Kinematic interaction of piles in laterally spreading ground

Abstract

All current empirical approaches for pile design in liquefied soils agree that the ultimate soil pressure on the pile is drastically reduced relative to the reference ultimate pressures, in the absence of liquefaction. However, there is disagreement with regard to the extent of the aforementioned reduction and also controversy about the pile and soil parameters which control it. For instance, well documented experimental data from centrifuge tests show that significant negative excess pore pressures may develop due to the dilation of the liquefied soil that flows around the upper part of the pile, thus enhancing ultimate soil pressures well above the recommended values. In view of the above objective uncertainties, the problem was analyzed numerically using a 3D dynamic procedure. Namely, FLAC 3D was combined with the NTUA Sand constitutive model, for dynamic loading and liquefaction of cohesionless soils, and was consequently used to perform parametric analyses for various pile, soil and seismic excitation characteristics. To ensure the validity of the predictions, the numerical methodology was first verified against the afore mentioned centrifuge experiments. It is thus concluded that dilation-induced negative excess pore pressures are indeed possible for common pile and soil conditions encountered in practice. As a result, apart from the relative density of the sand, a common parameter in most empirical relations, a number of other dilation related factors influence also the ultimate soil pressure, such as: the effective confining stress, the permeability of the sand and the predominant excitation period, as well as the pile diameter and deflection. Furthermore, it is shown that dilation effects are more pronounced at the upper and middle segments of the pile, having an overall detrimental effect on pile response. Finally, a preliminary evaluation of numerical results shows that the development of a new methodology for the evaluation of p–y response in laterally spreading soils which would incorporate the above effects is feasible.