上海超深基坑地下连续墙的空间变形特性实测分析

Field tests on performance of diaphragm wall for an ultra-deep excavation in Shanghai soft ground

基于大量现场实测数据,对上海城区某大型通道工程超深基坑在开挖和降水耦合作用下地下连续墙的空间变形特性及规律进行了详细分析与研究。结果表明:①当基坑长深比、长宽比较小时,受坑角效应影响,地下连续墙变形呈现显著的三维效应,且局部出现较大横向挠曲,其中靠近坑角两侧的局部挠跨比大于中部,靠近坑角两侧的横向挠跨比平均值约为(0.32~0.56)δ_(h)/H_(e),跨中位置横向挠跨比平均值约为(0.15~0.23)δ_(h)/H_(e);②地下连续墙最大侧移δhm随着开挖深度H的增加而呈非线性变化,特别地当开挖深度H超过12m之后,其变化率明显增加;③大幅度坑内超前预降水的卸荷效应会造成地下连续墙先期变形显著增加,并导致后续累计变形的大幅增加,受此影响,基坑开挖期间地下连续墙最大侧移及竖向变形分别为0.7%He,0.1%He(回弹),坑中立柱回弹变形最大为0.2%He,约为地下墙回弹的两倍,各变形量均显著大于上海地铁标准车站基坑(深度16~20 m)的实测统计数据。因此,在超深基坑施工过程中应尽量采用分步降水方式,避免一次性预降水。

Based on the field observation,the spatial characteristics of the diaphragm wall for a 31.5 m-deep excavation of a passageway project in Shanghai downtown under the coupling effects of excavation and dewatering are investigated.The results show that:(1)With the small length-depth ratio and length-width ratio,due to the corner effects,the lateral deflection of the diaphragm wall exhibits dramatical spatial effects,and the induced flexure along the length is great.The local deflection-span ratio(DSR)near the pit corner exhibits larger volume than that in the middle,with the average local DSR about 0.15δ_(h)/H_(e) to 0.23δ_(h)/H_(e) for the middle-span position and 0.32δ_(h)/H_(e) to 0.56δ_(h)/H_(e) for the position near the corner.(2)The maximum wall deflection increases non-linearly with the excavation depth H,and its rate of change raises especially when the excavation depth exceeds 12 m.(3)The excessive pre-dewatering dramatically enhances the deformation of the diaphragm wall,leading to a significant increase in the cumulative deformation of the subsequent excavation.Consequently,during the excavation,the maximum wall deflections and the maximum vertical displacement of wall top are 0.7%He and 0.1%He(uplift),respectively,while the uplift of column is 0.2%He,twice to the wall uplift.All of them are significantly larger than the statistics of normal Shanghai metro stations(excavation depth of 16 to 20 m).Thus the unloading effects of pre-dewatering should be paid great attention to during the construction of ultra-deep excavations,which should be substituted by step dewatering.