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Abstract The pit at the north side of the Hangzhou Qingchun Road river-crossing tunnel，which is the first one that crossed beneath the famous Qiantang River，is typical silt-based pit，with the maximum excavation depth of 16 m. In this project，retaining structures of steel supports with SMW(soil mixing wall) method are mainly applied，of which the longest piles are up to 27 m. According to the analysis of on-site monitoring data，it could be obtained that：(1) the maximum horizontal displacement of piles is closely related to the excavation depth and time，and the set up of piles and the internal structures played a good role in restraining the piles' displacement；and (2) changes of external conditions such as temperature，rainfall，etc. also have considerable influence on axial force. In addition，the removal of steel support of adjacent pit would also impact axial force significantly. The measured values of steel support axial force are much smaller than the design values，so it is necessary to optimize the preliminary design. The changes in effective stresses resulted from dewatering and seepage are the main factors inducing settlement of ground surface，and at the same time，the construction of adjacent pit has a certain impact on surface subsidence. The variation of groundwater levels could reflect the watertight effect of the retaining structures，which could be used as an important indicator to find out whether there is water leakage. For silt-based pit within excavation depth，it is significant to maintain the pit safety by effectively controlling the changes of water around the pit.
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Received: 08 January 2010
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