A Fixed End Composed of Long Bolts and Mold Bag Concrete for Prefabricated Inner Support in Foundation Pit and Experimental Investigation on its Bearing Capacity

In the internal support system of foundation pit engineering, reinforced concrete support is widely used in foundation pit due to high load-bearing capacity, unrestricted arrangement, cast-in-place end nodes, and good integrity. However, the disadvantages of reinforced concrete support in design and construction are also significant, including oversized design sections, long construction periods, strong pollution, high costs, and severe waste. Considering the advantages and disadvantages of concrete supports in foundation pit engineering and combining with the characteristics of deep foundation pits in subway cut-and-cover stations, a new type of prefabricated composite steel tube concrete internal support is designed. From the perspectives of improving the stiffness and strength of the end nodes and achieving fixed connections at the ends, a matching new type of end node is proposed, called the “A fixed end composed of long bolts and mold bag concrete.” By elucidating the design scheme, structural characteristics, and working principles of the fixed end composed of special components such as embedded steel plates, hollow flanges, mold bag concrete, and long bolts, it is demonstrated the fixed end functions as a fixed support in the support system. Through the analysis of the mechanical characteristics, failure modes, and evolution laws of the fixed end, the analytical methods for compressive and flexural load-bearing capacities of the fixed support end are obtained. To verify the constructability and construction effect of the prefabricated internal support system, the field technique test of the prefabricated internal support system was conducted.