Performance of a deep excavation in Lanzhou strong permeable sandy gravel strata

A large air shaft engineered between the Olympic Sports Center Station and Century Avenue Station of the Lanzhou Metro Line one, China, occurred in strong permeable sandy gravel complex strata. The short distance between the Yellow River and the pit (shaft) resulted in many challenges during construction. These challenges mainly included the quick recovery of the water level and the difficulty of plugging the water. Some measures for monitoring the structural safety and response of the surrounding environment were taken. As the data showed, the design combined a bottom-up method with a top-down method using a peripheral cut-off wall for anti-seepage treatment, the underground diaphragm wall and the concrete bracing had a large rigidity, good impermeability and little influence on the surrounding environment in the process of dewatering and excavation, the external plastic concrete cut-off wall could contribute to the groundwater drawdown inside the pit and reduce the soil pressure on the retaining structure of the pit.