Liquefaction Behavior of Typical River Channel Deposit in Kolkata City

The aim of this paper is to evaluate the applicability of the available three different models to predict excess pore water pressure generation in non-plastic silty sand mixtures during cyclic loading. Cyclic triaxial tests were performed to investigate the effect of silt content on the pore pressure generation in sand. These tests were carried to 200 cycles or to onset of initial liquefaction, whichever occurred first. Several stress-controlled cyclic triaxial tests were performed to measure excess pore water pressure generation at different levels of cyclic stress ratios for the specimens prepared at six different silt contents (FC=0% to 100%). The specimens were tested under 100 kPa confining pressures at two relative densities of 25% and 50%. Results of these tests were used to investigate the behavior of silty sands under undrained cyclic triaxial testing conditions. Seed et al. [3], Booker et al. [2] and Polito et al. [8] pore water pressure generation models based on test results are also presented in this paper. An attempt to estimate the pore pressure model coefficient a as a function of silt content, relative density, cyclic stress ratio was made. Keywords: Pore water pressure, non-plastic silt-sand mixtures, cyclic triaxial tests, bootstrap method

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.

Excess pore pressure generation in sand under non-uniform cyclic strain triaxial testing

The amount of energy dissipated in the soil during cyclic loading controls the amount of pore pressure generated under that loading. Because of this, the normalized dissipated energy per unit volume is the basis for both pore pressure generation models and energy-based liquefaction analyses. The pattern of energy dissipation in the soil in load-controlled cyclic triaxial and load-controlled cyclic direct simple shear tests and displacement-controlled cyclic triaxial and displacement-controlled cyclic direct simple shear tests is quite different. As a result, the pattern of pore pressure generation associated with load-controlled tests is markedly different from that in displacement-controlled tests. Pore pressure generation patterns for each of the four test types were proposed based upon the manner in which the load was applied during the test and the soil’s response to that loading. The results of four tests, two load controlled and two displacement controlled, were then used to verify these patterns. Pore pressure generation rates in load-controlled and displacement-controlled tests are different when plotted against their cycle ratios. Conversely, the tests produce nearly identical patterns when plotted against energy dissipation ratio. This occurs because of the relationship between energy dissipation ratio and pore pressure generation is independent of the loading pattern.

A qualitative understanding of the liquefaction process and the pore water pressure generation in sandy soils has considerably enhanced by various researchers for the last few decades. Laboratory investigation carried out by these researchers have highlighted that liquefaction and pore water pressure generation in granular materials depends on many factors, such as sample preparation, initial anisotropy, confining pressure, void ratio, loading condition, amplitude of shear strain etc. Also, in these laboratory experiments there are limitation of repeatability (with in lab) and reproducibility (lab to lab) in preparation of sample. In this paper three dimensional discrete element method (DEM) was employed to understand the liquefaction and pore pressure generation of granular materials. In DEM modeling a good control of sample preparation can be exercised and it is easy to prepare samples repeatedly with similar fabric. A series of strain controlled undrained cyclic triaxial test has been carried out on isotropically compressed assembly of spheres. The undrained tests have been simulated by maintaining a constant volume condition of the sample throughout the cyclic triaxial tests. The pore water pressure has been computed taking the difference of mean pressure between the undrained (effective) and drained (total) stress path during strain controlled cyclic triaxial tests. The study has been planned to emphasize the effect of confining pressure, void ratio, initial anisotropy and amplitude of shear strain on the liquefaction and pore water pressure generation in the sample. As observed from the results, numerical simulation using DEM has captured liquefaction and pore water pressure generation very similar to the experimental results. Further, evolution of micromechanical parameters such as average coordination number, deviator anisotropic coefficients during cyclic triaxial loading has also been reported and discussed.

