Abstract: The waterrich sandy stratum is characterized by large permeability coefficient, noncohesion, easy liquefaction and poor selfstability. When constructing a foundation pit in such area, accidents would be easily induced due to improper design and construction. Hence, the main causes for foundation pit leakage of a deep foundation pit of Nantong Metro are summarized from the aspects of geological conditions, design and construction of foundation pit by data monitoring on leakage section and analyzing on retaining structure design. The causes include: (1) The foundation pit is located in a deep and thick waterrich sandy stratum with high groundwater level and permeability. (2) The seepage channel existing in the encrypted section of the diaphragm wall reduces the water tightness of the diaphragm wall. (3) The variation of monitoring data did not receive serious attention.

Key words: deep metro foundation pit, waterrich sandy stratum, leakage, groundwater