Study On The Mechanism Of Cyclic Softening And Thixotropy Hardening Of Strong Structural Deep-sea Soft Clay

The construction of offshore platforms and other offshore structures is very important for the effective use of marine space and the rational development of marine resources.The proper functioning of offshore structures depends much on the safety and stability of the subsea anchoring system.As the foundations of piles,anchor or pipelines,marine soft clay has unique engineering properties which are quite different from onshore soft clay.In addition,the stress conditions of seabed soil are very complicated and changeable due to the varied marine environment.Therefore,intensive research on the mechanical properties and inner mechanism of marine clay under loadings,especially the deep-sea soft clay,has important engineering value and scientific significance.In this study,numerous intact deep-sea soft clays are recovered from the South China Sea.The cyclic softening and thixotropy hardening characteristics of them are investigated by various indoor test methods.The microstructural evolutions during the change of strength are qualitative described and quantificative analysed by using microscopic observation method and digital image processing technology.Furthermore,a novel parameter called microstructural entropy is proposed to build the relationship between the macroscopic mechanical properties and the microstructure evolution.The main research contents and conclusions are as follows:(1)Through systematically studying the basic physical and mechanical properties of deep-sea soft clay,it is found that it has some special properties with high nature water content,high void ratio,high clay content,high Attergerg limits,high compressibility,low consolidation rate,low permeability and strong structure.In addition,they contain a large number of microbial remains and other special sediments.The undrained shear strength of deep-sea soft clay is measured using an indoor T-bar full-flow penetrometer.The results show that the strength of deep-sea soft clay has significant stratification and regional differences.The strength in the shallower depth is much high and it shows an "apparent overconsolidation"phenomenon,which is related to its special sedimentary environment.(2)T-bar cyclic penetration tests are carried out on both intact and reconsolidated remoulded samples to investigate the strength degradation of deep-sea soft clay.The results show that the T-bar penetration resistances decrease significantly before 5 cycles and keep almost stable after 8 cycles.The cyclic hysteresis loops become more symmetrical with the increase in cycles,and the positions of the symmetry point gradually tend close to the vertical axis.The initial penetration resistances of intact samples are all higher than those of reconsolidated remoulded samples.However,as the number of cycles increases,the following penetration resistances of intact samples become lower than those of the latter ones.The normalized penetration resistance curves are well fitted by exponential function,and the softening index can denote the degree of cyclic softening of soil.The higher the softening index,the higher the softening degree of soil.The softening degrees of intact samples are much higher than those of reconsolidated remoulded samples,which indicate that the deep-sea soft clays have strong structure.(3)The thixotropy hardening characteristics of deep-sea soft clay are measured by the fall cone apparatus combined with the bender element method.The results show that all the deep-sea soft clay can show strong thixotropy strength recovery and stiffness recovery capabilities.Generally,the increase rate is relatively fast in the first 10 days,after that it gradually keeps stable or slightly increases.The relationships between thixotropy strength recovery and basic properties or index are analysed.It is found that there are good correlations between thixotropy strength ratio and nature water content,plasticity index,liquidity index,sensitivity and activity,but not for over-consolidated ratio.The temperature effect on thixotropy is investigated and the results show that the increase in temperature will improve the thixotropy strength recovery of deep-sea soft clay.So it indicates that the change of strength measured in room temperature will overestimate the thixotropy strength recovery capacity of in-situ deep-sea soft clay.The DSS test method is used to study the effect of reconsolidation on the thixotropy of deep-sea soft clay.The results present that the reconsolidation process has a great influence on the thixotropy strength recovery,and the final strength of the sample will depend much on the reconsolidative level.(4)Scanning electron microscopy and digital image processing technology are used to analyze the evolution of the microstructure of the deep-sea soft clay during both the cyclic softening and thixotropy hardening.The results show that the microstructure evolutions of the intact and reconsolidated remoulded sample are quite different.For the intact soil,the change of microstructure includes the gradual destruction of the flocculation structure into smaller aggregates or clay pieces,and the gradual dispersion of large pores into small pores.While for the reconsolidated remoulded soil,the change of microstructure is mainly characterized by the gradual fracture of the dense packing structure,and the pore size and distribution changes accordingly.The microstructure changes in the thixotropic process are mainly reflected in the gradual tendency of dispersed particles to flocculate,and the adjustment of pore size and its distribution.During both the cyclic softening and thixotropy hardening,the grain size and arrangement characteristics of the structural units and pores change significantly.While the morphological characteristics donot change very much,but they all have the characteristics of self-similar fractal.(5)Based on the microstructural evolution tendency during the change of strength,as well as fully considering the grain size and arrangement of the structural units,a new indicator call"microstructural entropy" is define.It can better quantify the microstructural evolution of soil from the perspective of entropy changes.The larger the microstructural entropy,the more disorder of the soil microstructure system,and the more stable of the soil will be.Conversely,the lower the microstructural entropy,the more disorder and unstable the soil system will be.During the cyclic softening process,as the cycles of T-bar penetration increase,the microstructure entropy of the soil gradually decreases,and the soil structure develops from a disordered,balanced and stable state into a state with an ordered,unbalanced and unstable state.And this process is accompanied by the decrease in soil strength.During the process of thixotropy hardening,as the thixotropy time increases,the microstructure entropy of the soil gradually increases,and the soil structure gradually changes from an ordered,unbalanced,and unstable state to a disordered,balanced and stable state.And the soil is accompanied by an increase in the strength.(6)Using the microstructural entropy parameter as a link,the microstructural evolution model of deep-sea soft clay is preliminary built,and the relationship between the macro-mechanical properties and the microstructure evolution is further established.During the cyclic softening,there is an exponential relationship between the ratio of normalized strength and microstructural entropy and the number of cycles.While in the thixotropy hardening process,there is a logarithmic relationship between the ratio of normalized strength and microstructural entropy and thixotropy time.The research in this thesis can not only provide some reference and guidance for the design of deep-sea seabed foundations,as well as provide evaluations of its safety and stability at the engineering level,but also help to reveal the inner mechanism of both cyclic softening and thixotropy hardening of deep-sea soft clay at the theoretical level.