Centrifuge modeling of permeability and pinning reinforcement effects on pile response to lateral spreading.

摘要

Earthquake-induced lateral spreading continues to be a major cause of damage to deep foundations. Currently there is a huge uncertainty associated with the maximum lateral pressures and forces applied by the liquefied soil to deep foundations. Furthermore, recent centrifuge and 1 g shaking table tests of pile foundations indicate that the permeability of the liquefied sand is an extremely important and poorly understood factor. The first part of this work presents experimental results and analyses of six centrifuge tests that were conducted at the 150 g-ton RPI centrifuge to investigate the effect of soil permeability in the response of single piles and pile groups to lateral spreading. In the models that simulate a liquefiable coarse sand layer, the piles bounced back after a couple of cycles of shaking; however in the models that simulate a liquefiable fine sand layer the piles never bounced back, reaching maximum displacements and bending moments as large as 6 times the ones measured in the models saturated with water. Negative excess pore pressures developed close to the foundations in the models with lower soil permeability, stiffening the soil and increasing the effective area subjected to the liquefied soil pressure, hence explaining the large bending moments.; The vulnerability of highway bridges to earthquake-induced ground failures arising from liquefaction has been clearly demonstrated by the extensive damage observed in past earthquakes, particularly when a nonliquefied soil layer rides on top of the liquefied soil. Liquefaction induced lateral spreading may be reduced by the restraining forces provided by pile group foundations. This reduction in lateral displacement in fact reduces the loads and displacement demands that are imposed on the piles. The second part of this work presents experimental results and analyses of four centrifuge tests that were conducted also at the RPI centrifuge to study the reinforcing or pinning effect the pile groups have on the lateral spreading. An analytical approach developed in this study proved to be very useful to understand the reinforcement effect pile groups have in liquefaction induced lateral spreading and to give an estimation, at least in centrifuge modeling, of the expected pile group deformations.