INFLUENCE OF INITIAL CONFINING STRESS ON THE MECHANICAL RESPONSE OF NATURAL FRASER RIVER DELTA SILT

Transcription

1 Sea to Sky Geotechnique 26 INFLUENCE OF INITIAL CONFINING STRESS ON THE MECHANICAL RESPONSE OF NATURAL FRASER RIVER DELTA SILT Maria V. Sanin, Department of Civil Engineering University British Columbia, Vancouver, BC, Canada Dharma Wijewickreme, Department of Civil Engineering University British Columbia, Vancouver, BC, Canada ABSTRACT The earthquake response of natural Fraser River Delta silt was investigated using constant-volume cyclic direct simple shear (DSS) testing. Laboratory data from tests conducted on normally consolidated specimens was examined to assess the effect of initial confining stress on the monotonic and cyclic shear response of the silt material. Under monotonic shear loading, the DSS specimens deformed initially in a contractive manner followed by dilative response, with phase transformation from contractive to dilative occurring essentially at a unique mobilized shear stress ratio. The normalized effective stress paths from monotonic tests on specimens normally consolidated to different stress levels were almost coincident indicating a response similar to that typically observed for normally consolidated clays. Under cyclic DSS loading, the tested silt exhibited typical cyclic mobility type strain development mechanism. The cyclic shear resistance was relatively insensitive to the confining pressure, again, displaying behavioural patterns similar to those typical for normally consolidated clay. RÉSUMÉ La réponse aux séismes de silt naturel du delta du fleuve Fraser a été étudiée en utilisant des essais de cisaillement simple direct (DSS) cyclique à volume constant. Les essais effectués sur les spécimens normalement consolidés ont été examinées pour évaluer l'effet de l'effort de confinement initial sur la réponse monotonique et cyclique de cisaillement du silt. Sous le chargement monotonique de cisaillement, les spécimens de DSS se sont d abord déformés d'une façon contractante, suivi d'une réponse dilatante, avec la transformation de phase de contraction à dilatation se produisant essentiellement à un rapport mobilisé d'effort de cisaillement unique. Les cheminements efficaces des contraintes normales des essais monotoniques sur des spécimens normalement consolidés à différents niveaux d'effort étaient presque coïncidents indiquant une réponse semblable à ceux typiquement observés pour les argiles normalement consolidés. Sous le chargement cyclique de DSS, le silt examiné a montré un mécanisme de développement de déformation de "mobilité cyclique" typique. La résistance au cisaillement cyclique était relativement peu sensible à la pression de confinement, encore, montrant des modèles de comportements semblables à ceux typiques pour l'argile normalement consolidé. 1. INTRODUCTION Earthquake-induced ground displacements and associated geotechnical hazards are some of the main engineering concerns in the design of structures in regions of moderate to high seismicity with saturated loose/soft soil deposits. Liquefaction of saturated sands has been the topic of extensive research during the past 3 years, while the behaviour of silty sands and silts has been studied only on a very limited scale. It has been found that certain saturated fine-grained soils can be susceptible to earthquake-induced softening and strength reduction as much as relatively clean sands (Boulanger et al. 1998). Fine-grained silty soils with high levels of saturation are commonly found in natural river deposits such as the Fraser River Delta of British Columbia, Canada. Silty soils also originate as a man-made waste product in tailings derived from the processing of ore in the mining industry. There is significant controversy and confusion regarding the currently available approaches to assess the earthquake response of silts including clayey silts (Youd et al., 21; Seed et al., 21). For example, as noted during the session on liquefaction of Eighth NCEE (26), the Chinese Criteria (Wang, 1979; Marcuson et al. 199) that has been in common use to evaluate the liquefaction susceptibility of fine-grained soils is now clearly noted as unacceptable. Moreover, recent evidence of ground failure in fine-grained soils during strong earthquakes has emphasized the need for understanding