基坑支护设计中的若干问题探讨

 对基坑倾斜坡面的主动土压力计算方法、桩锚支护结构的锚杆水平弹簧系数的计算方法以及桩锚支护结构计算主动土压力应否考虑桩、土间的摩擦力等存在异议的问题进行理论分析和简化计算公式的推导，提出基坑倾斜坡面的主动土压力、桩锚支护结构的锚杆水平弹簧系数和考虑桩、土间的摩擦力的主动土压力计算公式。基坑倾斜坡面的主动土压力可以直接采用建立在极限平衡理论基础之上的计算公式计算，而无需采用先计算朗肯主动土压力再进行修正的计算方法。桩锚支护结构中的锚杆头部一般不可能发生竖向位移，可假定锚杆轴向伸长量等同水平伸长量，以此假定计算锚杆水平弹簧系数比较合理。用算例对所推导的计算公式和现行规程的计算公式进行计算比较，工程现象及算例结果均表明，不管是土钉墙还是桩锚支护结构，现行规程土压力计算公式未考虑墙(桩)、土间的摩擦力，计算的主动土压力偏大；现行规程计算的桩锚支护结构的锚杆水平弹簧系数偏小。因此，设计的支护结构均偏安全，不利于控制工程成本，应根据工程实际情况进行适当优化。

DISCUSSION ON SEVERAL PROBLEMS FOR RETAINING AND PROTECTION DESIGN OF EXCAVATION

The problems for calculational methods of active earth pressure of inclined excavation slope plane and the horizontal anchor spring coefficient of pile-anchor retaining and protection structure,and whether considering the friction between pile and soil or not on calculating active earth pressure are discussed,and these calculation formulas are deduced. The active earth pressure of inclined excavation slope plane can be directly calculated using the formula on the limit-equilibrium theory,no need first to calculate the Rankine's active earth pressure and then amend the result. The vertical displacement of pile-anchor retaining and protection structure generally does not occur to anchored bearing,so the axis extension of anchor is assumed equal to the horizontal extension of anchor,and it is rational to calculate the horizontal anchor spring coefficient on this assumption. The deduced formulas are compared with the formulas in code by calculation examples. Both engineering case and the calculating results of the examples show that the calculating active earth pressure using the formula in code is more than the calculating active earth pressure without considering the friction between pile and soil,and the calculating horizontal anchor spring coefficient using the formula in code is smaller than the calculating result using the formula for whether soil-nail wall or pile-anchor retaining and protection structure. Therefore,the retaining and protection structure of excavation should be properly optimized for the purpose of controlling engineering cost.