Study On CPT Penetration Mechanism And Soil Layer Division Method In Layered Soil Based On Transparent Soil

The analysis and design of geotechnical engineering need accurate stratum characterization and estimation of engineering soil properties.However,due to the difficulty of obtaining undisturbed samples of coarse-grained soil,engineers often rely on field tests to estimate the physical and mechanical properties of soil.Static cone penetration test(CPT)has become one of the most widely used field characterization tools because of its high repeatability,accuracy,simplicity,high data resolution,rapidity and robustness and industry familiarity.Now,it has become one of the most common field soil test equipment.It has the least interference to the ground and provides information about soil classification and geotechnical parameters.At present,according to the response of soil to the load imposed by probe penetration,the CPT theory of soil layer division mainly focuses on the soil behavior type(SBT),and the soil layer is not directly divided according to the continuity of CPT penetration resistance;There is a reasonable explanation for the interlayer interface effect reflected by the penetration resistance of static penetration test(CPT)of layered soil,and this part of the research is still in the stage of engineering experience;The "leading" and "lagging" effects reflected by CPT penetration resistance of layered soil are still in the stage of engineering experience,and there is no reasonable explanation;There is no test to verify the interaction between cone tip and layered soil,soil deformation and layered soil interface effect in the process of penetration.At present,it is more realized by numerical simulation.With the emergence of transparent soil technology,the visualization of penetration process can be realized and the internal deformation of soil can be observed,which is of great significance to reveal the penetration mechanism.In this paper,the model test of CPT penetrating into layered transparent soil is designed and carried out.The deformation and penetration resistance of CPT penetrating into layered soil are measured.Combined with theoretical analysis and discrete element numerical method,the penetration mechanism of CPT in layered soil and the influencing factors of interface effect in layered soil are studied.The main contents include:(1)CPT penetration test in layered soil based on transparent soil.Through the design and implementation of CPT penetration resistance transparent soil model test,the CPT penetration resistance curve and the internal deformation of soil under different working conditions are obtained,and the images obtained from the test are processed by digital image processing technology(PIV).The test results show that in single-layer soil,the influence of probe on sand is greater than that on clay;The direction and influence range of soil displacement change with the change of penetration depth;In layered soil,due to the existence of soil interface,the displacement and influence range of soil will be greatly affected.The soil with strong relative performance plays a leading role in the penetration process.(2)Theoretical study on CPT penetration mechanism of layered soil.On the basis of Mohr-Coulomb theory and the conclusion of reaming theory,the soil disturbance range in the process of probe penetration is obtained,the theoretical value of the applicable domestic probe geometric size and actual demand parameters on the cone resistance is deduced,and the relationship between the plastic zone influence range of penetrating layered soil and penetrating probe is obtained.Through this new method,the phenomenon of "leading and lagging" in penetrating layered soil is quantitatively explained.Compared with the complexity of the current soil layer division,the soil layer can be divided directly according to this method,which improves the efficiency and accuracy of soil layer division(3)Numerical simulation study.The discrete element software PFC2 D is used to establish the model of CPT penetrating into layered soil.The simulation results are compared with the theoretical analysis and test results to verify the rationality of the model test and theoretical analysis.On the other hand,by setting different probe radius,probe penetration speed and physical and mechanical parameters of upper and lower soil layers,the influence of CPT penetration into layered soil on Soil and interlayer interface effect are explored.The simulation results show that the "advance" depth increases with the increase of penetration speed,probe radius and overburden strength;The "lag" depth decreases with the increase of the strength of the underlying soil layer,while the penetration speed and probe radius have no significant effect on the "lag" effect.The conclusion of this paper can provide some reference for dividing soil layer according to CPT penetration resistance.