Experimental study on effects of lateral soil flow on group pile and its application to design procedure

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Model tests were conducted on a 1-G shaking table with a variety of group pile which was embedded in a liquefied sloping ground. Different from foregoing group pile tests with 4 or 9 constituent piles, the present study employed 36 or 121 piles in a square configuration.Upon shaking the model with different intensities and frequencies, the bending moment was recorded along the length of piles at several piles. Moreover, lateral ground displacement was studied by using data of embedded displacement transducers as well as distortion of soil markers in the ground surface. Records were taken of acceleration and pore water pressure as well. It was revealed that soil flows in the downslope direction upon liquefaction and large deformation occurs around piles. This large deformation makes it very difficult or even inappropriate to analyze the pile-soil interaction by conventional FEM analysis.The interpretation of bending moment data revealed that the magnitude of bending moment (and consequently the lateral earth pressure) takes the maximum value at the upstream and downstream ends of the pile foundation. This implies that the existence of piles in the upstream direction reduces the earth pressure in the downstream direction, which is called the Shadow Effects, and that, in the downstream end, the ground elevation difference, which is produced by the prevented soil flow by piles, creates significant lateral earth pressure on end piles. The shadow effects vary with the pile spacing ; the narrower spacing makes the shadow effects more remarkable.Finally, a retrofitting measure for existing group foundation was proposed. That is installation of additional piles on upstream and downstream sides of a foundation. The earth pressure due to flow of liquefied soil is borne by those additional piles and the load on the existing and important piles is mitigated. These findings were examined in a numerical model as well in which liquefied sand is modeled as a viscous liquid.