Mitigation of lateral ground displacements of liquefied soils with underground barriers

Analyses of damage data from earthquakes in the last 35 years show that very high financial losses have resulted from cases where liquefaction of soils was associated with ground lateral displacements towards a free boundary such as a shoreline, a river channel, or an open trench. Lateral displacements in excess of 10 m have been documented in the literature [Bartlett and Youd, J. Geotech. Engng, ASCE 121 (1995) 316]. In fact, in many cases, displacements amounting to only a fraction of this number are capable of causing considerable disruption to man-made works. Several factors contribute to the extent of lateral spreading: surface and subsurface geometry, soil characteristics, and intensity of ground motion.

Ground displacements can be minimized or even arrested in practice with an underground structure properly designed to counter the driving forces, gravity and inertia combined. Mitchell et al. suggested practical guidance for the design of such structures, or barriers, in 1998 [Geotech. Spec. Publ. 75 (1998) 580]. However, to date there is no standard procedure to carry out the analysis of such barriers. The paper describes several recent designs of underground barriers that have been constructed in highly seismic environments. Three types of underground barriers are described: clay fill, a grid of structural piles, and a grid of cement-treated soil. The design of the cement-treated cell barrier is discussed in detail as it accounts for the most unfavorable combination of all forces acting on the structure: lateral stresses induced by liquefied soil, hydrodynamic effects, inertia forces, and loss of ground.     

Interpretation from large-scale shake table tests on piles undergoing lateral spreading in liquefied soils

Results from a benchmark test on full-scale piles are used to investigate the response of piles to lateral spreading. In the experiment, two single piles, a relatively flexible pile that moves together with the surrounding soil and a relatively stiff pile that does not follow the ground movement have been subjected to large post-liquefaction ground displacement simulating piles in laterally spreading soils. The observed response of the piles is first presented and then the results are used to examine the lateral loads on the pile from a non-liquefied soil at the ground surface and to evaluate the stiffness characteristics of the spreading soils. The measured ultimate lateral pressure from the crust soil on the stiff pile was about 4.5 times the Rankine passive pressure. The back-calculated stiffness of the liquefied soil was found to be in the range between 1/30 and 1/80 of the initial stiffness of the soil showing gradual decrease in the course of lateral spreading.     

Parametric analysis of single pile response in laterally spreading ground

Extensive damage to pile-supported structures has been witnessed in several recent earthquakes (Chi-Chi, 1999; Kobe, 1995, etc.), as a result of liquefaction-induced lateral spreading of slightly sloping ground or free-face topographic irregularities. This paper presents a parametric analysis of the basic pile and soil parameters, as well as the pile-soil interaction mechanisms affecting the response of single piles subjected to such lateral spreading, based on numerical simulation with the nonlinear P-y method. In parallel, a set of design charts and analytical relations is established, for approximate computation of maximum pile deflections and bending moments, using a “theory guided” multi-variable statistical analysis of the numerical predictions. Three different combinations (design cases) of pile head constraints and soil conditions were considered, which are commonly encountered in practice. The overall accuracy of the proposed analytical relations is evaluated against experimental results from seven centrifuge and five large shaking table experiments.     

Evaluation of pile foundation response to lateral spreading

The effects of liquefaction on deep foundations are very damaging and costly, and they keep recurring in many earthquakes. The first part of the paper reviews the field experience of deep foundations affected by liquefaction during earthquakes in the last few decades, as well as the main lessons learned. The second part of the paper presents results of physical modeling of deep foundations in the presence of liquefaction conducted by the authors and others at the 100g-ton RPI centrifuge. In the last decade centrifuge modeling has been identified as a key tool to identify and quantify mechanisms, calibrate analyses and evaluate retrofitting strategies for pile foundations. Results are presented of centrifuge models of instrumented pile foundations subjected to lateral spreading, including single pile and pile groups, 2- and 3-layer soil profiles, mass and stiffening elements above ground to incorporate the effect of the superstructure, and evaluation of proposed retrofitting strategies. Interpretations of these centrifuge experiments and their relation to field observations and soil properties.