Modeling the rate dependent dynamic response of submarine soft clay deposits

The extraction of natural resources on the continental shelf and continuing coastal development have contributed to the increasing need to better understand the triggering of submarine mass movements and their potential consequences. Submarine landslides can involve hundreds to thousands of cubic kilometers of material and trigger significant local tsunamis. As a result, the instability of submarine slopes is considered to be one of the most serious threats to offshore installations, including communication cables, pipelines, production wells, oil platforms and coastal facilities.; This work examines the rate dependent dynamic response of soft clays prevalent in the continental shelf to provide new insights into the triggering mechanisms of submarine slope failures. The research work addressed three main aspects of the problem: a) Observed material response in simple shear, b) constitutive modeling of single element tests and, c) numerical simulation of seismic site response. Experimental results on soft clays show that multidirectional effects are significant and generally result in larger deformations and excess pore pressure than those from equivalent uni-directional testing. An existing simple effective stress constitutive model was improved by incorporating rate effects and using new laws to better describe the response of anisotropically consolidated clay under multidirectional cyclic simple shear loading. The new model, referred to as MSimpleDSS, is able to describe the effect of consolidation stress history on the stress-strain-strength characteristics of soft clayey soils. In particular, the model captures the rate-dependent response through simple adjustments of a limited number of material parameters.; The research then focuses on the dynamic response of submerged slopes under multidirectional excitation. An existing dynamic finite element program was enhanced to allow the simulation of sloping ground with two independent input ground motions oriented in the direction of the dip and strike of the slope. The MSimpleDSS model was implemented in the new code, referred to as AMPLE2D, and numerical simulations were performed to determine key factors affecting seismic response, namely: earthquake motion characteristics, shear strain dependent stiffness and damping, thickness of soil profile, and consolidation stress history. Numerical simulations show that submarine slopes can become unstable during or after earthquake loading. If the earthquake intensity is high enough and the applied shear stress exceeds the available undrained shear strength at a given depth, the slope can fail during the earthquake. In many instances, the earthquake intensity is not enough to trigger failure during the earthquake but it induces large amounts of excess pore pressure and the slope fails afterwards as a result of pore pressure redistribution and/or drained or undrained creep processes. This study suggests that the amount of pore pressure generated at the end of the earthquake is a key parameter for identifying post-earthquake instability of submarine slopes. Through the incorporation of multidirectional loading and rate effects, the amount of excess pore pressure developed at the end of the earthquake can be 20 to 30% higher than that under uni-directional shaking. These results will significantly impact the assessment of risk for submarine slopes and suggest that current State-of-the-Practice analyses yield unconservative results.