Areal subsidence under pumping well-curtain interaction in subway foundation pit dewatering: conceptual model and numerical simulations

Subway foundation pit dewatering may contribute to the regional land subsidence in a built-up area when aquifers are too thick to be cut off by a curtain. The land subsidence induced by subway foundation pit dewatering was divided into local subsidence and areal subsidence. The former was managed by construction organizations, while the latter was managed by a land resource, urban management, and hazard prevention department. The areal subsidence could not be recovered after dewatering because of its vast influence area and contribution to regional land subsidence. The boundary between local subsidence and areal subsidence was defined as the location of three times the foundation pit depth to the foundation pit boundary. The subsidence within the boundary belonged to local subsidence, whereas the subsidence outside the boundary belonged to areal subsidence. With Shanghai as background, the conceptual and mathematical models that considered hydrogeological conditions, curtain depth, and pumping well screens were established, and numerical simulations were performed. When the pumping well screens were enveloped by a diaphragm wall, the best vertical distance of about 1.0-4.0 m was obtained between the screen bottoms and curtain bottom. The decreasing rate of areal subsidence was the highest under the interaction of diaphragm wall and pumping well screens. Further increasing the penetration depth of the diaphragm wall and shortening the length of the pumping well screen were both less efficient.