A study on collapse law of soil cave with different rainfall rates based on FLAC3D
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摘要: 针对桂林市临桂区岩溶塌陷易发区域，采用FLAC3D模拟不同降雨速率下的强降雨入渗过程，探究不同直径土洞在强降雨作用下的致塌规律，结果表明：（1）强降雨条件下，不同直径土洞最大位移均出现在洞顶部。降雨速率相同，洞顶竖向位移增长速率随土洞直径的增加呈整体加快的特点；加快降雨速率，竖向位移增长明显，竖向位移与土洞大小呈正相关。（2）相同降雨速率下，土洞直径增大会引起土洞底部剪切破坏区域进一步扩展。上覆土层在强降雨初期主要受到潜蚀作用，加快降雨速率，土洞底部水位剧烈波动对上覆土体产生的水击气爆成为主导作用，剪切破坏速率加快，洞趾剪切应变明显增加，当土洞直径达到3 m时，水位波动愈加剧烈，加速上覆土层破坏。（3）降雨速率的变化对土洞塑性区拓展范围具有不同程度的影响，较大直径的土洞在加快降雨速率时塑性区拓展范围明显扩大，即土洞大小、降雨速率对上覆土层稳定性具有较大的影响。研究结果为定量研究强降雨与上覆土层塌陷的关系提供了依据，对有效、合理地预警岩溶塌陷具有一定的意义。Abstract:
Frequent heavy rainfall can lead to large-scale collapse of karst soil caves. Quaternary strata are widely distributed in the karst area of Lingui district, Guilin City, among which the subsidence area of the silty clay overlay with a one-dimensional structure formed by the Upper Pleistocene alluvial-pluvial layer Q3al-pl under the action of heavy rainfall is the largest in the collapse area of the entire study area. This subsidence area accounts for the largest proportion, posing a huge risk on security to the people. Heavy rainfall has aroused extensive attention and research on the collapse law of karst soil caves. Many scholars have revealed the mechanism of rainfall collapse, the effect laws of erosion, and water level fluctuations in the process of karst collapse from qualitative and quantitative perspectives. In order to better explore the collapse law of soil caves of different sizes with different rainfall rates, a generalized geological model was constructed for the study area, and the calculation models of karst soil caves of different sizes were established in the modeling software Rhino. Besides, the FLAC3D finite difference software was used to simulate the infiltration process of heavy rainfall with different rainfall rates. In this study, a karst calculation model—Mohr-Coulomb constitution and the parameters of physical and mechanical properties of the overlaying soil layer were firstly assigned. Then, the boundary conditions, the rainfall rate, the superimposed force field and the flow field were set to reproduce the cloud map of changes in stress, displacement and plastic zone during the evolution of karst soil caves in the process of heavy rainfall infiltration. The numerical simulation results show that, (1) Under the condition of heavy rainfall, the maximum displacements of soil caves with different diameters all appear at the top of the cave. With the same rainfall rate, the larger the diameter of the soil cave is, the faster the growth rate of vertical displacement becomes. The increase of rainfall rate may result in obvious increase of vertical displacement, and the vertical displacement of the cave top is positively proportional to the size of the soil cave. (2) Given the same rainfall rate, the expansion of the soil cave diameter has caused the further expansion of the shear failure area at the bottom of the soil cave. The overlying soil layer is mainly subjected to subsurface erosion in the early stage of heavy rainfall. When the rainfall rate is accelerated, the water-hammer gas explosion of the overlying soil caused by the violent fluctuation of the water level at the bottom of the soil cave becomes the dominant effect. The shear failure rate is accelerated, and the shear strain at the toe of the cave increases significantly. When the diameter of the soil cave reaches 3 m, the water level fluctuates more and more violently, which accelerates the destruction of the overlying soil layer. (3) The change of rainfall rate poses, in different degrees, the influence on the development range of plastic zone in the soil cave. In the soil cave with a larger diameter, the development range of plastic zoon is significantly expanded with the accelerated rainfall rate. The size of the soil cave and the speed of rainfall have a great influence on the stability of the overlying soil layer. These results provide a basis for the quantitative study of the relationship between heavy rainfall and collapse of the overlying soil, which is crucial to effective and reasonable warning to karst collapse. -
Key words:
- heavy rainfall /
- karst collapse /
- FLAC3D /
- numerical simulation /
- soil cave
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图 5 降雨速率为6.94×10−4 m3·s−1时土洞竖直方向位移图
Figure 5. Vertical displacement of soil cave when the rainfall rate is 6.94×10−4 m3·s−1
图 6 不同降雨速率下土洞竖直方向位移图
Figure 6. Vertical displacement of soil cave with different rainfall rates
图 7 不同降雨速率下土洞最大剪应力图
Figure 7. Maximum shear stress of soil caves with different rainfall rates
图 8 降雨速率为6.94×10−4 m3·s−1时土洞最大剪切应变增量图
Figure 8. Maximum shear strain increment of soil cave when the rainfall rate is 6.94×10−4 m3·s−1
图 9 不同降雨速率下土洞最大剪切应变增量图
Figure 9. Maximum shear strain increment of soil caves with different rainfall rates
图 10 降雨速率为6.94×10−4 m3·s−1时土洞塑性区分布
Figure 10. Plastic distribution of soil cave when the rainfall rate is 6.94×10−4 m3·s−1
图 11 不同降雨速率下土洞塑性区分布
Figure 11. Plastic distribution of soil cave with different rainfall rates
表 1 土体基本物理、力学参数
Table 1. Basic physical and mechanical parameters of soil
覆盖层类型 密度/g·cm−3 孔隙率 剪切模量/kpa 体积模量/kpa 内摩擦角/° 粘聚力/kPa 渗透系数/cm·s−1 粉质黏土 1.72 0.47 1.354 $ \times $106 4.22 $ \times $106 8.8 25.3 3.22 $ \times $10−4 
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表 2 模拟方案设计
Table 2. Simulation scheme design
方案 土洞直
径/m降雨强
度/mm·d−1集雨面
积/m2降雨历
时/h降雨速
率/m3·s−11 0.5 200 50 2 1.39 $ \times $10−3 1.0 1.5 2.0 2.5 3.0 2 0.5 2.5 1.11 $ \times $10−3 1.0 1.5 2.0 2.5 3.0 3 0.5 4 6.94 $ \times $10−4 1.0 1.5 2.0 2.5 3.0
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