Inverse Analytical Method For Strength Parameters Of Soils Around The Cast-in-place Concrete Pile

The bearing capacity of pile foundation mainly depends on the limit shear strength of soil around the pile. So, it is main factor for shear strength index of soils to determine the bearing capacity of pile foundation. The soil strength parameters consist of cohesion c and internal friction Ψ. These two parameters are usually tested by soil test, including direct shear test, triaxial compression text, and so on. However, the soil samples are needed to be separated from its working environment, and its real fracture plane can not be accurately simulated, so the parameters errors are larger. So far, experts all over the world has tried a series of improving tests to simulate the real working environment of soil, but failed to achieve the goal fully.On the basis of the mechanical characteristics relation of the soil samples around the pile and the soil samples of triaxial compression test, the soil stress state around pile and under pile tip were researched respectively. Using the elastic mechanics theory, on stress boundary condition of the normal uniform stress and shear uniform stress at one side,1/4 infinite plane stress solution were solved. The Mindlin’s stress solutions were adviced to solve the stresses of soil element under pile tip. And then, the soil element fracture surface, the first principal stress and the third principal stress were determined. According with the no continuum of geotechnical material, the stress attenuation coefficient of pile side soil was proposed. Then, using the Mohr-Coulomb criterion and stress path theory, author proposed the inverse analytical method to solve strength parameters of soil, and this method was named Mohr stress circle diagram method. For soil on pile side, using the stress solution method of soil element on pile side in this paper and the stress attenuation formula, through the stress path theory, the location of the limit equilibrium state was determined. For soil under pile tip, the stresses in the effective compression layer were calculated by the Mindlin’s solution, through the stress path theory, the location of the limit equilibrium state was determined. Then, the Mohr stress circle method was used for clay material and cohensionless soil separately to determine the cohension and internal friction angle of soil.After establishing the inverse analytical method, the strength parameters of the soil around cast-in-place concrete pile (CCP) were solved by the loadings and settlements of CCP static load test. Calculations were used in limit analysis of FEM, strength reduction method (SRM) and load increment method (LIM). The P-s curves of LIM were fit for P-s curves of static load test. Then, the ultimate loading was researched by SRM, it further proves that the Mohr circle graphic method is correct.