深大基坑首层盆式开挖对基坑变形影响分析

以上海中心大厦裙房逆作施工基坑的首层土方开挖为背景，针对盆式开挖盆边预留土堤大小对基坑变形影响进行了深入分析。结果表明，围护墙变形计算值与实测值较为吻合；在逆作法深大基坑中，围护墙厚度在0.8～1.6 m之间变化及是否考虑地墙落低作用对首层开挖的基坑变形影响不明显；采用梁单元模拟围护墙比采用实体单元的围护墙变形计算值要大10%～21%左右，原因是采用后者能考虑土体与墙之间摩擦力所产生的抵抗弯矩作用。预留土堤所承受的水平抗侧压力F随土堤宽度b的减小，先缓慢减小，当b达到为20 m左右时，F值达到最小值，接着随b的减小F值逐渐增加；对特定的盆边开挖深度 ，b值存在一临界值 ，当b减小到 时，F达到峰值，此时如再减小b值，墙体位移将迅速增加；且 值随 值增大而增大。最后根据分析，给出了盆式开挖预留土堤尺寸设计建议。

Analysis of effect of first-level basin excavation on deformation of deep and large foundation pits

On the backgroung of the first-level soil excavation on the podium pit of Shanghai Tower which is constructed with top-down method, the effectiveness of earth berms on pit deformation is analyzed. Analytical results show that the results of calculation agree with field measurements for the lateral deformation of soil. In deep and large top-down excavations, the effects of the changes of retaining wall thickness range from 0.8 m to 1.6 m; and whether to consider lowered wall on the pit deformation are not significant when the first level soil is excavated. When the retaining wall is modeled with beam elements, the calculated value of wall lateral deformation is 10%-21% larger than that when the wall is modeled with solid elements; the reason is that the latter can consider the effect of resistance moment which is generated by the friction between the wall and the soil. The lateral anti-pressure F firstly decreases slowly with the decrease of the width of berms b. When b reaches about 20 m, F reaches the minimum, then gradually increases with the decrease of b. To the given excavation depth of berm boundary , there is a critical value for b; when b decrease to , F reaches peak; if b is further reduced, the wall deformation is increasing rapidly. And increases with the increase of . Finally, based on these results, the design method for earth berms in basin excavation is given.