超深基坑减压降水引发地面沉降的估算及其影响因素分析

Influence factors analysis and calculation of land subsidence caused by dewatering of ultra-deep foundation pit

为了深入认识厚层软土地区超深基坑减压降水引发地面沉降的过程,并为地面沉降防治及基坑工程安全提供技术参考,以上海地区为例,从工程水文地质特性、影响因素、综合分区、沉降估算等方面开展了前瞻性研究.超深基坑减压降水需求估算结果表明,超深基坑减压降水将会涉及更多、更深的承压含水层;在分析总结超深基坑减压降水引发地面沉降的主要影响因素的基础上,进行了综合分区研究;基于数值计算和多元回归分析,提出了距基坑3倍开挖深度处降水目标含水层水位降深值及地面沉降量计算公式,并以上海某在建典型超深基坑工程为例进行了验算应用探讨.

Soft soils with high water content,high porosity ratio,high compressibility,high sensitivity and low strength are widely distributed in the southeastern coastal areas in China,which are very easy to cause land subsidence. With the continuous utilization of underground space in soft soil development area,many foundation pits engineering have been implemented,and the depth of excavation is getting deeper and deeper. The land subsidence caused by dewatering in foundation pit to prevent surge and guarantee construction safety will become more complex. In order to understand the process of land subsidence caused by dewatering of ultra-deep foundation pit and provide technical reference for the land subsidence prevention and engineering safety,therefore a prospective study which took Shanghai as an example has been carried out on the hydrogeological characteristics,influencing factors,comprehensive zoning,subsidence estimation and so on. First,the groundwater level drawdown of several confined aquifers was estimated with the regional hydrogeological characteristics in Shanghai. The estimation results show that dewatering of ultra-deep foundation pit would involve more and deeper aquifers,such as the second and third confined aquifers. With the increase of excavation depth,the groundwater level drawdown increases correspondingly which would exacerbate the impact on the surrounding environment. Second,the main influencing factors of land subsidence caused by dewatering of ultra-deep foundation pit were summarized. The main influencing factors are stratum combination,excavation depth of foundation pit,insertion depth of waterproof curtain into target aquifer,area of foundation pit and shape of foundation pit. With these influencing factors,the study on comprehensive zoning has been conducted. Then,the formula of drawdown and land subsidence at the point of 3 times the depth of foundation pit excavation were proposed based on numerical calculation and multiple regression analysis. The calculation coefficients of different partitions were also fitted out. Finally,a ultra-deep foundation pit in Shanghai was chosen to do some discussion about the study results. The excavation depth of this ultra-deep foundation pit exceeds 40 m,which must consider the influence of second and third confined aquifers.