Study On Anisotropic Undrained Shear Strength Of Soft Clays And Its Application In Excavations

Natrual soft clays with high water content,low permeability and shear strength usually exhibit prominent anisotropy subjected to the effects of deposition and consolidation and the complex stress.In the soft clay engineering,such as excavation process,the soils are often under undrained conditions.Therefore,the studies on the anisotropy of undrained shear strength soft clay engineering problems are of great importance.However,few experimental and theoretical research results on this subject has been reported in the literarture and the researches about its engineering application also needs to be more abundant.In this thesis,a formula for the anisotropy undrained shear strength of the undisturbed soft soil was obtained by experiment and theoretical analysis,and the influence of anisotropy on the basal heave stability and deformation characteristics of soft soil foundation excavation was discussed.The main contents and achievements of this thesis are presented as follows:1?An semi-empiricial formula for the anisotripy which takes into account the soil perpertities(effective internal friction angle)was obtained on the basis of the undrained shear strength tests conducting on undisturbed soft clays in Hangzhou.2?The theoretical formulas for undrained shear strength of soft clays in triaxial compression and tensile stress state were deduced based on the anisotropic constitutive model of undisturbed soft clay in hangzhou.Combining with the semi-empircal formula mentioned above,an anisotrpic shear strength theoretical formula containing less paramrters which can take into account the effect of the internal friction angle have been obtained.3?In the analysis of basal stability with upper bound method and circular sliding method,not only was the anisotropic strength formula mentioned above employed,but the reduction between the shear stress and undrained shear strength on the failure surface has been taken into consideration.4?By the analysis of basal stability,the results calculated using the formula in the theisis was better than that of classic Casagrande formula.5?A subroutines for the anisotropic ideal elastics constitutive model that is based on the above formula was implanted in the UMAT secondary development module in the ABAQUS finite element analysis software to simulate the excavation constructing process.Calculation results show that the displacements of the excavation taking into account the anisotropy are larger than that in the cases of isotropy,and the effect of anisotropy may be more siginificant when the soil is approaching failure state.The development of soil horizontal displacement and vertical displacement and the influence degree of anisotropy on both of them are associated with the ratio of depth and width.The development of the horizontal displacemnt is greatly influencd by width,and the virtical displacemnt is greatly influenced by depth.When the ratio of depth and width is large,the vertical displacement of the soil is larger and the influence of anisotropy on the safety factor will be significant,and in this condition,basal heave failure will most likely take place and a reasonable and accurate result will not be obtained without considering the effects of anisotropy.