Effects of Sample Preparation on Sand Liquefaction

New perspectives on preshearing history in granular soils

The design of deep dump slopes for opencast mines usually requires information about the soil resistance to liquefaction during earthquakes. This resistance depends not only on the initial stress, the initial density, and the amplitude of the cyclic loading, but also on the preshearing, that is, the deviatoric stress path applied to the soil before the cyclic loading. To explore the influence of preshearing on the subsequent soil behaviour, a set of triaxial tests with a combination of undrained preshearing and drained stress cycles using two sample preparation methods is presented. It is shown that the preshearing as well as the preparation method have a major influence on the strain accumulation upon cyclic loading. Simulations of the experiments with four advanced constitutive models reveal that neither the long-lasting effect of preshearing nor the preparation method can adequately be captured by all of the models. This deficiency of the constitutive models can lead to unsafe designs due to the overestimation of the cyclic resistance to liquefaction and to the underestimation of long term settlements.

Microstructural differences between naturally-deposited and laboratory beach sands

The orientation of, and contacts between, grains of sand reflect the processes that deposit the sands. Grain orientation and contact geometry also influence mechanical properties. Quantifying and understanding sand microstructure thus provide an opportunity to understand depositional processes better and connect microstructure and macroscopic properties. Using x-ray computed microtomography, we compare the microstructure of naturally-deposited beach sands and laboratory sands created by air pluviation in which samples are formed by raining sand grains into a container. We find that naturally-deposited sands have a narrower distribution of coordination number (i.e., the number of grains in contact) and a broader distribution of grain orientations than pluviated sands. The naturally-deposited sand grains orient inclined to the horizontal, and the pluviated sand grains orient horizontally. We explain the microstructural differences between the two different depositional methods by flowing water at beaches that re-positions and reorients grains initially deposited in unstable grain configurations.

Static and dynamic characterization and response analysis of soils from northern India

Experimental investigations on soil samples collected from the sites of Baraut, Noida and Mandi in North India have been performed to study the static and cyclic behavior of soil. Liquefaction potential of the sites near Himalayan range is also carried out. The SPT and downhole seismic tests have been performed for the estimation of static and dynamic properties of soil. Strain-controlled cyclic triaxial tests are also carried out on soil samples collected from the borehole at various depths up to 30 m for the evaluation of dynamic soil properties. The study examined the effects of parameters such as cyclic shear strain, loading frequency and overburden pressure on the cyclic behavior of the soil. One- and two-dimensional dynamic response analysis has been performed on three sites. The initiation of initial liquefaction ranged from 202 to 752 cycles for the Baraut site, 212 to 722 cycles for the Noida site, and 121 to 617 cycles for the Mandi sites. The final design response spectra of the sites are obtained from 1D ground response analysis. The results from the response analysis have been compared with the reported results. The soils from the present study areas are prone to liquefaction at 10 m to 15 m from the ground surface.

Behavior of nonplastic silty soils under cyclic loading

The engineering behavior of nonplastic silts is more difficult to characterize than is the behavior of clay or sand. Especially, behavior of silty soils is important in view of the seismicity of several regions of alluvial deposits in the world, such as the United States, China, and Turkey. In several hazards substantial ground deformation, reduced bearing capacity, and liquefaction of silty soils have been attributed to excess pore pressure generation during dynamic loading. In this paper, an experimental study of the pore water pressure generation of silty soils was conducted by cyclic triaxial tests on samples of reconstituted soils by the slurry deposition method. In all tests silty samples which have different clay percentages were studied under different cyclic stress ratios. The results have showed that in soils having clay content equal to and less than 10%, the excess pore pressure ratio buildup was quicker with an increase in different cyclic stress ratios. When fine and clay content increases, excess pore water pressure decreases constant cyclic stress ratio in nonplastic silty soils. In addition, the applicability of the used criteria for the assessment of liquefaction susceptibility of fine grained soils is examined using laboratory test results.