砂性土中自平衡试验转换系数取值研究

Research on conversion factor of self-balanced loading test in sandy soil

自平衡法测试基桩抗压承载力时转换系数取值的合理性直接影响到检测结论的准确性。为确定砂性土中转换系数?的准确性,选取3个工程,主要土层均为砂性土,同一场地均进行了自平衡法和传统静载试验。每个工程中选取自平衡检测的基桩与传统静载的基桩为对比组,其地质条件、施工工艺、桩的几何尺寸、桩顶和桩底高相近或一致,以传统静载法测得结果为基准,采用Matlab编程对自平衡法测得等效荷载-位移曲线进行拟合,得出与传统静载法结果拟合度最佳的?取值。将?的拟合值和?=0.6、0.7和?原工程取值分别得到的等效位移荷载曲线与传统静载试验结果进行对比。结果表明,(1)3个工程中所得转换系数γ拟合值取值范围在0.42~0.71之间,与原工程中采用的γ值相比均偏小;(2)?取值对等效受压桩上段桩总摩阻力和弹性压缩量的影响很大;(3)采用转换系数γ拟合值计算的等效转换曲线与原工程中采用的?值相比更接近传统静载法所得结果,表明3个工程中?的取值均过大,导致所测极限承载力过于保守;(4)砂性土中自平衡转换系数建议取值为0.6~0.7左右。

In the self-balanced loading test, reasonable determination of conversion factor γ, ratio of downward shaft resistance to upward shaft resistance, is critically important for the accuracy of bearing capacity and final decision of pile loading test. To study the appropriate value of γ in sandy soil, 3 typical construction projects are chosen. All of their main soil layer are sandy soil; and the self-balanced loading test and head-down loading test are conducted at every site. And then a pile from self-balanced loading test and one from head-down loading test are picked as a group. In these groups, the geometrical sizes, geological condition and construction method of the two piles are same or similar. Because the results from traditional loading test are more convincing than ones from self-balanced loading test in general, the results from top-down loading test are taken as references; and then the Matlab is used to find the optimal fitting solution of γ, latterly, returning the value to the simplified capacity equation of the self-balanced loading test, and comparing the result of equivalent top-loaded settlement curve with ones from γ used in actual project and from head-down loading test, ? = 0.6 and 0.7. The results show that:(1) The fitted values of ? from 3 projects are ranged from 0.47 to 0.71, which are rather smaller than that used in actual projects.(2) The accuracy of ? has large effect on the values of Qs(skin friction) and sss(elastic compression from skin friction distribution) of upper piles of the equivalent pile.(3) Compared with the top-loaded settlement curve from traditional loading test, the results from the optimal fitting solution of ? fit much better than of which used in actual project,which means that values of ? used in these projects are overly conservative, which cause overly conservative judgement of bearing capacity.(4) It is suggested that the value of conversion factor ? in sandy soil about 0.6 to 0.7.