Study On K₀ Consolidation Undrained Strength Of Structured Soft Clays

Soft clays are widely distributed all over the world.In the long-term sedimentary and post-sedimentary process,soil structure is formed due to various physical and chemical and factors such as leaching,salt solvent extraction,thixotropy,etc.Accordingly,a significant difference in mechanical properties between natural sedimentary undisturbed soft clays and the corresponding reconstituted soils.Most of the existing soil mechanics theories,however,are based on the study of reconstituted soils and not reflected the effect of soil structure.To this end,most studies on soil structure have been made by comparing the differences in mechanical behavior between undisturbed and reconstituted soils from domestic and foreign scholars.In addition,natural soils are in the state of K₀ consolidation instead of isotropic consolidation.K₀is the earth pressure coefficient at rest,which is defined as the ratio of the soil in-situ horizontal stress to the vertical stress.Therefore,K₀ consolidation undrained shear tests have been done on undisturbed and reconstituted clay samples of typical natural soft soil in this study.The effects and corresponding mechanism of consolidation mode,initial water content,stress ratio on the undrained shear strength of soils have been discussed and revealed.Main contents are summarized as below:?1?Investigate the undrained shear strength properties of soft soils under K₀consolidation.Experimental investigations have been made to discuss the effect of K₀consolidation on undrained shear behavior on natural undisturbed and reconstituted soils by conducting isotropic consolidation and K₀ isotropic consolidation undrained triaxial shear tests.Test results show that The K₀ consolidation undrained shear strengths for both undisturbed and reconstituted soil samples under K₀ consolidation are higher than that of the isotropic consolidation.In the pre-yield state,the consolidation undrained shear strength of undisturbed soils increased slightly with consolidation pressure due to soil structure,which is higher than that of reconstituted soil.When the triaxial consolidation stress is higher than the yield stress,the undrained shear strength of undisturbed soil specimen gradually tends to the reconstituted soil specimen with increasing p'resulting from the gradually loss of soil structure.When the influence of soil structure is completely disappeared,the variation of undrained shear strength with increasing p'for undisturbed sample shows the same trends in accordance with the undrained strength for reconstituted soil sample.shear strength indexes obtained from K₀ consolidation are different from that of isotropic consolidation,which is caused by the different soil particle arrangement between from K₀ consolidation and isotropic consolidation?2?Investigate the undrained shear strength properties of soft soils under different consolidation paths with different stress ratios.The influence of stress ratio on the consolidated undrained shear strength of reconstituted soil samples is investigated through consolidation undrained shear tests at different stress ratios.Test results show that the water content at the end of consolidation decreases with the increasing stress ratio.The compression index computed from consolidation under different stress ratios increases linearly with the increasing stress ratio.Under lower effective mean stress p',the relationship between the deviator stress and the axial strain shows strain hardening.With the increase of p',the relationship between the deviator stress and the axial strain become strain softening.The consolidation undrained shear strength increases linearly with the increase of p'.Under the same p',the higher the stress ratio,the higher the consolidation undrained shear strength S_u is.A quantitative expression between S_u and p'can be fitted through the test data.For total strength index,the stress ratio has little effect on the cohesion of soil and the internal friction angle increases linearly with the increase of stress ratio.For effective stress strength index,the cohesion and internal friction angle shows little change with the increasing stress ratio.?3?Investigate the undrained shear strength properties of soft soils with different initial water contents under K₀ consolidation.One-dimensional oedometer test and K₀consolidation undrained shear test were carried out to investigate the effect of different initial water content on undrained strength properties of reconstituted soil samples with different initial water contents.Test results show that the one-dimensional compression curve for reconstituted soil sample with higher initial water content is above the soil sample with lower initial water content.With the increase of the consolidation pressure,the compression curves tend to be consistent gradually.Compression curves for reconstituted soil samples with different initial water contents can be well normalized to intrinsic compression line for reconstituted soil samples.The variation of the K₀consolidation undrained shear strength with p'for soil sample with different initial water content is a straight line through the origin.The undrained shear strength for soil samples with different initial water content shows a good linear relationship with the normalized water content?w/w _L?in log-log scale.The effect of the of initial water content on total shear strength index is mainly reflected in the difference of the total internal friction angle.The total internal friction angle?_cuu decreases linearly with the increase of the initial water content.The initial water content has little effect on the total cohesion,effective cohesion and effective internal friction angle.The ratio of undrained shear strength to the effective vertical stress?Su/?'_v?is close to 0.33 for reconstituted soil samples with different initial water contents under different p'.The undrained shear strength of reconstituted soil sample with different water content falls on the intrinsic strength line?IS_uL?.