Mechanical Characteristics And Deformation Of The Interface Between Double Layers Of Soil

Natural foundations and slopes consist of heterogeneous and anisotropic soils after a long geological deposition and tectonism. Mechanical properties of soils are affected by comprehensive factors, in which the contact and friction effect is an important factor. Weak and discontinuous soil interfaces influence load transmission and deformation of foundations and slopes, such as the “Code for design of building foundation”(GB50007-2011), which reflects the effects of doublelayer soil stress diffusion on foundation bearing capacity. Research on the mechanical properties and the deformation law of double-layer soils on the laboratory-scale is the basis of soil deformation and failure on the engineering-scale. Taken double-layer soils(the upper part is hard and the lower part is soft) as the research objects, this thesis selects reasonable test plans, adopts the self-designed large-size direct shear test apparatus, combines with PIV technology and establishes PFC models by PFC2 D to study the macroscopic shear strength, the shear interface deformation law, the micromechanical stress distribution and particle displacements of doublelayer soils.(1) Problems of interface contact mechanics are generally studied by direct shear tests and PFC2 D shear models to establish the effects of different shear forms, shear sizes and shear displacements on the shear deformation and the shear stress. Forces between the interfaces are uniform if the area of the shear plane is constant. Shear stress versus displacement curves and particle velocity vector distributions at different model sizes show that 400 mm×200 mm is the optimal model size. The ratio of the model length to the model height being 2 can eliminate the boundary effect and the shear stress can reach its peak within the shear displacement of 35 mm.(2) The interface morphology, the preloading stress and the stiffness ratio between the two soil layers are analyzed. The interface morphology includes the plane interface, the interface with sawteeth of 2.5 cm and the interface with sawteeth of 5 cm. The complicated interface morphology can increase the shear strength of the double-layer soils, and lead to sawteeth moving but not be cut. Compared with the specimen without preloading, the hardening stage of the specimen with preloading(100 kPa and 200 kPa) increases and the interface cementation is strengthened, exhibiting fluctuations as well. Considering stiffness ratios of 3.98, 5.36 and 6.42, the greater stiffness ratio leads to the stronger interface cementation and the greater internal friction angles. The shear strength parameters and deformation law of double-layer soils have great discrepancies compared with those of the single layer soil. Analysis of soil shear deformation law by PIV technology shows that the upper soil near the shear interface has greater displacements with fluctuations and the volume is vertically compressed. The increase of the preloading stresses and the decrease of stiffness ratios can result in greater horizontal displacements of the upper soil near the shear interface. Compared with the initial stage of shearing, the rate of horizontal displacements of the upper soil decreases.(3) On the basis of the calibrations of friction coefficient μ, normal strength of contact-bond model n_bond and contact normal stiffness between particles kn, compared with laboratory tests, PFC2 D is adopted to model the large-size direct shear tests. Analysis of shear stress – shear displacement curves and shear strength parameters shows that the PFC shear model can reflect strength softening and the shear stress reaches the peak at the late stage of shearing under interfaces with sawteeth. Increase of the friction coefficient can strengthen interface cementation and the residual shear stress increases after reaching the shear displacement of 3.5 cm. The specific color strips were used to analyze the soil deformation, which agrees well with that obtained by the PIV analysis. With the stiffness ratio between the two layers and the interface friction increasing the width of shear failure zone decrease. The contact forces among particles transmit through the shear plane and concentrate on the contact zone between the two soil boxes. The interface cementation is strengthened, and cylindrical oriented force chain in parallel with the shear plane appear at the interface. 20 measure circles are used to monitor the particle number at the interface during the shearing and the qualities of meso-structures(connection failure rate etc.), which shows that greater interface friction can increase the particle contact at the interface and decrease the connection failure rate. The measure circles,which monitor the normal stress of the interface, evenly distribute at the initial stage. Then the normal stress increases and decreases afterwards with stress concentration on the right of the interface.