Seepage-induced sand-leakage karst collapse mechanism considering the soil arching effect
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摘要:
渗流作用下砂漏型岩溶塌陷在“上黏下砂”二元结构覆盖层隐伏岩溶区经常发生，此类塌陷宏细观力学机理和漏砂启动力学评价模型的研究还不充分。以武汉市二元结构覆盖层岩溶区由岩溶水位降低诱发的陆家街岩溶塌陷为例，采用物理模型试验和PFC（particle flow code）-CFD（computational fluid dynamics）耦合数值试验探索塌陷破坏的宏细观力学机理，并构建渗流作用下漏砂启动的土拱极限平衡力学评价模型。结果表明：（1）物理模型试验和数值模型试验得到的陆家街塌陷临界水位差分别为7.2 m和8.0 m，误差约10%，漏砂启动后砂层底部孔隙水压力骤降，地表沉降增加；（2）在到达临界水位差之前，岩溶开口处砂层流速增大而孔隙水压力减小，土拱水平应力、竖向应力以及侧压力系数增大，且水力梯度增大速率明显大于侧压力系数；（3）所构建的渗流作用下土拱极限平衡模型表明，随着水位差增加，土拱中致塌力和抗塌力都在增加，但致塌力增大更明显。研究结果对提高砂漏型岩溶塌陷防灾监测水平具有重要意义。
Abstract:Sand-leakage karst collapse often occurs in the hidden karst area of the binary structure covering layer due to seepage. However, further research is needed to understand the macro and micro mechanical mechanisms of such collapse and the mechanical evaluation model for sand-leakage initiation. This study used the Lujia Street karst collapse, induced by the lowering of the karst water level in the binary structure covering layer karst area of Wuhan City, as a case study. The macro and micro mechanical mechanisms of collapse failure were investigated through physical model tests and PFC (particle flow code)-CFD (computational fluid dynamics) coupled numerical experiments. A soil arch limit equilibrium mechanical evaluation model was developed to assess sand leakage initiation under seepage conditions. The results show that the critical water level differences for the of Lujia Street karst collapse revealed by physical model experiments and numerical model experiments are 7.2 m and 8.0 m, respectively, with an error of approximately 10%. After sand leakage begins , the pore water pressure at the bottom of the sand layer drops sharply, leading to an increase in surface subsidence. Before reaching the critical water level difference, the velocity of the sand layer at the karst opening increases while the pore water pressure decreases. The horizontal stress, vertical stress, and lateral pressure coefficient of the soil arch increase, and the rate of hydraulic gradient increase is significantly greater than the lateral pressure coefficient. According to the constructed limit equilibrium model of soil arch under seepage, it is found that as the water level difference increases, although the collapse force and anti-collapse force in the soil arch increase, the increase in collapse force is more pronounced. This study is of great significance for improving the level of disaster prevention and monitoring of sand-leakage karst collapse.
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表 1 数值模拟细观参数
Table 1. Numerical simulation mesoscopic parameters
材料类型 密度/（kg·m−3） 颗粒半径/m 孔隙率 法向刚度/（106 N·m−1） 切向刚度/（106 N·m−1） 法向黏结强度/kPa 切向黏结强度/kPa 摩擦系数 黏滞系数 砂土 2650 0.06~0.30 0.21 20 10 — — 0.7 — 黏土 2700 0.06~0.30 0.24 20 6.7 40 40 0.5 — 水 1000 — — — — — — — 0.001 注：二维数值模拟孔隙率经验公式： $ n_{2\mathrm{D}}=0.42n_{实际}^2+0.25n_{实际} $ 。
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