覆盖型岩溶土洞形成的临界水流速度数值模拟研究

Numerical simulation of critical flow velocity in the covered karst soil cave formation

覆盖型岩溶塌陷灾害具有隐蔽性、突发性、不确定性、机理复杂性等特点,致使其预防、治理工作十分困难。覆盖型岩溶塌陷过程中,覆盖层土体形成土洞,土洞发育、扩展直至地表塌陷。现阶段土洞塌陷研究方法多为有限单元法等数值模拟,但有限单元法无法解决颗粒迁移、土体崩落等岩体大变形问题。在浙江省江山市岩溶塌陷地质条件分析的基础上,利用颗粒流数值模拟方法(PFC)对覆盖型岩溶土洞的形成过程进行了模拟,从微观角度揭示土洞形成过程中特殊土体颗粒的位移、裂纹的发展趋势和系统不平衡力的变化情况等,再现外力作用下岩溶土洞形成的多场变化特征。结果表明:(1)岩溶土洞发育过程可概化为裂纹产生、连通面形成、颗粒剥落3个阶段;(2)岩溶土洞开始发育时,土体颗粒位移曲线、裂纹的贯通程度、不平衡力三者之间具有良好的一致性,且垂直方向扩展速度大于水平速度;(3)覆盖层土体为黏土时初始土洞形成的临界水流速度为0.104 cm/s。研究结果可为岩溶塌陷治理提供参考数据。

The covered karst collapse has the characteristics of concealment,suddenness,uncertainty,and mechanism complexity,which leads to its prevention and treatment more difficult.In the process of covered karst collapse,soil caves formed in the overlying soil,develop and expand until the surface collapse.Numerical simulation such as the finite element method is an effective method of soil cave collapse;however,the finite element method can not solve the problem of large deformation of rock mass such as particle migration and soil collapse.Based on the analysis of the geological conditions of karst collapse in Jiangshan city,Zhejiang province,this study used the particle flow code(PFC)to simulate the formation process of covered karst soil cave and then to reveal the displacement of special soil particles,the development trend of cracks,and the change of system imbalance force on a micro level.The multi-field variation characteristics of the formation of covered karst soil cave under external force conditions were reproduced.The results show that the development process of karst soil cave could be summarized into three stages including crack generation,connected surface formation,and particle peeling.As the karst soil cave began to develop,there was a good consistency among the displacement curve of soil particles,the penetration degree of cracks,and the unbalance force,and the vertical expansion speed was larger than the horizontal speed.The critical flow velocity of the initial soil cavity is 0.104 cm/s when the overburden soil is clay.This study can provide basic information for the treatment of karst collapse.