Soil liquefaction is one of the most detrimental forms of earthquake-induced ground failure that can result in catastrophic damage to engineering structures. For the seismic safety evaluation of foundations and high-rise structures, it is the most critical way to assess liquefaction resistance. In this paper, an array number of isotropically consolidated undrained cyclic triaxial tests (CTT) and cyclic hollow cylinder tests (CHCT) have been performed to evaluate soil liquefaction resistance. Thirty-seven isotopically consolidated undrained cyclic triaxial tests and thirty-seven cyclic hollow cylinder tests were run on the uniform medium Monterey No. 0/30 sand and it is with four different percentages of fine content. By using cyclic triaxial and cyclic hollow cylinder tests for evaluating liquefaction resistance, it helps us better understand the relationship between two types of tests on uniform clean Monterey No. 0/30 sand and soil sample with five different percentages of fines content. Four different relative densities of 30%, 45%, 50%, and 60%, two confining pressure of 103 kpa and 207 kpa, and five cyclic stress ratios (0.15, 0.2, 0.25, 0.3, and 0.4) have been used for a series of cyclic triaxial tests and cyclic hollow cylinder tests. At the same relative densities of 30% and 60%, the correction factor between CTT and CHCT evaluated ranged from 0.46 to 0.63 on the uniform clean Monterey No. 0/30 sand. Statistical analyses were performed to formulate functional relationships for predicting the correction factor of soil liquefaction resistance between CTT and CHCT tests.

Soil liquefaction has conventionally been studied as a pure undrained condition subject to cyclic shearing in the laboratory. However, in the field, ground settlement and expulsion of pore water in the form of sand boils are frequently observed, which conventional triaxial and simple shear apparatus are unable to replicate. In this study, a novel insight of coupled pore pressure generation and dissipation in liquefiable clean sand is discussed with the use of a modified cyclic triaxial testing set-up. Stress-controlled cyclic triaxial tests with controlled pore water drainage far lower than the soil's permeability were carried out to investigate ‘near-perfect undrained’ conditions, which are more representative of field conditions. Comparison of results from three sets of stress-controlled cyclic triaxial testing were carried out: (a) conventional shearing under an undrained condition; (b) shearing under controlled drainage; and (c) undrained shearing until liquefaction, and thereafter continual shearing under controlled drainage. The critical cyclic shear stress ratio where excess pore pressure dissipation dominates excess pore pressure generation is identified from the coupled cyclic shear loading and pore water drainage tests. In addition, this study also noted a critical void ratio where the soil is no longer susceptible to liquefaction even under considerable shearing amplitudes. These findings may offer guidance on the tolerable cyclic shear stress and minimum degree of densification to avoid occurrence of soil liquefaction in the field.

To overcome current limitations in predicting in situ pore-pressure generation, a new field testing technique is used to measure directly the coupled, local response between the induced shear strains and the generated excess pore pressure. The pore-pressure generation characteristics from two in situ liquefaction tests performed on field reconstituted specimens are presented, including the pore- pressure generation patterns at various strain levels, the observed stages of pore-pressure generation, and pore-pressure generation curves. Comparisons of the in situ pore-pressure generation curves with data in the literature and from laboratory strain-controlled, cyclic direct simple shear tests support the in situ testing results. In addition, the effects of effective confining stress on threshold shear strain and pore- pressure generation curves are discussed. Comparisons of the rate of pore-pressure generation among the in situ tests, laboratory strain-controlled tests, and a model based on stress-controlled tests reveal that in situ pore pressures generated in reconstituted soil specimens during dynamic loading develop more similarly to those from cyclic strain-controlled laboratory testing. This observation implies that the evaluation of induced strains rather than induced shear stresses may be more appropriate for the simulation of pore-pressure generation.

The paper examines the effect of small percentages of bentonite on pore pressure generation in loose sands, from small strains all the way to liquefaction. It relies on resonant column, static triaxial and cyclic triaxial tests conducted on specimens prepared by dry-mixing Ottawa sand and bentonite, at a skeleton relative density of 35% ± 5%. Two main variables are investigated: the percentage of bentonite (3% and 5% by dry mass of the sand), and the duration of the ageing period preceding shear (1 to 10 days). The presence of bentonite increases the shear strain needed to initiate the generation of excess pore pressures in resonant column tests; in cyclic tests it reduces the mean pore pressure generated per loading cycle and allows the specimen to sustain an increased loss of effective stress before liquefaction initiates, both effects contributing to an increased resistance to liquefaction. These effects are further enhanced with prolonged pre-shear ageing. Additionally, under static conditions, the behaviour of the sand is found to become increasingly stable as the ageing duration is extended. Given the role played by the ageing period, the effects observed cannot be simply ascribed to the increased bulk density of the specimens with bentonite. Instead, they are attributed to the pore fluid formed in presence of bentonite: a concentrated clay gel with solid-like properties. This pore fluid increases the sand's threshold shear strain, which is shown to have a strong correlation with the resistance to liquefaction.