of the response of silts under cyclic loading in a more fundamental manner (Bray et al. 24). The response of a given soil to cyclic loading is controlled by many parameters such as packing density, microstructure, fabric, level and duration of cyclic loading, confining stress, initial static bias, etc. These parameters have been noted to primarily govern development of excess pore water pressures, stiffness, and strength in a soil mass during the occurrence of an earthquake, in turn, controlling the overall seismic response. With this background, a laboratory research program has been undertaken at the University of British Columbia (UBC) to study the mechanical response (both static and cyclic) of different types of natural silt from the Fraser River Delta of British Columbia, Canada. As a part of this work, a series of laboratory element testing has been conducted on samples obtained from a relatively recent 252

2 Sea to Sky Geotechnique 26 channel-fill silt zone of the Fraser River Delta. This paper presents the results from some of the direct simple shear tests conducted to study the effects of confining stress level on the monotonic and cyclic shear response. 1 Clay Silt Sand Grav el 2. LABORATORY TESTING DETAILS The silt material for this research was obtained from a site located on the north riverbank of the South Arm of the Fraser River, at the southern foot of No. 3 Road of Richmond, B.C., Canada. The choice of this native channel-fill silt as the test material for the current program was judged reasonable because of its presence in large parts of the highly populated areas of Fraser River Delta. The Fraser Delta sediments have a thickness of up to 2 m, and consist of: overbank silts extending up to 6 m in thickness, overlying up to 2 m in thickness of deltaic sands, which are underlain by a thick deposit of fine sand and clayey silts. The samples used for the tests examined herein were retrieved from a depth horizon of 7.9 m to 8.5 m below the ground surface at the test location (Sanin and Wijewickreme 26). The silt material in this depth zone is judged relatively uniform based on the available data from in situ cone penetration testing (CPT testing) conducted at the site, and the gradation of Fraser River silt determined from several tests as shown in Figure 1. Electron micrographs indicated that the silt had very thin (<1 mm) interbedded sand layers (Sanin and Wijewickreme 26). Physical parameters for the soil derived from index testing combined with data available from in situ testing are summarized in Table 1. Table 1. Index parameters of Fraser River silt. Index Property Values Water content, w c (%) 38.8 Liquid limit, LL (%) 36.5 Plastic limit, PL (%) 27.1 Plasticity Index, IP 9.4 Liquidity Index, I w 1.4 % of particles <.2mm 1% % of particles >.75mm 5% Unified soil classification ML Specific gravity, Gs 2.69 An assortment of constant-volume monotonic and cyclic shear tests conducted on the above silt samples was considered for this study. The tests conducted on specimens initially consolidated to different effective vertical confining stress (σ vo) levels, between 1 to 5 kpa, are examined herein. The preconsolidation pressure of the silt was estimated to be about ~1 kpa. Therefore, consolidation of DSS specimens to σ vo levels above 1 kpa essentially assured that all the tests were on specimens that were normally consolidated. Percent Passing by Mass Particle Diameter (mm) Figure 1. Grain size distribution of channel-fill Fraser River Delta silt The tests were conducted using NGI-type (Bjerrum and Landva, 1966) cyclic direct simple shear test (DSS) device at UBC, with no applied static shear stress (i.e. τ=, level-ground) prior to commencement of constantvolume (monotonic or cyclic) shear loading. The constant-volume monotonic shear tests were conducted at a strain rate of approximate 1 per cent per hour. The constant-volume cyclic shear loading was applied at a frequency of.1 Hz. This consisted of a symmetrical sinusoidal pulse at constant cyclic shear stress (τ cyc) amplitude [i.e., constant cyclic stress ratio CSR = (τ cyc/σ v)]. A continuous record of test data was obtained using a computer interfaced data acquisition system. The test variables monitored consisted of full time-histories of horizontal shear stress (τ), decrease in vertical stress (equals