Study On The Mechanism Of Cone Penetration Test By Numerical Simulation

Cone Penetration Test(CPT)is an in-situ test method to classify soil layers and to determine the mechanical properties of soil.The CPT works by pressing the metal probe into the soil and analyzing the penetration resistance.Piezocone penetration test(CPTU)is an enhanced test method based on the CPT.By measuring the excess pore water pressure and its dissipation using sensors,the CPTU can be used to infer the permeability and consolidation characteristics of soil and hence produce better results.Owning to its advantages of continuity,rapidness,multi-function and easy automation,the application of CPT/CPTU in geotechnical engineering is becoming more and more extensive.However,most engineering practices are now based on empirical or semiempirical formulas,the understanding of its mechanism is still limited.Research focused on the mechanism of cone/piezocone penetration test is of great significance to the geotechnical engineering of China.In this work,the finite element software ABAQUS is used to simulate the process of cone/piezocone penetration tests in sand,stratified sand and clay,and the penetration mechanism is analyzed.The following conclusions have been reached:(1)The coupled Euler-Lagrange method is used to simulate the penetration process in sand and the variations of tip resistance and side friction with depth are obtained.The numerical result is compared with existing calibration chamber test result.The distributions of stress,velocity and plastic deformation in the surrounding soil are analyzed and an area with near zero radial stress is found right below the tip of the cone.Comprehensive analysis of the simulation results suggests that the penetration process of the probe in sand should be seen as a cavity expansion problem,rather than a bearing capacity problem.(2)The penetration process of the probe into stratified sand is simulated.The changes of the penetration resistance near the interface of adjacent soil layers and the deformation of the interface itself are analyzed under different conditions.A method to find the location of soil layer interface based on the trend of the probe resistance is proposed.The penetration process of the probe into sand with weak interlayers is also simulated.The changes of penetration resistance and stress distribution in the weak interlayer are analyzed and the minimum thickness of a weak interlayer that can be identified from CPT measurement is discussed.A method is proposed to calculate the thickness of the interlayer based on its influence depth on the probe resistance.(3)A numerical simulation is carried out for the CPT in clay with the arbitrary Lagrange-Euler method.The distributions of stress,displacement and excess pore water pressure(including its dissipation process)in clay near the probe are emphatically analyzed.The interaction between the probe and clay,the deformation mechanism of clay and the movement of pore water are discussed.The difference of excess pore water pressure at the shoulder and tip of the cone is analyzed.The penetration mechanism in clay is compared to that in sand.Due to the excess pore water pressure in clay during penetration,the distributions of effective stresses in clay and sand are different.After dissipation of the pore water pressure,the overall distribution of stress becomes similar.However,there is no area with near zero radial stress in clay below the tip of the cone,which indicates that the penetration mechanism in clay and sand may be different.