Seismic Performance of a Piled Raft Foundation with Grid-Form DMWS Considering Softening of Stabilized Soil-Paper

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The study evaluates the seismic performance of a piled raft foundation reinforced with grid-form deep mixing walls (DMWs), accounting for post-peak softening of the stabilized soil. The analysis models a base-isolated, 12-story building in Tokyo using a detailed three-dimensional nonlinear finite-element model previously calibrated against observations from the 2011 Tohoku earthquake. A strong, code-defined Level 2 artificial earthquake, based on the Hachinohe record, is applied. The DMW model includes tensile and shear failure, with tensile softening governed by fracture energy.<br /><br />Analyses show that earthquake-induced tensile failure occurs mainly in the lower portions of the DMWs, and the affected zone is larger when softening is included. Despite this partial damage, the DMWs limit ground deformation beneath the raft and substantially reduce pile bending moments, particularly near the pile heads. With DMWs, calculated pile moments remain below the allowable design criterion; without them, moments approach the ultimate criterion.<br /><br />The results indicate that grid-form DMWs can protect piles effectively even when parts of the stabilized soil crack and lose strength during strong shaking. The authors conclude that performance-based seismic design can account for partial DMW failure, provided that nonlinear three-dimensional analysis incorporates the softening behavior of the stabilized soil.

Keywords

seismic performance

piled raft foundation

grid-form deep mixing walls

deep mixing walls

post-peak softening

nonlinear finite-element model

base-isolated building

tensile failure

pile bending moments

performance-based seismic design