts. coefficient)and (ef& [ = e/[fc+( I-fc)/Rd”, m = a coefficient, Rd = D5,Jd50.DsO = 50% passing diameter of sand portion, and dt;o Posr-Li~uefclction Densification: Quantification of post- = 50% passing diameter of fines portion] (Thevanayagam, liquefaction densification is an important aspect in 2000), respectively, the data for each group fall in a narrow performance evaluation of liquefiable soil sites. At present, band (Figs. 6b and c). there is only limited data available on this subject (Lee and Albaisa, 1974, Pyke et al., 1975, Silver and Seed, 197la,b, It is also interesting to note that the above respective Tokimatsu and Seed, 1987). The data are primarily limited to equivalent intergranular void ratios have also been found to clean sands. The data from the current study sheds further correlate with the number of cycles to cause liquefaction light on this subject. (Thevanayagam et al. 2000b) and the energy required to cause liquefaction (Thevanayagam et al. 2000a) for sands and silty sands, and sandy silts and silts, respectively. 10, I CONCLUSION

Laboratory undrained cyclic tests followed by dissipation were conducted to study the pore pressure generation, and post- liquefaction dissipation and densification behavior of silty sands and sandy silts. The limited data show the following.

1 (a) Pore pressure generation characteristics (r, vs N/N,) for 0 0.5 1 e sand and silty sand up to 25% silt content follow the same trend found for sand by Seed et al. (1976). The generation rate for silt and sandy silt (silt > 25%) is somewhat faster than that of sand. This needs further study.

(b) Soil skeleton is completely remolded during liquefaction l OS00 and as a result post-liquefaction compression line (e vs. n OSl5 Go’) almost parallels the pre-liquefaction compression line. ros25 It behaves like a freshly deposited soil.

(c) For the same soil, post-liquefaction volume compressibility (m,) and coefficient of consolidation (c,) 0.4 0.6 0.8 1 values are similar to those of the normally consolidated @,I,, virgin soil at the same stress level.

(d) Coefficient of consolidation for silty soils is lower by more than one or two orders of magnitude and is primarily affected by the silt content (viz. permeability of the soil). For the same soil, coefficient of consolidation values of soils at low confining stresses (10 kPa) immediately following liquefaction are about an order of magnitude less than that at 100 kPa confining stress. It is significantly stress dependent. oJ 0.4 0.6 0.8 1 (e) At the same void ratio, a silty sand (silt content<25%) (efh densifies more than clean sand following liquefaction. Post-liquefaction volumetric densification of sand and Fig. 6. Post-liquefclction volume change data silty sand up to 25% silt content correlates well with equivalent intergranular void ratio, (e,),,. Post-

Paper No. 4.28 liquefaction volumetric compression of silty soils correlates well with equivalent interline void ratio, (et),. Silver, M.L., and H.B. Seed. (1971a). “Deformation characteristics of sands under cyclic loading.” Journal of the Ground improvement schemes for liquefaction mitigation in and Foundations Div., ASCE, Vol.97, No. silty soils based on densification and drainage methods need to SM8, Proc. Paper 8334. ~~1081-98. take into consideration of the differences in the above behavior characteristics of silts compared to sands. Primarily, Silver, M.L., and H.B. Seed. (1971b). “Volume changes in permeability (and silt content) affects the dissipation sands during cyclic loading.” Journal of the Soil Mech. and characteristics of silty soils compared to sands. This requires Found. Div., ASCE, Vol.97, No. SM9, Proc. Paper 8354. much closer spacing of dynamic compaction grids or stone pp I 17 l-82. columns or provision of additional means such as supplementary wick drains to expedite dissipation of pore Singh, S. (1994). “Liquefaction characteristics of silts.” pressures developed during ground improvement operation in Ground failures under seismic conditions. Proc., ASCE silty soils. Convention, GSP. 44, S.Prakash and P.Dakoulas, eds., ASCE, Reston, VA. pp. 105 116. ACKNOWLEDGEMENTS Thevanayagam, S. (2000). “Liquefaction potential and Financial support for this research was provided by MCEER undrained fragility of silty soils.” Proc. 121hWorld Conf. Highway project, sponsored by the FHWA. Earthq. Eng., New Zealand, Feb.2000.

REFERENCES Thevanayagam, S., J. Liang, and T.Shenthan. (2000a). “Contact index and liquefaction potential of silty and Andrews, D.C.A. (1998). “Liquefaction of silty soils: gravely soils.” Proc. 14’h ASCE Eng. Mech. Conf., Austin, susceptibility, deformation, and remediation.” PhD Texas, May 2000. Dissertation, Dept. of Civil Eng., USC, CA. Thevanayagam, S., M. Fiorillo, and J. Liang (2000b). Baez, J.I., and G. Martin. (199.5). “Permeability and shear “Effect of non-plastic fines on undrained cyclic strength wave velocity of vibro-replacement stone columns.” Soil of silty sands.” Proc. ASCE Conference, Denver, CO, improvement for earthquake hazard mitigation, GSP No.49, August 2000 (in press). Proc ASCE Convention, San Diego, CA. ~~66-81. Tokimatsu, K., and H.B. Seed. (1987). “Evaluation of Coduto, D.P. ( 1999). Geotechrzicnl .&gineering, Prentice Hall, settlements in sands due to earthquake shaking.” J. Geotech. Inc.. Upper Saddle River, NJ. Eng. Div., ASCE, Il3(8). pp.861-78.

Dise, K., M.G. Stevens, and J.L. Von Thun. (1994). “Dynamic compaction to remediate liquefiable foundation soils.” GSP No.45, ASCE National Convention, Oct. 1994.

Lee, K.L., and A. Albaisa. (1974). “Earthquake induced settlements in saturated sands.” J. Geotech. Eng. Div., ASCE, Vol.lOO(4). pp.387-406.

Luehring, R., B. Dewey, L. Mejia, M. Stevens, and J. Baez. (2000). “Liquefaction mitigation of a silty dam foundation using vibro-stone columns and drainage wicks - a test section case history at salmon dam.” No.OO-0748, Unpublished report.

Pyke, R., H.B. Seed and C.K. Chan. (1975). “Settlement of sands under multi directional shaking.” J. Geotech. Eng. Div., Vol. IO I(4). pp.379-397.

Seed, H.B, P.P. Martin and J. Lysmer. (1976). “ Pore water pressure change during soil liquefaction.” J. Geotech. Eng. Div., ASCE, Vol. 102(4). pp.323-346.

Seed, H.B., and J.R. Booker. (1977). “Stabilization of potentially liquefiable sand deposits using drains.” J. Geotech. Eng. Div., ASCE, Vol. 103(7). pp.757-68.

Paper No. 4.28