Accumulation capacity of soluble rocks as the main stability index in covered karst areas

For an engineering geologist, covered karst areas are of special interest, not only because these are the most widespread on the Earth. More important, unlike the areas of bare karst, these areas are more favorable and suitable for economic development and human life as a whole. And the study shows that the greater is the thickness of the cover series, the more true is this statement. The local forecast of sinkholes appears to be the most important and difficult engineering geological problem in these areas. It is especially acute there, where collapses and subsidence are absent, not registered, or poorly expressed in the relief, that is, in the areas of unrealized (potential, expected) sinkhole hazard. One of the possible approaches to solving this problem is discussed in the paper. It is based on comparing the volume of incoherent and fractured rock removed from the cover deposits with the accumulation capacity of the karst massif, i.e., the volume of hollow space in the karstic massif capable of receiving and accumulating clastic material. The most common result of soils outflow in fissure-karst reservoirs is their loosening in the deformed-destructed area of the overburden. For the three canonical schemes of this area structure, the critical values of clastic matter volume were obtained from the equation for soil mass balance before and after loosening, the further removal of which will lead to the failure of the earth's surface. Some techniques and methods for assessing the accumulation capacity of soluble rocks are considered and serious differences in the use of the discussed approach in predicting the stability of karstified and undermined territories are revealed. In particular, it is shown that in karst areas, when performing calculations, one should be guided by the maximum values of the coefficient of primary loosening of soils. Otherwise, the predicted stability of the overburden will prove to be unjustifiably underestimated. The results obtained and the procedure for assessing the stability of covered karst terrains by the proposed method are exemplified by the construction site of the main buildings of the Nizhny Novgorod NPP (3.25 km2), where there are no collapse and subsidence sinkholes. However, the entire construction site (20 km2) is located in the area of active sulphate-carbonate karst. From the engineering-geological point of view, this is the main feature of the facility design.