Study On The Mechanical Properties Of Deep Clay Under Various Stress Level

The mechanical characteristics of saturated remolded clay under different stress level and its constitutional relationship and intrinsic mechanism were studied deeply and systemically by means of three methods which include laboratory test, theoretical and numerical analysis. The clay used in the experiment is the typical deep clay distributed in East China. The quantitative analysis of mineral composition and the measurement of basic physical index for the clay show that it is a kind of high liquid limit clay including 68.6 percents of Montmorillonite /illite / kaolinite.The drained direct shear test of saturated remolded clay was carried out using the DRS-1 high normal stress direct and residual shear apparatus under 16 different consolidation pressures which vary from 0.2 MPa to 15 MPa.The experiment result shows that the compression index of saturated clay in high pressure was lower than the one in medium-low pressure.The compression curve of saturated clay in high pressure can be fitted by double broken line and the meeting point was at approximately 1.6 MPa.The relationship computed by experiments between clay's permeability coefficient and void ratio had an exponential relation, from which it can be seen that the permeability coefficient change rate underwent a turning point at 1.6 MPa.The shearing strength of the saturated clay used in experiment went up with increase of vertical pressure,however the internal friction angle in medium-low pressure is 9°higher than the one obtained in high pressure.The shearing strength envelope curve can also be fitted by double broken line and the meeting point was at about 1.6 MPa.At the same time,the direct shear test also proved that the shear deformation turned from plasticity to brittleness gradually with vertical pressure increasing. The volume deformation during shearing showed compression without exception and the compression volume decreased with increase of vertical pressure.Then TSZ30-2.0 medium pressure triaxial experiment apparatus and TATW-500 high pressure triaxial experiment apparatus were introduced to carry out triaxial consolidated undrained shearing test for saturated clay under 11 different consolidation pressures which vary from 0.2MPa to 9 MPa.The experiment results indicated the critical shear stress ratio decreased with rise of confining pressure and the trend can be approximately described by bilinear equation with a dividing point of some 1.8 MPa.The shear strength of saturated clay in high pressure includes cohesion.The critical shear stress ratio would be slightly larger without consideration of cohesion. The final failure form turned from plastic outward squeeze to brittle failure with significant fracture plane with confining pressure increasing.The comprehensive analysis of direct shearing experiment and triaxial undrained shearing experiment showed that critical shear stress ratio that was back-calculated from peak of shear stress in direct shearing experiment corresponds quite well with the one measured in triaxial experiment,in which the former one is 1.5 times lower than the latter. The critical shearing stress ratio of saturated remodeling clays in general and medium pressure was 2 times of the one in high pressure.The compression index of saturated remodeling clays in medium-low pressure was 1.11 times of the one in high pressure. The dividing point of strength and compression characteristics in high pressure and low pressure was about 1.6~1.8 MPa.It seemed that the origin of mechanics property difference for saturated clay in higher or lower pressure lied in the change of pore water. The dividing point of shear deformation characteristics in high pressure and low pressure was about 0.8 MPa, which is different from strength and compression with the reason that shear deformation's intrinsic mechanism is related to the cementation between clay particles.Based on the result of mercury injection experiment of this paper and electric double layer theory, the present algorithm which can be used to predict compression curve in high pressure was modified. It showed that the clay's compression property was determined by its inherent physiochemical mechanism. With the elastic adhesive friction theory,it showed that the friction coefficient in 0.2 MPa was 2 times of the one in 1.6 MPa,which is consistent to the measured result.According to the analysis aiming at mechanics property of saturated clay in high pressure,the paper put forward the CASM-hc model applicable to the saturated clay with cohesion which was based on the critical soil mechanics model CASM. The author embedded these models into the ABAQUS. The numerical simulation of the triaxial undrained shearing test based on the program showed that the two models above can simulate the critical strength property of saturated clay in medium and high pressure quite well, however it presented some deficiency on the analysis of the plastic deformation and pore water pressure changing, which required further research.