光纤监测和离散元分析为地面沉降机制带来新的认识-南京大学

自2013年以来在苏州地面沉降区布设监测钻孔，利用分布式光纤应变监测技术开展地面沉降监测。连续6年的监测数据表明，主要开采含水层上方的隔水层中存在巨大的压缩应变。进一步，开展了室内模型箱试验，发现在砂层排水期间，上覆粘土层中也观察到明显的压应变。同时，在粘土层中存在低于大气压的负孔隙水压力，与压缩应变的变化显著相关。采用改进的离散元模型来模拟负压作用下土层的变形。模拟结果与试验结果吻合，说明负孔隙水压力增强了隔水层的压缩。在长江三角洲南部地区，由于负孔隙水压力的影响，在排水和回灌循环中，加剧了隔水层的压缩。

Abstract:Land subsidence in the south Yangtze River delta area did not cease when the ground water table has risen during the last 20 years. This issue was generally interpreted as a result of the slow release of excess pore water pressure in the aquitards and the creep of soil. Recent distributed strain monitoring data in boreholes shows a great compressive strain in the aquitard above the main exploited aquifer. In laboratory tests, such great compressive strain also is observed in the clay layer during the draining of the underlying sand layer. Furthermore, a negative pore water pressure is observed in the clay layer, which significantly correlates with the change of the compressive strain. An improved discrete element model was used to simulate the deformation of soil layers under the effect of the negative pressure. The simulation results coincide with the tests, and the negative pressure enhances the compression of aquitard. Due to the effect of the negative pore water pressure, aquitards may be compressed during the draining and recharging cycles in the south Yangtze River delta area.

Figure 1. (a) Stratigraphic section of the 200 m borehole and the monitored strains from 2013 to 2019. (b) Strain curves between 40 and 88 m, seven typical curves are marked.

‍‍Figure 9. Strain ZZ of the model during the numerical simulation. 

(a) 300 steps; (b) 500 steps; (c) 1,000 steps.

Liu, C., Shi, B., Gu, K., Zhang, T., Tang, C., Wang, Y., & Liu, S. (2021). Negative pore water pressure in aquitard enhances land subsidence: Field, laboratory, and numerical evidence. Water Resources Research, 57, e2021WR030085.