Experimental Study On The Influence Of Sturucture Of Soft Soil On Shear Band

As an intrinsic property of natural soil,structure has a great influence on the engineering mechanical properties of soil.Especially for coastal soft soil foundation with strong structure,under the influence of construction disturbance such as foundation pit excavation and shield tunneling,the soil structure weakens,the strength decreases,and the deformation increases continuously,which easily leads to progressive instability and failure of the soil.In the case of progressive instability of soft soil,the failure of the soil usually proceeds from uniform deformation to strain concentration,then to local instability,and then to macro-localization band(usually shear band),which eventually leads to progressive failure of the overall instability of the soil.Therefore,understanding the structure of soft soil and its relationship with the onset of shear band will be the key to deduce the gradual instability catastrophic process of coastal soft soil engineering.At the same time,because it is difficult to obtain a large number of soil samples with the same structure under natural conditions and the repeatability requirements of the tests,the model tests often need to rely on the artificially structured soft soil.In this paper,the typical undisturbed Ningbo silty clay,a group of artificial structured soft soil with different structural strength by adding a small amount of cement and salt particles into the reconstructed silty clay and completely remolded silty clay are studied.The simulation effect of the artificial structured soil on undisturbed soil and the differences in mechanical properties of different structural Soils are analyzed by using consolidated undrained shear triaxial tests(CU)under four different confining pressures.And the influence of structure change on the shear band of soft soil are revealed by particle image velocimetry(PIV).The purpose of this paper is to lay a foundation for the study of the influence of soil with different disturbances on engineering and the preparation of artificial structural soil in model tests.The main conclusions are as follows:(1)A series of structured soft soils with structural differences were constructed by adding a small amount of cement and salt grains to the reconstructed Ningbo coastal silty clay.The CU triaxial test showed that the structure of the sample with 2%cement content are similar to the undisturbed Ningbo silty clay.The goal of artificially structured soil to simulate the undisturbed soil was achieved.(2)With the increase of cement content from 0%to 2%,the effective cohesion c' of the artificially structured soft soil increases in a approximate exponential form,and there is no obvious change in the effective internal friction angle ?',and the initial deformation modulus E0 increases.(3)Under the same confining pressure,with the enhancement of soil structure,the specimens are more likely to be localized in strain and destroyed in the form of shear band;the stress-strain curve is more probably to be strain-softened,and the peak stress increases and the corresponding strain should be less.(4)Under the condition of CU triaxial test,the shear band of structured soft soil occurs in the stage of strain-softened and the occurring point is close to the peak point of stress-strain curve.With constant confining pressure,the corresponding axial strain of the occurring of shear band under CU triaxial loading decreases with the increase of the structure of soft soil.For the same structural soft soil,the corresponding axial strain of the occurring of shear band under CU triaxial loading decreases with the increase of confining pressure.(5)For the same kind of soil with different structure,the inclination angle of shear band will not be different significantly.The Mohr-Coulomb theory agrees well with the measured value of the inclination angle of shear band.However,both the Roscoe theory and the Arthur theory underestimate the inclination angle of the shear band of soft soil.