Residential instability caused by earthquake-induced degradation of volcanic ash cohesive soils

Abstract

Cyclic and post-cyclic undrained triaxial tests were conducted on undisturbed volcanic cohesive soils taken from residential sites in Mashiki Town, Kumamoto, which were struck by and severely damaged by the 2016 Kumamoto earthquake. Special emphasis was placed on elucidating the degradation of the undrained strength and stiffness of volcanic cohesive soils based on test results used for predicting earthquake-induced settlement and displacement of residential foundations. Results of laboratory tests and analyses of instability of residences and foundations that had experienced the Kumamoto earthquake indicate the points listed below.
i) Decreased strength and stiffness as well as strength parameters were observed after cyclic loading. Particularly, the post-cyclic stiffness reduction was more marked than strength reduction.
ii) Strength parameters, cohesion c, and internal friction angle φ, were reduced both on a total stress and an effective stress basis after undrained cyclic loading.
iii) Instability and residual deformation of residences on volcanic ash cohesive soils were verified using both strength and stiffness degradation and a decrease in strength parameters.