过江隧道深基坑中SMW工法加钢支撑围护结构现场监测分析

杭州庆春路过江隧道是"钱江第一隧",其江北岸基坑是典型的粉性土基坑,最大开挖深度16m,主要采用SMW工法(劲性水泥土搅拌连续墙)加钢支撑的围护结构体系,围护桩最长达27m。基坑开挖过程监测数据表明:围护桩的最大水平位移与开挖深度及时间密切相关,支撑的架设及内部结构能很好限制桩体变形;气温、降雨等外界条件的变化对支撑轴力的影响较大,临近基坑支撑的拆除也会产生重大影响;钢支撑轴力均未达到设计值,应对设计方案进行优化;基坑降水及由此引发的渗流会改变土体有效应力,是基坑周围地表沉降的主要原因,同时相邻基坑的施工也会产生一定影响;地下水位的变化能很好反应围护桩的止水效果,可作为判断基坑是否出现漏水的指标。对于粉性土基坑,有效控制基坑周围水的变化,对保持基坑安全有重要意义。

IN-SITU MONITORING ANALYSIS OF RETAINING STRUCTURES OF SMW PILES PLUS STEEL SUPPORT IN DEEP FOUNDATION PIT OF A RIVER-CROSSING TUNNEL

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.