Failure Mechanism Of Submarine Slope And Solid-Fluid Unified Constitutive Model

Submarine landslides are landslides in the marine environment that transport sediment across the ocean.It can cause significant damage to offshore structures such as oil and gas exploitation platforms,submarine cables,submarine pipelines,etc.Submarine landslides may even trigger tsunami,resulting in a series of secondary disasters.The development of marine resources and the frequent occurrence of extreme weather have increased potential risks of submarine landslides,posing serious threats to both human beings and facilities in the sea.It has been a very challenging issue for offshore geotechnics associated with the failure mechanism,the post-failure movement pattern and the numerical simulation of submarine landslides.This dissertation interprets the mechanism of pockmark and liquefaction in submarine slopes involving high pore pressure through centrifuge modeling,investigates the landslide movement from its initiation,evolution,to final depostition based on the theory of continuum mechanics,and then implements numerical simulations of the whole process of the landslide based on the material point method.The main novel achievements of the dissertation are as follows:1.Centrifuge modeling tests were carried out to examine the triggering mechanism of pockmark and liquefaction in low permeability ocean sediment involving high pore pressures associated with gas hydrate dissociation.Two failure mechanisms are highlighted:(1)Tensile failure results in major pockmarks in gentle slopes and thick clays where accumulation of high pore pressure can be up to 2.2 times of the vertical effective stress of the sediment.The failed mass will be intact and showed low mobility.(2)Fractures forming in steep slopes or thin clay layers during its downward movement reduce the amplitude of the accumulated pore pressures to close to or less than the vertical effective stress of soil.The failed mass may liquefy and attain high velocity due to mixing with water.2.A concept of the deviatoric tensor direction is introduced in terms of the second invariant of the deviatoric tensor.Based on the hypothesis that the deviatoric stress is coaxial with the deviatoric deformation rate in the critical state,a plastic constitutive relation is established,and the previous inconsistent description of the critical state in elastoplastic and viscoplastic mechanics is pointed out and unified.The plastic constitutive relation can reasonably characterize the basic properties of plastic flow,well describe the intrinsic relationship between the three concepts of plasticity,elasticity and viscosity.It is of universal significance,and is suitable for describing the solid-fluid transition state of landslides.3.A rate-constitutive relationship between plasticity and viscosity has been proposed in terms of the objective time derivative.A concept of objective stress rate decomposition is introduced to establish a general method for developing elastic-plastic and elastic-viscous constitutive models.This method does not need to decompose the deformation into elastic and plastic parts.It improves the mathematical structure of the plastic theory,thus solving the unreasonable initial instability in traditional plastic calculation.This method enables a unified constitutive description of solid and fluid.A general constitutive model is thus derived for granular media covering solid-like and fluid-like flow regimes,which is suitable for continuum mechanical modeling of landslides.4.The material point method has been adopted to simulate the landslide.A numerical algorithm is proposed for the aforementioned solid-fluid unified constitutive model.In order to reproduce the stress-strain characteristics of the granular medium subjected to tension,a material point volume correction algorithm is made.The calculation results show that the proposed numerical algorithm can effectively simulate the landslide and reproduce the mechanical behaviors of the solid,the fluid or the solid-fluid transition state of landslides.