Earthquake induced foundation failure has occurred in a number of events worldwide, such as the 1964 Niigata earthquake and 1995 Kobe earthquake. Fabric anisotropy has been considered as one of the factors that affect liquefaction characteristics. In this study, dynamic centrifuge tests were conducted for studying the influence of sand fabric anisotropy on seismic response of circular foundations. For this purpose, sub-angular Toyoura sand was used as the most anisotropic material, while sub-rounded Nevada sand was used for the comparison tests. Each group of tests were carried out in both dry and saturated conditions. These tests are considered complementary to similar centrifuge tests that the authors performed using rectangular foundations. The tests show that sand fabric anisotropy has significant impact on the seismic response of circular foundations as well, especially in saturated conditions. Specifically, as the deposition angle of the sand layer increased, the data show a remarkable decrease in acceleration amplitude concurrently with a considerable decrease of pore pressure ratio and foundation settlement. These effects were more pronounced in the sub-angular Toyoura sand. These effects of increased deposition angle are consistent with the corresponding decrease of bearing capacity and increase of footing settlements in static centrifuge tests from the literature.

Introduction

In 2015 alone, magnitude seven and greater earthquakes happened 19 times in the world, including two major ones in Nepal. The April 2015 Nepal earthquake (Gorkha earthquake) killed over 8000 people and injured more than 21,000. Liquefaction is a phenomenon that occurs both in natural and man-made events, in which the strength and stiffness of a soil are significantly reduced. It is typically induced by earthquakes or other rapid loading processes. Foundation failure due to soil liquefaction during earthquakes has been and continues to be a major type of damages. Soil anisotropy is widely observed in the natural deposited sand. It is determined by particle shape, contact, particle roughness, deposition history and many other more. It affects a few important parameters in soil properties including shear modulus, friction angle and bearing capacity. In previous studies, researchers found that fabric anisotropy of soil grains is an important factor in dynamic soil response as well. Brewer [1] introduced the definition of anisotropy as the property of being directionally dependent, and in most cases, the result of weathering of hard rock or sedimentary solids. Several researches were conducted using true triaxial tests, nonlinear models and their applications in 1970s. Saada and his colleges at Case Western Reserve University investigated the mechanical behavior of anisotropic materials. They used one-dimensional consolidation of the test specimens in a tri-axial cell to simulate field condition [2]. Oda stated that sands deposited in air or in water show anisotropic shear strength in their triaxial compression tests [3].