天津市承压层应力状态及减压引发沉降规律研究

Study of stress state and settlement induced by pressure relief of confined aquifers in Tianjin

基坑工程中坑内承压水降水引发的坑外土体沉降与含水层土体性质及应力状态密切相关,但在天津地区承压含水层的土体应力状态及其沉降变形规律均缺乏系统地研究。首先通过天津市第一、二含水层组的历史水位变化,分析各含水层组土体的应力状态。再结合2组现场抽水试验,分析了各承压层降水-恢复过程土体沉降变形规律。根据对历史数据的分析表明,受补给关系和自然条件影响,天津市第一含水层组中上部水位变化较小,处于正常固结和轻度超固结状态。而20世纪60~80年代天津市深层地下水由于大规模开采形成了以市区为中心的降深漏斗,造成了严重的地表沉降。80年代后限制开采,天津市区的地下水位得以部分恢复,导致第一含水层组底部和第二含水层组处于严重的超固结状态。抽水试验结果表明,基本处于正常固结状态的第一承压层在抽水-恢复过程中变形以塑性变形为主,处于超固结状态的第二承压层变形以弹性变形为主,在反复降水-恢复中仍然会产生一定量的塑性变形,即当水位恢复时承压层压缩变形并不能完全恢复;受'土拱'作用影响,承压含水层降水引发的土体沉降最大值并不位于地表,而是位于抽水含水层上覆弱透水层附近。

In the excavation engineering, the soil settlement outside the excavation caused by pressure reduction in the excavation is closely related to the soil properties and stress state of the aquifer. However, the soil state and the settlement regularity of confined aquifers in Tianjin still lack of systematic research. Firstly, the stress states of the soils in the first and second aquifer groups of Tianjin are analyzed through the historical groundwater level changes. On this basis, combined with two groups of field pumping tests, the settlement and deformation laws of confined aquifers in the process of dewatering and recovery are analyzed. According to the analysis of historical data, the upper part of the first aquifer group in Tianjin is in the normal consolidation and light over-consolidated state affected by recharge and natural conditions. However due to the over-exploitation from 1960 s to 1980 s, the drawdown cone centered on the urban area was formed in deep aquifers of Tianjin, which caused serious surface subsidence. Now the groundwater level has been partially recovered due to the limitation of exploitation from 1980 s. Therefore, the bottom of the first aquifer group and the second aquifer group are seriously over-consolidated. Based on the analysis of the pumping test results, the results show that the deformation of the first confined aquifer in the normal consolidation state is mainly plastic deformation during the pumping-recovery process, while the second confined aquifer in the over-consolidated state is mainly elastic deformation;and the plastic deformation will still occur in the cycle pumping-recovery process. When the groundwater level was recovered, the settlement of the soil cannot be fully recovered. Meanwhile, affected by 'soil arching', the location of the maximum settlement caused by the aquifer pumping was not on the ground surface, but near the aquitard over the pumping aquifer.