Study On Conversion Coefficient Of Negative Friction Resistance Of Self-balancing Test Pile In Expansive Soil

Due to its unique loading method,the self-balancing test pile method is an advanced testing technology at present,which makes the static load test simple and easy to carry out,safe to operate,free from site constraints,and can greatly save testing costs.Since the stress mechanism of the self-balancing test pile is different from that of the traditional static pressure pile,when determining the ultimate bearing capacity of the self-balancing test pile,the ultimate bearing capacity of the upper and lower sections of the pile should be determined according to two completely independent tested piles,and then the ultimate bearing capacity of the test pile can be determined by using the equivalent conversion method.Among them,the value of negative friction resistance conversion coefficient is the key to the accuracy of bearing capacity of self-balancing test pile,which should be determined according to the actual situation through the comparison test under similar conditions and regional experience.At present,there are few comparison tests on special soils,and there is no reference value of conversion coefficient.Comparing the self-balancing test in special soil area and obtaining the value law of negative friction resistance conversion coefficient in this area will be beneficial to the popularization and application of self-balancing test pile method.In view of the value problem of negative friction resistance conversion coefficient of self-balancing static load test in expansive soil area,this paper carried out the model comparison test of traditional static pressure pile and self-balancing test pile under the same boundary condition of expansive soil,and analyzed the value of ultimate bearing capacity and negative friction resistance conversion coefficient.Expansive soil,as a special soil,has "uplift" effect on pile body due to expansion in water,which increases negative friction resistance.Therefore,model tests under two working conditions of immersion and non-immersion are adopted to analyze the value of negative friction resistance conversion coefficient and moisture content of expansive soil.At the same time,in order to further expand the influence factor analysis of the value of conversion coefficient,based on the theoretical basis of the temperature field equivalent humidity field,the finite difference software FLAC3D was used for the numerical analysis of traditional static load test and self-balancing static load test.The main conclusions are as follows:(1)When expansive soil is not immersed in water,its self-balancing negative friction resistance conversion coefficient is 0.63,which is similar to that of clay soil.However,the self-balancing negative friction resistance conversion coefficient of expansive soil is 0.82,which is larger than the recommended value of clay.The self-balancing test curve with equivalent conversion coefficient has a higher matching degree with the traditional static load test curve.(2)There is a polynomial relationship between moisture content of expansive soil and negative friction resistance conversion,which is y=7.441 ×10-4x2-0.0502x+1.471,and the correlation coefficient is R2=0.9771.(3)The free expansion rate and shear strength not only have a great influence on the ultimate bearing capacity of test pile,but also increase the value of negative friction resistance conversion coefficient,which is due to the "uplift"effect of expansive soil.Therefore,in the expansive soil with strong expansion potential and large cohesive force,the measured value of the pile on the upper section of self-balancing test pile is close to the actual bearing capacity,so the negative friction resistance conversion coefficient can be taken as a large value.(4)The pile length and diameter have a great influence on the ultimate bearing capacity of the test pile,and as the pile length and diameter increase,the negative friction resistance conversion coefficient also increases,indicating that the loading value of the upper segment of the self-balancing test pile with large diameter and super-long length is close to the real bearing capacity,that is,the negative friction resistance has little influence on the self-balancing test pile with large diameter and super-long length.