Both stress-controlled and strain-controlled cyclic triaxial tests were conducted on samples of Monterey No. 0 sand of 30 percent relative density to assess the frequency effect on its liquefaction potential. The result indicated that the liquefaction potential of a loose sand was affected significantly in the stress-controlled cyclic triaxial test with stress reversal and the cyclic triaxial extension test when the frequency is greater than 0.01 Hz. The effect increases with the frequency. In a low frequency cyclic triaxial test, a sample was allowed to undergo severe contraction in the extension phase of the test. As a result, the sample was substantially weakened due to the generation of a large excess pore water pressure. Thus, the extension phase of a stress-controlled cyclic triaxial test was found to be more damaging and responsible for the strength reduction and liquefaction of a loose sand than the compression phase. As the frequency increases from one-hundredth of a hertz, the amount of damage per cycle of loading reduces, and the number of cycles required to cause liquefaction also increases. In a strain-controlled cyclic triaxial test, however, the above effect was found to be negligible.

Recent developments in studies of soil response to earthquake loadings have made it possible to incorporate the rates of pore water pressure build-up in soils in to nonlinear response analyses of the grounds. Such pore pressure changes help in computing the changes in stress-strain behaviour of soils in the deposit progressively as the earthquake progresses. The rate and magnitude of pore pressure generation in soils during seismic loading will have important effects on the shear strength, stability, and settlement characteristics of a soil mass, even if the soil does not liquefy. The results in terms of pore pressure response in soils from a series of experimental investigations using strain-controlled cyclic triaxial tests on soils samples collected from liquefied sites are presented in this paper. The effect of relative density, amplitude of cyclic shear strain, number of loading cycles, confining pressure and frequency of cyclic loading on the pore pressure build-up are studied. Analytical expressions are proposed using regression analysis to define mean relationships between normalized pore water pressure and normalized cycles for the prediction of pore water pressure build-up in silty sands. Also, the pore water pressure build-up in soils is independent of frequency of loading.

Cyclic undrained behavior of silty sand

When loose, saturated sands and non-plastic silts are subjected to undrained cyclic loading, they will generate positive pore pressures. This increase in pore pressures leads to a decrease in effective stress with a corresponding decrease in shear strength and increase in liquefaction susceptibility. For combinations of sand and non-plastic silt, the threshold fines content can be defined as the non-plastic silt fines content at which the soil changes from sand-like behavior to silt-like behavior. Soils below the threshold fines content behave like sands and soils above the threshold fines content behave like silts. During cyclic triaxial and cyclic direct simple shear tests performed on specimens of sand and silt prepared to the same relative density but different fines contents, two rates of pore pressure generation were observed. When compared at five cycles of loading, soils with silt contents above the threshold fines content were found to produce pore pressure ratios as much as 50% higher than those observed for soils with silt contents below the threshold fines content. When evaluated in terms of cycles, cycle ratio, and dissipated energy ratio, the rate of pore pressure generation was found to be more rapid for soils above the threshold fines content than for soils below the threshold fines content.

An analysis of a saturated sandy subsoil behaviour due to cyclic loading is presented. Cyclic loads create cyclic shear strains in a subsoil, and subsequently cause pore pressure generation that leads to the reduction of the shear strength of a subsoil and, in an extreme case, to the failure of structures founded on such soils. A pore pressure generation is accompanied by a process of pore pressure dissipation which reduces a rate of pore pressure generation. The assumption of undrained behaviour (no dissipation effects) allows for obtaining the “lower bound estimate” of subsoil behaviour, since the pore pressure dissipation effects make the rate of pore pressure generation smaller, and then we need more loading cycles to get the onset of liquefaction. An analysis of the problem of simultaneous pore pressures generation and dissipation is very much complicated because the differential equations describing these phenomena are coupled. We have proposed a numerical procedure which allows for studying this problem in a relatively simple manner. The pore pressure generation in a subsoil is computed with the help of the compaction theory which is formulated in terms of the cyclic stress and strain amplitudes. The pore pressure dissipation is computed with the help of a simplified version of the Biot theory. The process of resolidification starts when the action of cyclic loadings is over. The excess pore pressures now freely dissipate down to the equilibrium. We can also study this behaviour. In our contribution we present the fundamentals of the compaction theory, numerical algorithm of dealing with boundary value problems, as well as some numerical examples regarding the behaviour of a subsoil beneath a breakwater.

The effect of plastic fines on the pore pressure generation characteristics of saturated sands