Abstract
Liquefaction of saturated sandy soils is one of the most significant aspects of earthquake triggered natural hazards. The main mechanism deals with the loss of effective stress due to rapid pore water pressure generation during earthquake shaking. This chapter involves with the fundamental mechanism and impacts of liquefaction. Liquefaction susceptibility of geological environments are briefly represented for preliminary assessment. Standard procedures of liquefaction are summarized. The dynamic response of sands are also reviewed. A case of anisotropic loading is considered, using three different particle sized sands below a shallow footing. Such sandy soils are subjected to anisotropic consolidation before performing undrained cyclic triaxial testing along limited cycles. Variation of axial strain, pore water pressure and related parameters are investigated. Main outcome of this study is to review the initial liquefaction state of sands by anisotropic loading case.
TopBackground
The term “mobility” as offered by Terzaghi (1925) has been used to describe the condition of sands during liquefaction (Castro, 1969). Casagrande (1936) has introduced the concept of “critical void ratio” to explain the response of sands to varying shear stress conditions. This study is not sufficient to describe the liquefaction of soils due to dynamic loading. conditions. Maslov (1957) has offered “complete liquefaction” to state the behavior of saturated sands, subjected to vibrations. Pressure distribution in saturated sands during and after liquefaction has been presented by Florin & Ivanov (1961). Housner (1958) has investigated the mechanism of sand movement from a liquefied layer to ground surface. Most extensive work on liquefaction has been carried out by H.B. Seed and his colleagues (Seed and Lee, 1965; Lee and Seed, 1967; Peacock & Seed, 1968; Silver & Seed, 1969). Castro (1969) has evaluated the mechanism of liquefaction with several types of tests on different types of sands.
The standard method for evaluating in situ liquefaction potential of soils has been offered by Seed and Idriss (1971) using the Standard Penetration Test (SPT) data, followed and widened by case studies (Iwasaki, et al.1978; Seed, et al. 1984, 1985; Tokimatsu & Yoshimi, 1983; Iwasaki, et al.1981; Seed & DeAlba, 1986; Ishihara, 1993). Recent procedures have been introduced to evaluate the liquefaction mechanism of soils based on Cone Penetration Test - CPT (Shibata & Teparaska, 1988; Stark & Olson, 1995; Robertson & Wride, 1998; Robertson, 2009) and Shear wave velocity -Vs (Kayen, et al. 1992; Andrus & Stokoe, 2000). Moreover, standard method of liquefaction potential is still updated (Seed & Idriss, 1982; NRC, 1985; NCEER 1997; Youd, et al. 2001; Cetin, et al. 2004, Idriss & Boulanger, 2008).
Key Terms in this Chapter
Shear Stress: A stress state where the stress is parallel to the surface of the material, as opposed to normal stress when the stress is perpendicular to the surface.
Relative Density: Ratio of the difference between the void ratios of a cohesionless soil in its loosest state and existing natural state to the difference between its void ratio in the loosest and densest states.
Deviator Stress: Difference between major and minor principal stresses in a triaxial test which is equal to the axial load applied to the specimen divided by the cross-sectional area of the specimen.
Skempton's A Parameter: Pore water pressure parameter “A” depends on stress conditions, varying with over-consolidation ratio of the soil and also with the magnitude of deviator stress.
Pore Water Pressure: Pressure of groundwater held within a soil or rock, in gaps between particles.
Permeability: Measure of the ability of soils to transmit fluids.
Axial Strain: A strain equal to the ratio between the change in length of an object and its original length.