中密砂土地基中H-T组合作用下单桩承载特性研究

Bearing behavior of a single pile in ground of medium-dense sand under the combined H-T loads

为探讨中密砂土地基中桩顶水平力(H)与扭矩(T)组合作用单桩的承载特性,研制了适合于室内砂箱模型试验的H-T组合加载装置,并通过对比试验获得不同H-T大小组合与加载顺序(H→T或T→H)下的桩身内力变形与承载力,及其对桩身承载特性的相互影响曲线,结果表明:相比单一受荷时的水平或扭转极限承载力(H_u或T_u),H-T组合作用会导致桩身承载力减小,且H→T要比T→H的影响要明显,如先施加2/3T_u时桩身水平承载力减小约12.4%,而先施加2/3H_u后的桩身扭转承载力减幅达48.5%,故不能简单基于叠加原理评估组合受荷时的承载力。在此基础上,引入能反映水平力对桩身扭转承载力影响的桩侧扭转土抗力放大因子α_(TH),基于杆系有限元和m法建立了考虑桩周土约束及H-T耦合效应的桩身单元综合刚度矩阵,进而用MATLAB编制了H-T组合受荷桩的改进杆系有限元法计算程序,并将其结果与模型试验值进行对比分析。

To study the bearing behavior of a single pile in ground of medium-dense sand under the combined loads of horizontal force H and torque T, a loading device of applying the combined loads of H and T on the pile top in indoor model tests was designed. Based on a series of comparative tests, the bearing capacities, inner forces and deformation along the pile shaft under various combined load magnitudes and in different applied sequences （i. e., H→T or T→H） as well as the influencing curves were obtained. The results show that compared with the ultimate bearing capacity under a single load of H or T （i. e., Hu or Tu ）, the combined action of H-T loads may lead to an obvious decrease of the bearing capacity. For example, applying 2/3Tu in advance may reduce the horizontal bearing capacity by 12.4%, while applying 2/3Hu, in advance may result in a reduction of the torsional capacity up to 48.5%. Therefore, traditional superposition principles can＇t be simply used to evaluate the bearing capacity of piles under the combined H-T loads. Finally, based on the FEM and the m method for bar system, an amplification factor aTh reflecting the effect of H on the torsional capacity was introduced to form the unified stiffness matrix of pile shaft, the calculation program of FEM was coded in MATLAB, and then the calculated results of FEM were compared with those of model tests.