CENTRIFUGE RESEARCH OF COUNTERMEASURES TO PROTECT PILE FOUNDATIONS AGAINST LIQUEFACTION-INDUCED LATERAL SPREADING

Abstract

Repeated experience of foundation and superstructural damage during earthquakes associated with ground failure and liquefaction, have shown the importance of lateral spreading and especially of the presence of a shallow nonliquefiable soil layer as a cause of damage. Typically such strong nonliquefiable layer or crust, riding on the liquefiable soil where the lateral spread actually occurs, pushes laterally against the piles and pile cap, inducing large lateral forces, deformations and bending in the foundation. The effects depend on the free field deformation, characteristics of the nonliquefiable layer and foundation, and characteristics of the superstructure. For an existing pile foundation, the first step is to verify if the pile foundation has enough stiffness and strength to resist the soil lateral forces, and especially the passive thrust applied by the nonliquefiable layer, with acceptable foundation displacement and rotation and no damage to the foundation elements, in which case no countermeasures are needed. If this evaluation concludes that the foundation does not possess the required stiffness and strength to prevent distress, three mitigation strategies are often used: (i) Remediation countermea-sure of the liquefiable soil at the site to prevent the free field liquefaction and lateral spreading from occurring in the first place; (ii) retrofitting of the foundation by reinforcing it, for example by adding piles or strengthening the existing piles; and (iii) isolation of the foundation, and especially of the pile cap, from the nonliquefiable layer in the free field by a retrofitting countermeasure where a “soft fuse” such as a soft soil or other material or device are installed at shallow depths near the foundation, hence limiting the lateral force applied to the piles and pile cap to a low value which minimises the deformation and bending of the foundation. The paper discusses these countermeasure strategies through several series of centrifuge model tests of single piles and pile groups conducted with the participation of the authors at the RPI geotechnical centrifuge.