Design Methods of the Concrete-filled Steel Tube Inner-bracing System for Deep Excavation

Centered on the characteristics of deep excavation engineering for open-cut subway station projects, a fabricated inner-bracing system of concrete-filled steel tube, which consisted of normal straight components, short components, connection joints, etc., was developed. Based on the features of inner-bracing stress and structure, the internal force calculation formulas were derived from all the components. Referring to the specifications of concrete-filled steel tube and the formulas of column stability, several calculation methods of bearing capacity for the supports and ring-shape purlin were proposed. The mix proportion of filled concrete was designed, and the concrete physical, working and mechanics properties were detected by material tests, and then the lightweight, slight expansion and self-compacting concrete were prepared as the inner-bracing filling material. Taking a deep excavation engineering of a station project in Beijing Subway as a case, the inner-bracing system of normal section foundation pit was designed and studied. The results show that the calculation methods of inner-bracing system design is reasonable and reliable, and the designed inner-bracing system meet the requirements of excavation support force and foundation pit stability.