Study On Strength Properties Of Marine Clay And The Effects On The Seabed Secondary Failure Mechanism

Submarine landslides are a major offshore geohazard,it can not only lead to the destruction of submarine pipelines,cables,marine foundations and other facilities,but also cause tsunamis leading to huge loss of human life and property.The triggering mechanism of the submarine landslide is very complex.The long distance and large volume submarine landslide is usually results from gradual accumulation under certain triggering conditions,and it needs to experience a progressive failure process.When a landslide just occurs,the slip surface is difficult to appear in the whole slope at the same time,but propagate along with a local shear band.Meanwhile,the soil strength gradually decreases with the increase of shear displacement at the shear zone,and then presents the overall slope failure.Submarine landslide is mainly caused by two factors:one is the increase in load;The second is the decrease of soil strength in the slope.The volume of the sliding mass caused in the initial small slope failure is generally limited,but the small sliding mass may affect the secondary instability of the seabed during its sliding process.To study the secondary failure mechanism of clay seabed,it is necessary to study the strain softening characteristics of marine clay.Strain softening properties of clay are also the basis of safety evaluation for marine engineering,and are very important to geotechnical engineering problems,such as seabed stability evaluation and bearing capacity analysis of marine foundation.For this,based on the large deformation finite element method(LDFE)and model test method,the evaluation methods for the soil strength,strain softening parameters,and weak layer parameters are studied.A pressurized penetration test method is put forward to ensure the full-flow mechanism of the full-flow penetrometer.The optimized strain softening model is established,and the weak layer identification method is proposed.Besides,the seabed secondary failure mechanism is revealed caused by the sliding mass.The main achievements are as follows:First,considering the strain softening behavior of soil,the penetration of the T-bar into the seabed with clay is simulated based on the LDFE method,a relation to estimating the critical depth of T-bar just achieving the full-flow mechanism is established,and a strength estimation method under shallow penetration conditions(with uncomplete full-flow mechanism)is proposed.By studying the characteristics of the T-bar's cyclic penetration resistance curve and the corresponding soil flow mechanism under the in-complete full-flow state,it is found that a cavity may be formed in T-bar tests,thus affecting measured resistance.This feature shows the importance of ensuring the full-flow state in cyclic T-bar tests.Second,as the application limitation of the full-flow penetrometer test in high strength soil caused by the full-flow mechanism difficult to be established,a pressurized T-bar test method is proposed,and the supporting test equipment is invented.Two laboratory test methods,i.e.observing the cavity evolution and measuring the strength degradation relation of soil,are used to verify the effectiveness of the pressurized T-bar test method.By observing the cavity evolution during the T-bar extraction,it is found whether the cavity is generated is related to the diameter of the T-bar.A series of cyclic T-bar tests are completed both for the consolidated kaolin clay and Guangzhou offshore clay under different overburden pressure.The test results show that sufficient overburden pressure can ensure T-bar under the full-flow mechanism during tests.After that,the resistance degradation relations are nearly unaffected under different overburden pressures,which verifies the effectiveness of the proposed test method.To optimize the process of the pressurized T-bar test method,the LDFE method is used to simulate the process of the T-bar pre-penetration and applying overburden pressure.The optimal prepenetration depth(2D)and overburden pressure estimation methods are proposed.Third,considering the influence of the weak layer on the seabed stability,the identification method of the weak layer is studied based on the T-bar penetration test.Based on the ABAQUS/CEL method,the process of the T-bar penetration in layered soil is studied,and the influence of the weak layer on the penetration resistance of the T-bar is analyzed.Based on the numerical results under different strength ratios and weak layer thicknesses,it is found that the parameters of the weak layer are mainly affected by the top layer soil.Besides,a method for estimating the parameters of the weak layer is proposed,which can be applied to engineering practice.Fourth,based on the resistance degradation relations obtained from cyclic penetration tests,the strain softening model of marine clay is studied.Considering that the initial brittleness of soil is not sufficiently considered in the previous study,an optimized exponential strain softening model is proposed.The proposed model can flexibly reflect the initial brittleness and fully remolded state of soil by introducing a parameter ?.20 cyclic penetration test results were used to verify the proposed model.Taking the process of pipeline penetration as a background,the numerical simulation results show that the constant value of ?(?=1.0 for one-stage model,?=2/3 for enhanced-stage exponential model)will lead to underestimation or overestimation of the pipeline resistance,with the maximum error up to 22.8%.Last,considering the strain softening characteristics of soil and the existence of a weak layer,the secondary seabed failure mechanism is studied.Based on the finite element method(FEM),the effect of sliding mass on the seabed stability is studied,and the failure mechanism of seabed under the effect of strain softening behavior and the weak layer is revealed.An envelope about the critical sliding loads is proposed to evaluate the seabed stability.By studying the friction conditions between the sliding mass and the seabed,it is found that the compressive stress(i.e.the normal stress component for the load of the sliding mass)on the seabed surface is the main reason for the seabed failure,but the interface the friction conditions have a relatively little effect on the critical load of the sliding mass.The gravity loading applied on the elastomer is used to simulate the sliding mass acting on the seabed in static,and the process of the sliding mass gradually acting on the seabed is studied based on the CEL method.The research found that the sliding mass in a dynamic propagation state is more likely to lead to the seabed failure(dynamic critical load is 15%higher than that in the static analysis model),but the velocity of the sliding mass has little influence on the critical loads.