Necessity of Inverted Supports in the Inclined Strut Section of the Foundation Pit with Strip Bottom Plate and Diaphragm Wall

[Objective] In order to save project investment, strip bottom plates that are not fully closed are used for the foundation pit bottom sealing in some foundation pit inclined strut sections. After the removal of the lowest supports in such foundation pits, there will be a large abrupt change in the diaphragm wall. It is necessary to study the necessity of constructing the inverted support in foundation pit inclined strut sections with diaphragm wall and strip bottom plates, and the deformation control effects of the foundation pits with different inverted supports. [Method] In a case study of a metro line foundation pit in Beijing without inverted support after the support removal, an early warning of the retaining structure deformation occurred. The deformation of the retaining structure of the foundation pit after the support removal is analyzed through the monitoring data. Based on numerical simulation, the deformation control effects of different inverted support types on the foundation pit are studied. [Result & Conclusion] After the support removal in the inclined strut section of the foundation pit with an ultra-wide and thin-walled diaphragm wall, the deformation of the foundation pit exceeds the design threshold. For such foundation pits, it is necessary to standardize the construction of the inverted support. The deformation rates of the foundation pits with horizontal inverted support and inclined inverted support are 0.078 and 0.366 respectively, which differ by 4.68 times. The deformation control effect of the horizontal inverted support is significantly better than that of the inclined one. When the full-height scaffold is used as an alternative scheme for the inverted support, the foundation pit deformation rate is 0.166, with a better effect than that of the inclined inverted support but weaker than that of the horizontal inverted support. Among different types of inverted supports, only the horizontal one and the full-height scaffold can control the deformation of the foundation pit after the support removal within the design threshold. The fully closed bottom plate structure produces sufficient effects in the stress conversion stage of the foundation pit after the support removal, and the deformation rate of the foundation pit is 0.304.