单层土体结构岩溶土洞的形成机理

Formation mechanism of karst soil-void in single-layer soil structure condition

岩溶土洞是岩溶区工程路基施工中的主要地质灾害。本文首先对广西桂林阳朔高速公路雁山段、西气东输二线工程广西贵港段两个典型工程路基单层土体结构的岩溶土洞进行详细剖析,然后通过室内模型试验、崩解试验的验证,研究岩溶土洞的形成机理。结果表明:单层土体结构的岩溶土洞发育是先从基岩面逐渐往地面发展,在发展过程中,对于较厚的土层,岩溶土洞的发育与扩展分为3个阶段:(1)基岩面附近的土洞形成与地下水位的承压状态和地下水位初始下降速度密切相关,土洞的形态主要以缝(洞)的形式出现,规模较小,多形成于土体中的裂隙面或非均质部位;(2)在地下水包气带(季节变动带)土洞的扩展以崩解为主,规模较大;(3)近地表(垂直渗入带)土洞扩展机制以上部降雨入渗、洞顶剥落为主,崩塌块体较大。

Subsurface soil-voids are main potential geologic hazards being faced in engineering construction in karst regions. In order to analyze the formation mechanism of soil-voids, laboratory tests including physics model experi- ment and slaking test have been taken based on site investigation. For the regions with single-layer clay or silty clay, the authors divide the process of soil-void development into three phases corresponding to the void＇s position and its relationship with groundwater table. In the first phase, the initial soil-void forms usually along the weak zones of soil such as fractures and bedrock surface, and seepage deformation induced by karst water flow plays an important part in this process. In the second phase, the soil-void exists in the belt of phreatic fluctuation, and the soil disintegration for repeat soak is the key reason in the formation of soil-void. In the last phase, the soil-void is generally within the unsaturated zone and closes to ground, so the rainfall and surface water leakage along the soil fracture will influence the stability of the soil-void greatly.