深基坑水下开挖变形特性及坑底分仓优化

依托北京地铁8号线永定门外站深基坑工程,介绍了适用于水位高、厚度大、透水性强的富水砂卵石地层深基坑水下开挖工法。采用数值模拟方法构建基坑水下开挖数值模型,结合实测数据对模型进行了验证,并利用该模型分析了深基坑水下开挖过程中坑外地表沉降、墙体水平位移变形的特性。结果表明:对于富水砂卵石地层的基坑工程,采用水下开挖及坑底分仓工艺能够较好地控制基坑变形; 坑外地表沉降主要发生在干开挖及疏干开挖阶段,水下开挖阶段引起的地表沉降量只占总变形量的7%左右; 分仓墙的设置可有效限制坑底隆起及基坑中下部变形; 墙体变形在上部支撑及下部分仓墙作用下呈“弓”字形分布,墙体变形大多发生在干开挖及疏干开挖阶段,水下开挖引起的墙体变形只占总变形量的10%左右; 分仓数量及仓位布置形式影响墙体中下部变形,仓位增加至一定数量(20仓)后可明显控制墙体变形; 在满足抗浮要求的情况下,仓位可减少至12仓; 分仓数量一定的情况下,横向布置较纵向布置形式更有利于控制墙体变形。

Deformation Characteristics of Underwater Excavation of Deep Foundation Pit and Optimization of Sub-warehouse at Foundation Pit Bottom

Based on the deep foundation pit project of Yongding Gate Outside Station of Beijing Metro Line 8, the underwater excavation method of deep foundation pit in water-rich sandy pebble stratum with high water level, large thickness and strong permeability was introduced. By means of numerical simulation, the numerical model of underwater excavation of deep foundation pit was constructed. The model was validated by the measured data. Based on the model, the characteristics of surface settlement outside the pit, horizontal displacement and deformation of the wall in the process of underwater excavation of deep foundation pit were analyzed. The results show that for foundation pit engineering in water-rich sandy cobble stratum, the underwater excavation and sub-warehouse technology at the bottom of pit can effectively control the deformation of foundation pit. The surface settlement outside the pit mainly occurs in the stage of dry excavation and dredging excavation, and the surface settlement caused by underwater excavation only accounts for about 7% of the total deformation. The setting of sub-warehouse wall can effectively limit the uplift of the bottom of pit and the deformation of the middle and lower part of the foundation pit. The wall deformation is bow-shaped under the action of the upper support and the lower part of the sub-warehouse wall. Most of the wall deformation occurs in the stage of dry excavation and dredging excavation. The wall deformation caused by underwater excavation only accounts for about 10% of the total deformation. The number of warehouses and the layout of warehouses affect the deformation of the middle and lower part of the wall. When the number of warehouse increases to a certain number(20 warehouses), the wall deformation can be controlled obviously. The warehouse can be reduced to 12 warehouses when the requirements of anti-floating are satisfied. When the number of warehouses is fixed, the transverse layout is more advantageous to control the wall deformation than the longitudinal layout.