基坑坑底施工阶段围护墙变形监测分析

 控制基坑变形已成为深基坑工程中的核心问题，而坑底开挖施工阶段围护墙体的变形在基坑开挖施工中占有相当的比重。结合上海轨道交通某在建地铁车站基坑开挖时的变形监测数据及跟踪工况，对不同坑底土体暴露时间所对应的围护墙体变形情况进行了对比分析。分析结果表明，坑底土体暴露时间对土体暴露期间及整个坑底施工阶段的基坑变形有着显著影响，随着坑底土体暴露时间的延长，坑底施工阶段墙体变形量可达总变形的30%，缩短土体暴露时间是控制坑底施工阶段围护墙变形的关键。同时，素混凝土垫层对围护墙体变形具有一定的支撑效应，在一定程度上能起到类似结构底板的作用。施工中应尽快完成混凝土垫层的浇筑工作，尽早发挥垫层对围护墙体的支撑作用。另外，垫层对墙体的支撑效果与其自身的平直度等因素有很大关系，破坏垫层的整体性，则会削弱其对围护墙体的支撑作用。提出对坑底施工阶段基坑变形的控制措施，尽可能缩短坑底土体暴露时间及整个坑底阶段的施工时间，提高垫层的整体性，加快底板施工，尽早形成底板混凝土对围护墙体的支撑作用，从而控制开挖阶段的基坑变形，对其他深基坑工程具有一定的参考价值。

MONITORING AND ANALYSIS OF LATERAL DEFORMATION OF RETAINING WALL DURING BOTTOM EXCAVATION IN DEEP PIT

Controlling the lateral deformation of retaining wall is one of the hot issues in deep excavation，and the lateral deformation during bottom excavation occupies considerable proportion of that in the whole excavation works. Based on measured data and practical work of a metro station construction in Shanghai City，lateral displacement with different soil exposure times during excavation is analyzed. As a result，the exposure time of bottom soil has notable influence on the retaining wall¢s lateral deformation during the periods of both soil exposure and bottom excavation. As the increasing of soil exposure time，incremental lateral deformation of retaining wall on the stage of bottom excavation construction can reach 30 percent of the total displacement. Shortening soil exposure time is the key point of controlling lateral displacement on the stage of bottom excavation. In addition，concrete cushion plays critical bracing function on controlling lateral deformation of the retaining wall，which is similar to the reinforcement concrete bottom plate. Accelerating the construction of concrete cushion as soon as possible can make full use of bracing effect on controlling the lateral deformation of retaining wall. Bracing effect of concrete cushion depends on several factors，such as concrete cushion¢s strength，stiff and flatness. The effect can be weakened with debasement of concrete cushion¢s entirety. Some proposals for lateral deformation controlling during bottom excavation of deep pit are offered. Shortening bottom excavation soil exposure time and bottom construction time are favorable of lateral deformation controlling. Increasing concrete cushion¢s entirety and promoting construction progress of the reinforcement concrete bottom plate can reduce retaining wall¢s lateral deformation on the excavation stage. The results can provide references to other similar deep excavation engineering.