Dimitris K Karamitros

Analysis of Liquefaction Effects on Ultimate Pile Reaction to Lateral Spreading

Current design methods for piles in liquefied ground assume that the ultimate soil pressures acting on the pile are drastically reduced relative to the reference values in the absence of liquefaction. However, there is controversy about the adopted design parameters and their effects. Furthermore, it has been experimentally shown that soil pressures are not always reduced, but they may increase well above the recommended design values because of flow-induced dilation of the liquefied soil around the upper part of the pile. In view of this, the pile response is simulated in this paper using a three-dimensional, fully coupled dynamic elastoplastic numerical analysis. The methodology is first verified against results from centrifuge experiments and consequently applied parametrically for various pile, soil, and seismic excitation characteristics. It is thus shown that dilation-induced negative excess pore pressures are indeed possible for common pile and soil conditions at the upper segments of the pile, having an overall detrimental effect on pile response. It is further found that, apart from the commonly considered effect of relative sand density, ultimate soil pressures are affected by a number of other dilation-related parameters, such as the effective confining stress, the permeability of the sand, and the predominant excitation period as well as the pile diameter and bending stiffness. To quantify the relevant effects, new multivariable relationships are established and subsequently evaluated against the empirical methodologies that are currently used in practice.

FLIQ: Experimental Verification of Shallow Foundation Performance Under Earthquake-Induced Liquefaction

The seismic performance of a square footing, resting on a liquefiable sand layer, with a non-liquefiable clay crust, is examined herein, with the aid of three centrifuge experiments. Emphasis is given on the seismic settlements of the foundation, while it is for the first time attempted to measure its (degraded) post-shaking bearing capacity, with the aid of a hydraulic piston, specially programmed to push the footing to failure, immediately after the end of shaking and before the dissipation of excess pore pressures. Aimed to examine the effect of clay crust thickness H on foundation performance, the experiments were performed for H = 2/3B, B and 5/3B, with B being the width of the footing. Following a brief presentation of the testing configuration, soil properties and excitation characteristics, the experimental results are presented and evaluated through comparison with relevant numerical and analytical predictions. Thus, the beneficial effect of the clay crust thickness H is quantitatively substantiated and the existence of a “critical clay crust thickness”, beyond which subsoil liquefaction does not deter foundation performance, is experimentally verified.