Research Of The Penetration Mechanism Of CPTU

To get the geotechnical parameters efficiently and accurately,it is very important for us to calculate settlement and bearing capacity reliably,shorten project time,reduce the project cost to ensure the engineering quality,and improve the social and economic benefit.Piezocone Penetration Test(CPTU)is used to investigate the properties of soil in situ,as the result,the effect is best and application is the most widely.For example,it can effectively divide soil types,measure the soil profile and judge the type of foundation soil,and so on.In the word,the main advantages of CPTU are the more parameters measured.This paper introduces the research progress and current status of CPTU at home and abroad and summarizes its application in engineering.While,the certain geotechnical mechanics is introduced in brief to discuss the theoretical analysis of CPTU.According to the characteristics of CPTU,the finite element theory of CPTU was introduced.Then,the strong nonlinear processing ability and unique ALE(Arbitrary Lagrangian Eulerian)adaptive meshing method of ABAQUS is fully used to numerical simulation of the CPTU.On the one hand,CPTU is different from the simple conditions,laboratory test,the complexity of the condition in-site with more influence factors,but the numerical simulation can analyze CPTU some factors which is isolated and extracted.On the other hand,CPTU mainly takes the feedback analysis for indirect parameters,which is generally not written directly by the test results.This statistical experience and semi-empirical formula just reflect a kind of relationship,in other words,it is a qualitative analysis.Therefore,numerical simulation of CPTU can overcome the shortcomings above.Whether bearing capacity theory or the cavity expansion theory,the boundary of CPTU penetration is approximately treated,it is difficult to accurately reflect CPTU penetration process.Although the experimental results show that the research efficiency is improved,the boundary and disturbance factor affect the test results.The CPTU numerical simulation can overcome this disadvantage of the model slot.For example,it can simplify the condition to make it easier to observe the comparison and find the causation.By numerical simulation of ABAQUS,the distribution of the stress and strain is studied,at the same time,influence factors in the process of penetration are analyzed.What's more,the sleeve pressurecoefficient of soil and penetration rate of CPTU test is explored on the influence of the CPTU.Combined with the scene in situ test and model test.the simulation results are analyzed.The analysis is as follows:(1)When the CPTU is more shallow,it is less than 0.4 m(about 10 probe diameters),the probe has a greater disturbance to the soil.The disturbance decreases with penetrating more deep.(2)In the process of penetration,the area of soil plasticity of the probe is approximately 1probe radius,and the stress bubble is formed near the probe head,its characteristics are in good agreement with the theory of the cavity expansion of Yu.(3)The soil comes to certain dilatancy behaviour and fine cracking with penetration,and the earth's radial pressure is the largest in the soil layer of the tip of CPTU.(4)The optimum position of pore pressure permeable elements is placed at the junction of the cone and cone.(5)The initial stress state of the earth has a significant effect on the resistance of the cone,and the tip pressure coefficient of the earth's stationary side is increased by 0.2 to 1.0,and the resistance of the cone is more pronounced.There is a good correlation between the yield strength of the soil and the resistance of the tip.(6)The change of the penetration rate does not affect the stress distribution of the soil,but the tip resistance decreases small as the penetration rate increases.When the penetrtion rate at 2~10 cm/s,the tip resistance keep steady.The cone tip resistance change obvious as the rate small enough.Therefore,the rate of 2cm/sis usually chosen.(7)Compared with the test of model or in situ,the penetration process simulation of CPTU keep the consistency,such as,the closer to the tip of CPTU,the greater the stress.It draws a conclusion that the numerical simulation appear that the strain of the soil layer located in the tip of CPTU is the biggest.