|Session 9.2 - Behaviour of Soils / Liquefaction|
Soil liquefaction phenomenon involves progressive intergrain contact deformation, slip, reorganization of contacts, and eventual collapse of soil skeleton. During this process frictional energy is lost along contacts. Resistance to liquefaction depends on the density of active intergrain contacts, contact friction, and confining stress. Soils with a higher density of active intergrain contacts (per grain) are more resistant to liquefaction. Higher confining stresses lead to greater energy loss along contact points and these also have a higher resistance to liquefaction. This paper presents an analysis of the evolution of intergrain contact friction, slip, and energy loss during undrained cyclic loading in sand. A theoretical framework for estimation of an index of active contact density for sands is presented. Theoretical results for mobilized intergrain friction and frictional energy loss are compared with experimental data. A new expression is presented for porewater pressure generated during undrained cyclic loading as a function of energy loss.
Keywords: liquefaction, energy, sand, silty sand, sandy silt, grain
K.J. Butterfield and M.D. Bolton
To date, most research on water films has been undertaken using relatively expensive, specialist equipment such as centrifuge models or large laboratory models. This paper describes a simple, inexpensive apparatus that allows the direct observation of pore-fluid migration within the liquefied soil. A review of other research on waterfilms is presented and frames from digital video footage of the model in operation will be shown. The model may prove useful in the study of water film formation as well as post-liquefaction settlement, consolidation of soils at very low effective stress and sediment transport within a liquefied mass.
Keywords: liquefaction, layered soil, water films.
Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated soil during strong shaking from earthquakes. We describe a simplified procedure for estimating ground displacements from seismic compression in compacted fill. The procedure has three steps: (1) estimation of shear strain amplitude within the fill soil mass from the peak acceleration at the ground surface and other seismological and site parameters; (2) estimation of volumetric strains within the fill mass based on compaction conditions in the fill, the shear strain amplitude, and the equivalent number of uniform strain cycles; and (3) integration of volumetric strains across the fill section to estimate settlement. The framework of the present procedure is similar to that of Tokimatsu and Seed (1987), which is strictly applicable only to clean sands. We update this widely used procedure to incorporate relatively recent material models for clean sands, and to extend the formulation to allow analysis of non-plastic silty sands and low-plasticity clays. The procedure is implemented for three field case history sites with measured settlements, and is found to generally provide reasonable, first-order estimates of ground settlements given the simplifying assumptions associated with this approximate method of analysis.
Keywords: seismic compression, compacted fills, ground failure
R.A. Green and J.K. Mitchell
In attempting to overcome several of the shortcomings inherent to the commonly used stress-based liquefaction evaluation procedure, several studies have proposed using Arias intensity to quantify the capacity and demand of the soil and earthquake motions, respectively. Originally developed by Arias (1970) to quantify the destructiveness of earthquake motions on buildings, Arias intensity is proportional to the integral over time of the acceleration time history squared, and therefore, is directly a function of the frequency content and duration of the earthquake motion. Presented herein are insights gained from a closer examination of the Arias intensity-based liquefaction evaluation procedures. For example, it is shown that the Arias intensity-based liquefaction evaluation procedure proposed by Kayen and Mitchell (1997) is actually an alternate formulation of the stress-based procedure, with however, several advantages and shortcomings over its counterpart. One advantage of using Arias intensity to quantify demand, as stated above, is that the frequency content and duration of the earthquake motion are directly taken into account. However, as a corollary, the use of Arias intensity to quantify capacity inherently implies that the capacity of soil is frequency dependent, which requires further validation. Further elaboration of these and additional issues are presented.
Keywords: liquefaction, Arias intensity, soils, earthquake