Experimental investigation of the undrained dynamic behavior of soft clay under equivalent earthquake loadings

Abstract

Soft clay is extensively distributed in the Yangtze River Delta of China. Many seismic events indicate that underground structures buried in soft soil may suffer severe damage from earthquakes. In this study, a series of bidirectional dynamic cyclic triaxial tests were conducted to investigate the dynamic behavior of soft clay, considering different confining pressures and consolidation stress ratios. A simplified equivalent seismic loading method based on the strain failure criterion was proposed. The obtained equivalent amplitude of soft clay calculating by the critical cyclic stress ratio is averagely 1.58 times that of the sand liquefaction method. Under equivalent seismic cyclic loading, the dynamic shear strain and excess pore pressure of soft clay increases with the increase of confining pressure. The relationship between the maximum excess pore pressure and the corresponding shear strain can be expressed by a hyperbolic function. Due to the weakening effect of seismic loading, the shear modulus decreases as the shear strain increases, with a sudden reduction of up to 45 %. The shear modulus and damping ratio increase with the increase of confining pressure and consolidation stress ratio. The research results may provide some valuable insights into the seismic design practices in soft clay areas.