Full-Scale Field Test and Application of Long Servo Steel Struts for Deep Excavation

Abstract Although servo steel struts are commonly used to restrict excavation-induced deformations, their application is typically limited to narrow excavations due to insufficient stiffness. Then, the long servo steel struts have been introduced to address this limitation. To verify the applicability of long servo steel struts, full-scale tests were conducted on-site before excavation, including tests on the steel struts under unilateral loading (single-side loading test) and bilateral loading (double-side loading test). During the full-scale test, the safety and stability of this technology were validated. For further understanding of the deformation control mechanism of long servo steel struts, they were applied to an excavation project. The strut displacements, axial forces, and deformations of the diaphragm wall and tunnel were analyzed from the full-scale test and excavation case. The results indicate that the loading condition of the hydraulic jacks in the servo strut system and the stiffness of the retaining structure had a significant impact on wall deformations. Under asymmetrical loading, the struts near the diaphragm wall with lower stiffness or axial force experienced greater radial displacements, risking excavation stability. The long servo steel struts better controlled deformations in the upper part of the diaphragm walls with lower stiffness. Although the lower part of these diaphragm walls had more deformations, they were still less than those of the diaphragm walls supported by concrete struts. The uplift deformations and convergences of the tunnel caused by excavation met the requirements. Long servo steel struts effectively controlled excavation-induced deformations and broadened their application. However, axial force, wall stiffness, and working conditions of hydraulic jacks must be carefully considered in design and construction.