三峡地区泥灰质岩石斜坡带岩溶作用及其对工程稳定性的影响

Karst Process and Its Impact on Construction Stability of the Slope Terrain within Muddy Limestone in the Three Gorges Region

泥灰质岩石岩溶是岩溶研究一个薄弱环节,将工程稳定性与岩溶作用相结合是岩溶地区地质灾害评价和治理所面临的新课题。三峡地区巴东组(T2b)泥灰质岩石的岩溶问题是新近发现的重大工程地质问题。由于构造破碎和河流切割引起岩石卸荷与松动,使岩石有利于水的渗入和岩溶作用。水是岩溶的溶剂,在峡谷斜坡地带,雨水是地表水和地下水的主要水源,地处亚热带的三峡地区雨水丰沛。通过对15个不同类型水样的化学成分测试和计算表明,雨水呈酸性,方解石的饱和指数(SI)低,具有强烈的溶蚀性,在地表和地下运移的过程中对泥灰质岩石产生了强烈的溶蚀作用。不同的地貌位置和构造层位具有不同的地表水和地下水状态,从而产生不同强度和不同形式的地表和地下岩溶。地质历史时期,岩溶作用使岩石发生成分和结构的一系列变化,从而导致岩石力学强度降低,岩体发生不均匀沉降、地裂缝、滑坡、崩塌、泥石流和地面塌陷。测试和计算还表明,不仅雨水及其转化成的地表水和地下水,而且江水和自来水均有一定的溶蚀性。将来,水的岩溶作用和水的诱发作用可能导致地基不均匀沉降、地裂缝、滑坡、崩塌、泥石流和地面塌陷,从而破坏工程稳定性。防治三峡地区地质灾害的关键在于防止水向地基的入渗。建议对三峡地区泥灰质岩石斜坡带岩溶进行深入研究。

The study of karst in mud limestone is a weak link in karst geology. The connection of construction stability with karstifi- cation is a new project for the evaluation and control of geohazards in the karst region. The karst in mud limestone of Triassic Badong Formation （T2 b） is an important geological problem newly discovered in the Three Gorges Region. Due to the unloading and loosening brought about by tectonic shattering and river cutting, the mud limestone becomes favorable for water infiltration and karstification. Water is the fluxing agent of karstification. The rainwater is the main resource of surface water and groundwater in the slope terrain of the gorges, and there is plentiful rainwater in the semitropical Three Gorges. Based on the analysis of chemical contents of 15 water samples of different types and related calculations, it is found that the rainwater is acid and has low saturation index （SI） of calcite, and therefore it bears strong dissolution capability and strongly dissolves the mud limestone in the process of its migration at or beneath the surface. There exist different conditions of surface water and groundwater in different landform parts and tectonic locations where there are different karstification intensities and types at surface and in the depth. Generally speaking, there exists strong karstification where the discharge is large, and at places of temporary flow there exists high karstification where the water flow is slow. Karstification has led to a series of variations in the composition and structure of rocks, which degraded the mechanical intensity, and brought about uneven subsidence, fissuring, landslide, collapse, mud flow and cave-in of rockmass in geological history. It is also shown by the test and calculation that all the rainwater, surface water, groundwater, river water and tap water have dissolution capacity and, in future, the karstification and hydro-induction may result in uneven subsidence, fissuring, landslide, collapse, mud flow and cave-in of the construction foundation. These factors, in turn, can destroy the eonstruction stability. To control the geo-hazards in the region, it is of key importance to prevent water from infiltration into the foundation within mud limestone. It is suggested that the karst in slope terrain within mud limestone in Three Gorges region should be investigated in detail.