induced excess pore water pressure, u) and horizontal shear strain (γ). The DSS device is considered to closely simulate seismic loading conditions. In constant-volume DSS tests, the diameter and height of the soil sample is essentially constrained against changes while the vertical stress (load) on the sample is continuously monitored during the testing process. It has been shown that the decrease (or increase) of vertical stress in a constant-volume DSS test is essentially equal to the increase (or decrease) of pore water pressure in an undrained DSS test (where the constant-volume condition is maintained by not allowing the volume of pore water to change, Finn et al., 1978). 3. TEST RESULTS AND DISCUSSION 3.1 Constant-volume Monotonic DSS Loading Response Typical stress-strain and the stress-path response obtained from the constant-volume monotonic tests conducted on specimens consolidated to σ vo ~1 kpa 253

3 Sea to Sky Geotechnique 26 through ~5 kpa are presented in Figures 2 and 3, respectively σ' vo= 5kPa σ' vo= 4kPa σ' vo= 3kPa σ' vo= 2kPa σ' vo= 1kPa Shear Strain, γ (%) further examined by normalizing the stress paths in relation to σ vo as presented in Figure 4. Clearly, the normalized stress paths for all the tests appears to be almost coincident (or fall within a narrow range) indicating that the response of normally consolidated silt is similar to that typically observed for normally consolidated clays (Atkinson and Bransby, 1978). 3.2 Constant-volume Cyclic DSS Loading Response The stress path (σ v vs. τ) and stress-strain (τ vs. γ) relationships obtained from constant-volume cyclic tests conducted on specimens consolidated to σ vo ~1 kpa and ~4 kpa, under essentially identical CSR loading values of about.18, are presented in Figures 5 and σ'vo= 1kPa Figure 2. Stress strain curves from constant volume direct simple shear tests on Fraser River silt σ'vo= 2kPa σ'vo= 5kPa σ'vo= 4kPa 15 Phase transformation line τ/σ 'vo.2.15 σ'vo= 3kPa σ' v /σ' vo Figure 4. Normalized stress paths from constant volume direct simple shear tests on Fraser River silt Vertical effective stress, σ' v (kpa) Figure 3. Stress paths from constant volume direct simple shear tests on Fraser River silt. As may be noted from Figure 3, the specimens have deformed initially in a contractive manner followed by a dilative response. The points at which the phase transformation (from contractive to dilative) occurs are indicated by a dot symbol in the figure. The phase transformation seems to have occurred at the same mobilized shear stress ratio (τ/σ v). In terms of the stressstrain characteristics (Figure 2), all the samples can be considered to have exhibited a behaviour of no strainsoftening. It is also of interest to note that the stress paths from all tests seem to follow a consistent pattern. This could be Both the specimens initially show predominantly contractive response, and with increasing number of load cycles, the specimens display cumulative increase in excess pore water pressure with associated progressive degradation of shear stiffness. In a given cycle, the shear stiffness experiences its transient minimum when the applied shear stress is close to zero. All the specimens eventually experienced zero, close to zero, transient vertical effective stress conditions during cyclic loading. This cyclic mobility type response is generally similar in form to the undrained (constant-volume) cyclic shear responses observed from cyclic shear tests on natural silts, fine-grained mine tailings, and clays (e.g., Sanin and Wijewickreme 26; Wijewickreme et al. 25a; Zergoun and Vaid 1994) and compact to dense reconstituted sand (e.g., Sriskandakumar 24; Kammerer et al. 22). It is, however, of importance to note that the response of the two specimens is remarkably similar in spite of their significantly different initial consolidation confining stress levels (σ vo). 254

