Study About The Effects Of Lateral Pressure On Axial Force

The soil pressure and the pore water pressure close to the pile-soil interface are directly acting on the pile body,which is more important than the pressure away from the pile body.However,the test of the former pressure has not been reported in the references,limiting the deep study in pile-soil interface characteristics and bearing capacity Mechanism.The Lame formula of the elastic mechanics shows that the value of external radial pressure of the pile affects the value of the vertical axial force.In this paper,with the micro-FBG strain sensor,micro-silicon piezoresistive earth pressure sensor and pore water pressure sensor,static pressure pile Indoor model test was conducted in the cohesive soil foundation.The foundation of the model was prepared by "flooding method".The test studied the changes of soil pressure,pore water pressure and axial force at pile-soil interface during the pile driving and analyzed influences of lateral pressure on vertical axial force.The main research work and achievements are as follows:1 In this paper,we got the change laws of them,by installing the micro-silicon piezoresistive earth pressure sensor and pore water pressure sensor in the groove of the model pile.The laws show that: at the initial stage of penetration,the increasing trend of the excess pore water pressure is slow;with the increase of penetration depth,increasing trend of the pressure is approximately linear;when the pressure reaches the maximum and then it slowly dissipate.The earth pressure of the pile side gradually increases along the depth,initially change quickly and gradually slowed down with the increase of soil penetration depth.The lateral pressure of the same depth soil layer decreases with the increase of soil penetration depth.The distribution of effective lateral pressure is basically consistent with the distribution of lateral pressure.The value of excess pore water pressure during the pile driving process is small,accounting for 1 ~ 2% of the lateral pressure value and the maximum can reach 3%.2 In this paper,the micro-FBG strain sensor is used to test the axial force of the pile during the penetration process.The axial force of the pile can be calculated by the pile force and the pile lateral friction force.With the increase of penetration depth,The calculated and measured values of the force both increase and the measured value is greater than the calculated value.The lateral pressure produces both vertical and horizontal stresses to the pile.The increase of axial force is caused by the lateral pressure of the pile,which is about 1 to 2 times of the effective lateral pressure value under the same depth and the maximum can reach 2.7 times.For partially confined pile,the vertical stress of the pile is affected by lateral pressure and the increment of the vertical stress is considerable.Therefore,the influence of lateral pressure should be taken into consideration when studying the residual stress of piles.That is to say,there is side pressure producing some part residual stresses,which has never been involved in the previous study.3 FLAC 3D numerical simulation software is able to well solve the discontinuity large deformation problem of static piles.It is proved that when the diameter of bucket is more than 10 times of the diameter of model piles,the result of the experiment is not affected by boundary effect.4 In order to analyze influences of the boundary effect on the test,a second test was carried out at the center of the model at a distance of 20 cm from the boundary of the model barrel,which was equivalent to piling at the center of the model foundation with a diameter of 40 cm.The results show that the change law of the pile pressure and lateral pressure is almost same as the law in first test,but the numerical value increases;the penetration depth of the excess pore water pressure reaching its maximum value is deeper than that of the first test and the dissipation rate is slower;measured axial force is much larger than the calculated value which is close to the theoretical expectation,but it almost has nothing to do with the actual situation.Therefore,under the influence of the boundary effect,the influence of the lateral earth pressure on the axial force will be overestimated,which is not conducive to the analysis of the residual stress.