Liquefaction and Pore Water Pressure Generation in Sand: Cyclic Strain Controlled Triaxial Tests

Liquefaction and Pore Water Pressure Generation in Sand: Cyclic Strain Controlled Triaxial Tests

T. G. Sitharam, B. V. Ravishankar, S. M. Patil
Copyright: © 2012 |Pages: 29
DOI: 10.4018/ijgee.2012010104
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Cyclic strain controlled laboratory triaxial undrained tests were performed on sand samples collected from earthquake affected Ahmedabad City of Gujarat (India). To study the factors controlling the liquefaction potential and pore pressure generation, cyclic strain controlled triaxial tests were carried out on (a) base sand, (b) pure sand, and (c) pure sand and non plastic silt mixture. All the tests were conducted on reconstituted soil samples and consolidated isotropically to different effective confining pressures. Base sand, clean sand and sand with non-plastic fines were tested using cyclic strain controlled triaxial undrained tests for different combination of shear strain amplitudes, initial effective confining pressure, and relative density (RD). In case of base sand and pure sand both have qualitatively the same liquefaction and pore pressure generation behaviors. For sand with non plastic fines, basic concept of limiting fines content (LFC) is justified and shown that how the behavior of mixture undergoes transition before and after LFC. This transitional behavior is observed both in the liquefaction strength and pore pressure generation. To obtain a mean relationship between liquefaction strength, pore pressure generation on relative density, confining pressure and shear strain amplitude, approaches previously adopted by Talaganov (1996) are used.
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1. Introduction

It is widely recognized that earthquakes are among the most severe natural disasters causing significant damages such as failure of earth structure, settlement or tipping of buildings, sliding, and lateral spreading of sloping ground, densification or underlying soil failure causing vertical settlements. The reasons for these failures can be attributed either due to the compaction of loose deposits of soils or by a phenomenon called liquefaction. The phenomenon “soil liquefaction “can be observed under both monotonic and cyclic loadings. Due to its high potential to cause damages, this phenomenon of liquefaction during earthquakes has become a prime subject of concern in the geotechnical engineering. In addition, the problems such as post-liquefaction settlements and the monotonic strength of the liquefied sands began to attract increased attention. A major challenge facing the engineers is the residual strength of liquefied sands for uses in the analysis to check the post liquefaction stability of embankments and to predict the potential resistance of the liquefied sand to with stand the monotonically increasing loads. The assessment of post-liquefaction behavior of soils is also required as a controlling factor to mitigate many liquefaction hazards. Recent earthquakes such as Haiti (January 2010) and the great kanto earthquake of Japan (March 2011) have highlighted the importance of earthquake study and its mitigation. Although moment magnitude was 7.0 in Haiti when compared with Japan [9.0], the damage and death toll was severe in Haiti. In Haiti earthquake the extensive damage of the main port in Port-au-prince was due to the liquefaction caused by main shocks and as a result of aftershocks (USGS/EERI). And in Japan the widespread of liquefaction and subsequent settlement of ground has also been reported Japan (Takewaki, 2011).

In the present practice of cyclic testing on elemental soil samples there are two approaches (1) Cyclic stress controlled and (2) Cyclic strain controlled for carrying out tests to study the general dynamic behavior of soils. In a cyclic strain controlled tests one controls the amount of cyclic strain to be applied in per unit time and in the stress controlled case amount of stress applied per unit time remains constant (ASTM, 1996)

It was shown experimentally by Silver and Seed (1971) that the densification of dry sands are controlled by cyclic shear strain, γc = τc /G (where, τc is the cyclic shear stress and G is the secant shear modulus) rather than cyclic shear stress. The findings of the study Martin et al. (1975), Dobry (1982) and GovindaRaju (2005) strongly suggest that cyclic shear strain (γc), rather than τc, controls both densification and liquefaction in sands. Further, Seed and Silver (1971) performed strain controlled tests at small strains and showed that strain-controlled tests cause less water content redistribution in soil samples before initial liquefaction occurs and provides more realistic predictions of in-situ pore pressures than those obtained from stress-controlled tests. They also demonstrated that the fabric effect on pore pressure build up is practically non-existent if strain-controlled tests are performed. The behavior of soils subjected to cyclic loading is governed by what have come to be known as dynamic soil properties (shear modulus and damping ratio).In comparison with a strain controlled testing method the use of stress-controlled triaxial tests is less accurate due to the development of different strains during compression and extension phases. And also in a stress controlled test with each cycle of uniform loading, strains also increase as the number of cycle’s progress, increasing strains makes it less reliable for the determination of shear modulus and damping ratio values.

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