4 Sea to Sky Geotechnique 26 e c =.892 σ' vo = 1kPa CSR = Shear Strain, γ (%) Point of γ=3.75% (assumed triggering point of liquefaction) Vertical Effective Stress, σ'v (kpa) Vaid (1994) for normally consolidated clay in cyclic triaxial tests, and it is in accord with the typical behavioural frameworks noted for normally consolidated clay (e.g., Atkinson and Bransby 1978) e c =.844 σ' vo = 4kPa CSR = Shear Strain, γ (%) Point of γ=3.75% (assumed triggering point of liquefaction) Figure 5. Stress strain curve and stress path from constant volume cyclic direct simple shear test on Fraser River silt, σ vo=1kpa -6-8 Vertical Effective Stress, σ'v (kpa) In order to assess the cyclic resistance of the silt, the applied cyclic stress ratio [CSR = (τ cyc/σ vo)] is plotted against number of cycles to reach single-amplitude γ = 3.75% in Figure 7. The horizontal shear strain value of γ = 3.75% in DSS specimens is equivalent to reaching a 2.5% single-amplitude axial strain in a triaxial specimens, which also is a definition for liquefaction previously suggested by the National Research Council of United States (NRC, 1985). It is noted that cyclic resistance data points for specimens normally consolidated to confining stresses between 85 kpa (insitu stress) and 2 kpa have also been previously reported in Sanin and Wijewickreme (26). In addition, results from two cyclic tests conducted on specimens consolidated to σ vo = 3 kpa and 4 kpa are also superimposed herein. The data points seems to fall on a single trend-line suggesting that cyclic resistance is relatively insensitive to the confining pressure (and the changed initial void ratio due to consolidation) and that the response is influenced only by the mobilized shear stress ratio. Similar behaviour has been observed by Zergoun and Figure 6. Stress strain curve and stress path from constant volume cyclic direct simple shear test on Fraser River silt, σ vo=4kpa It is of interest to review the above observations with respect to the generally accepted response of relatively clean sands. In sands, the cyclic resistance generally increases with increasing density; for a given relative density, the cyclic resistance in sands has been noted to decrease with increasing confining stress (Seed and Harder 199). In addition to the effects of density and confining pressure, the differences in particle fabric alone could lead to significant differences in CRR (Wijewickreme et al. 25b). The observations presented herein suggests that, for the tested Fraser River silt, the dilative tendency arising due to stress densification seems to have overcome the possible contractive tendency due to the increase in confining stress. Park and Byrne (24) and Wijewickreme et al. (25b) have noted similar effects due to stress densification in their tests on sands. 255

5 Sea to Sky Geotechnique 26 Cyclic Resistant Ratio, CRR = τ /σ ' cy vo σ' v o = 3kPa. e c = No. of Cycles te reach γ=3.75% σ' v o = 1kPa. e c(av e) =.931 σ' v o = 85kPa. e c(av e) =.963 σ' v o = 2kPa. e c(av e) =.879 σ' v o = 4kPa. e c =.844 Not liquefied after 2 cycles Figure 7. Cyclic resistance ratio (CRR) versus number of cycles required to reach γ = 3.75% from constant volume cyclic DSS tests on Fraser River silt. 4. CONCLUSION A laboratory research program has been undertaken to assess the response of natural silts of the Fraser River Delta of Greater Vancouver Region of British Columbia, Canada. As a part of this work, data derived from a series of constant-volume direct simple shear (DSS) testing is examined to assess the effect of initial confining stress level on the mechanical response of a relatively recent channel-fill silt material. Under monotonic constant-volume shear loading, the DSS specimens deformed initially in a contractive manner followed by a dilative response. The phase transformation (from contractive to dilative) occurred essentially at the same mobilized shear stress ratio level. When normalized, the effective stress paths for tests conducted on specimens having different initial stress levels (ranging from 1 to 5 kpa) appeared to fall within a narrow range indicating that the response of normally consolidated silt is similar to that typically observed for normally consolidated clays. The tested Fraser River silt exhibited cumulative increase in excess pore water pressure and gradual decrease in shear stiffness with increasing number of load cycles. This cyclic mobility type strain development is generally similar in form to the undrained cyclic shear response typically observed for natural silts, fine-grained mine tailings, clays, and compact to dense reconstituted sand. The cyclic shear resistance, defined with respect to the number of cycles to reach single-amplitude horizontal shear strain of 3.75% in a soil