Observed performance of a deep excavation of Qingdao Metro Line 4 in China

Abstract

The L-shaped excavation of Anshan Road Station, with the maximum excavation depth of about 40 m, is one of the deepest excavations of Qingdao Metro Lines in Qingdao, Shandong Province, China. The deep excavation was constructed mainly in coarse-grained granite of different weathering degrees. A comprehensive field monitoring scheme was carried out to study excavation-induced ground movement and the response of nearby structures. Characteristics of ground surface settlement and pier settlement of Hang’an Expressway Viaduct adjacent to sub-zone D of the L-shaped excavation was studied based on the long-term monitoring data. A high-risk zone of rock failure at sub-zone D was successfully identified based on abnormal variations in the horizontal displacement of pile head and axial force of cable, concentration of microseismic events, and development of a 44-m-long longitudinal crack adjacent to the excavation boundary. A temporary bracing system, consists of two layers of steel struts and built-up steel columns, was designed and conducted to maintain the stability of the high-risk zone of rock failure at sub-zone D. Furthermore, effectiveness of the temporary bracing system in controlling rock failure of the L-shaped excavation was validated by analyzing the evolution of the axial force of steel struts, horizontal displacement of pile head and axial force of cable. This study may be valuable for monitoring, warning and controlling rock failure of deep excavations in hard rocks in dense and busy urban areas.