南通富水砂性地层地铁深基坑抽水回灌现场试验研究

Field tests on groundwater recharge of deep excavations in Nantong water-rich sandy stratum

针对南通地区特殊的深厚富水砂性地层,通过展开一系列单井抽灌和群井抽灌现场试验,对各地层之间水力联系进行了探究,并进一步分析了降水引起的周围地层沉降变化和水位恢复引起的地层回弹与时间的关系。试验结果表明:①在深厚富水砂性地层中,地下水位变化较快、短时间内便可趋于稳定,而地层土体沉降变化较慢,具有一定的滞后性;②降水(压)过程中,对上覆潜水层影响较大(沉降量37 mm),对承压水层和隔水层影响较小(沉降量6mm),且短时间内对承压水层进行抽水降压试验,对整个上覆土层影响并不明显;③回灌过程中,虽然水位上升较快,但土体回弹并不明显,地层沉降体现出一定的"塑性",其中,潜水层土体产生较大的"塑性"沉降量(20.2 mm),隔水层和承压含水层土体产生较小的"塑性"沉降量(2.9 mm);④在降水(压)过程中,建议潜水层地带双电源切换时间控制在7 min以内,承压水层地带控制在4 min内,并且后期回灌压力优选0.08 MPa,在敏感建筑物保护区域做到抽灌一体化,尽量减小因降水引起的附加变形。

A series of field tests of single-well pumping and group-well pumping are carried out in Nantong water-rich sandy soil to study the change of hydraulic characteristics of different soil strata.The mechanism of dewatering-induced ground subsidence and relationship of soil restoration over time caused by water level recovery are further analyzed.The results show that:(1)In the water-rich sandy stratum with great depth,the groundwater changes quickly and its level can return to a steady state in a short time.However,the development of soil settlement changes slowly due to the characteristics of hysteresis.(2)The process of group-well pumping tests has a great influence on the phreatic aquifer(settlement of 37 mm),but a small impact on the confined aquifer(settlement of 6 mm).Moreover,it has little effect on the overlying soil strata in a short time.(3)In the artificial recharge tests,the groundwater level rises obviously.However,the phenomenon of soil restoration is subtle and shows the characteristics of unrecoverable plastic deformation.The plastic deformation of 20.2 mm in phreatic aquifer is greater than that of 2.9 mm in confined aquifer.(4)During the whole process,it is recommended to control the switching time of double-power supply in the phreatic aquifer within 7 min and that in the confined aquifer within 4 min.Additionally,the later recharge pressure should be preferably 0.08 MPa.In the protected district of sensitive buildings,pumping and irrigation should be integrated to minimize the deformation caused by precipitation.