specimen, appeared to be relatively insensitive to the confining pressure (and changed void ratio), again reflecting behavioural patterns similar to those for normally consolidated clay. Since the data points corresponding to higher stress levels are limited, additional tests conducted on specimens consolidated to higher stress levels would be required to arriving at definitive conclusions in this regard. References Atkinson, J.H., and Bransby, P.L The mechanics of soils: An introduction to critical state soil mechanics. McGraw-Hill Book Co., London and New York. Bjerrum, L., and Landva, A Direct simple shear testing on Norwegian quick clay. Geotechnique, 1966; 16 (1): pp. 1-2 Boulanger, R. W., Meyers, M. W., Mejia, L. H., Idriss, I. M Behaviour of a fine-grained soil during the Loma Prieta earthquake. Canadian Geotechnical Journal; 35: pp Bray, JD., Sancio, RD., Durgunoglu, T, Onalp, A, Youd, TL, Stewart, JP, Seed, RB, Cetin OK, Bol, E, Baturay, MB, Christensen, C, and Karadayilar, T. 24. Subsurface Characterization at Ground Failure Sites in Adapazari, Turkey. Journal of Geotechnical and Geoenvironmental Engineering. ASCE 13(7): Eighth NCEE Session on Liquefaction (26), In Proceedings of 8th US National Conference on Earthquake Engineering (NCEE), 26, San Francisco, April Finn WDL, Vaid YP, Bhatia SK Constant volume simple shear testing. Proceedings of the second international conference on Microzonation for safer construction-research and application, San Francisco, U.S.A, Vol. II. p Kammerer A, Wu J, Pestana J, Riemer M, Seed R. 22. Undrained response of Monterey /3 sand under multidirectional cyclic simple shear loading conditions: Electronic Data Files, University of California, Berkeley, Geotechnical Engineering Research Report No. UCB/GT/2-2. Marcuson, W.F., Hynes, M.E. and Franklin, A.G. 199 Evaluation and Use of Residual Strength is Seismic Safety Analysis of Embankments. Earthquake Spectra. EERI; 6 (3). pp NRC Liquefaction of soils during earthquakes,. National Research Council Report CETS-EE-1, National Academic Press, Washington, D.C. Park, S.S., and Byrne, P.M. 24. Stress densification and its evaluation. Canadian Geotechnical Journal, 41(1): Sanin, M.V. and Dharma Wijewickreme, 26, Cyclic Shear Response of Channel-fill Fraser River Delta Silt. Soil Dynamics and Earthquake Engineering In Press Seed, R.B., Cetin, K.O., Moss, R.E.S., Kammerer, A.M., Wu, J., Pestana, J.M., and Riemer, M.F. 21. Recent advances in soil liquefaction engineering and seismic site response evaluation, Proc. 4th Int. Conf. on Recent Advances in Geotechnical Earthquake. Engineering and. Soil Dynamics., Paper No. SPL-2. Seed, R.B., and Harder, L.F SPT based analysis of cyclic pore pressure generation and undrained residual strength. In Proceedings of the Seed Memorial Symposium, Vancouver, B.C., Canada. Edited by J.M. Duncan. BiTech Publishers, pp

6 Sea to Sky Geotechnique 26 Sriskandakumar, S., 24, Cyclic Loading Response of Fraser River Sand for Validation of Numerical Models Simulating Centrifuge Tests, M.A.Sc. Thesis, Department of Civil Engineering, UBC. Wang, W Some findings in soil liquefaction. Earthquake Engineering Department, Water Conservancy and Hydroelectric Power Scientific Research Institute. Beijing: pp Wijewickreme, D. and Sanin, M. V. 24. Cyclic shear loading response of Fraser River Delta silt. In Proceedings of the 13th World Conference on Earthquake Engineering. Vancouver, B.C. August 1-6, 24; paper No Wijewickreme, D., Sanin, M.V., and Greenaway, G.R. 25a. Cyclic Shear Response of Fine-grained Mine Tailings, Canadian Geotechnical Journal. 42(5): pp: Wijewickreme, D., Sriskandakumar, S., Byrne, P.M., 25b, Cyclic Loading Response of Loose Airpluviated Fraser River Sand for Validation of Numerical Models Simulating Centrifuge Tests, Canadian Geotechnical Journal, 42 (2), pp: Youd, T. L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn, W.D.L., Harder Jr., L.F., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson III, W.F., Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K., Seed, R.B., and Stokoe II, K.H. 21. Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils. Journal of Geotechnical and Geoenvironmental Engineering. ASCE, (1): pp Zergoun, M., and Vaid, Y.P Effective stress response of clay to undrained cyclic loading. Canadian Geotechnical Journal, 31(5):