大直径扩底桩竖向端承力解析解与试验分析

Analytical solution and test analysis of vertical tip bearing capacity of large diameter belled pile

为了解决目前大直径扩底桩竖向端承力计算方法所存在的不足,将球孔扩张理论应用于扩底桩端承力的计算,建立了扩底桩的球孔扩张理论计算模型.考虑扩底桩桩端外边缘处力矩平衡以及几何对称性,建立极限球孔扩张压力、桩端中心以下主动土压力及扩底桩端承力之间的联系,推导出大直径扩底桩端承力的解析解,求得黏性土和无黏性土中扩底桩端承力的表达式.利用原型扩底桩试验对该公式进行验证,分析得出影响扩底桩端承力的主要因素包括土的粘聚力、内摩擦角、杨氏模量、泊松比、扩底桩的扩底半径和入土深度.试验结果表明:该解答精度满足工程要求;经验方法对扩底桩端承力进行折减只是经验的总结,物理意义不明确.

To solve the shortcomings of the methods for calculating the vertical tip bearing capacity of a large diameter belled pile,the spherical cavity expansion theory is applied to calculate the tip bearing capacity of the belled pile and a calculation model for the belled pile based on the cavity expansion theory is built.Considering the moment equilibrium and the geometric symmetry on the external border of the pile tip,the relationship among the ultimate expansion pressure,the active earth pressure under the tip center and the tip bearing capacity is set up.The analytical solution of the tip bearing capacity of a large diameter belled pile is derived.And the expressions for the tip bearing capacity of belled pile in cohesive soil and cohesionless soil are obtained.The solutions are verified by the prototype test results of the belled pile.The main factors influencing the tip bearing capacity of the belled pile are cohesion,internal friction angle,Yang's modulus,Poisson ratio,radius of the tip and embed depth.The experimental results demonstrate that the precision of the solution can satisfy the engineering demand.Meanwhile,it also shows that the empirical method multiplied by a coefficient to reduce the tip capacity is just a experience summary,whose physical meaning is uncertain.