Analysis of karst collapse development in Tai’anJiuxian water source area

Research area is located on the western edge of Tailai basin, Shandong Province, which is part of Taishan Mountain piedmont alluvial plain. Karst collapse as one of geological disasters in the area has occured since the 1960s. In recent years, with the development of the national economy, the karst collapse disasters has increasingly taken place at a high occurence frequency. In the research area, the succession of strata from old to young is the bottom Tarzan Group metamorphic rocks of the Archean era and then the Cambrian and Ordovician carbonate and shale rocks, overlaid by the Paleogene conglomerate and mudstone, the Neogene clay rock and unconsolidated sediments of the Quaternary. Daidaoan fault is the main geologic structure, which is of significance in controlling the karst collapse. In this area, the aquifers mainly consist of the Cambrian and Ordovician limestone and dolomite, where karst is well developed and is favorable for the rainwater and upper porous water to infiltrate. Groundwater extraction is the main route of excretion, and as a result mass exploitation has led to a large depression cone of the karst water in the research area. The change in hydrodynamic field is the most important factor that induces karst collapses.Based on previous studies and combining with the latest survey and the results of investigation and monitoring, this paper researches and analyzes the development conditions, distribution characteristics and genetic evolution of karst collapses, with the intention of putting forward countermeasures for the prevention and control of the karst collapses in this area. Results showed that the karst collapses mainly occurred in the following regions,(1) sections with shallowly well developed karst, mostly in the Ordovician and Cambrian limestone and dolomite; (2) the Quaternary sediments with a thickness of less than 20 m and a multi-layer deposit structure, namely, cohesive soil on the top, sandy soil in the middle and clay soil at the bottom; (3) fault zone; (4) areas within the depression cone of the karst groundwater and its influence range. Karst collapses in the research area occur due to all kinds of adverse factors, of which the most important one is change of hydrodynamic condition. With all available data including hydrology, geology, disaster data, it analysed the relationships between the occurence of karst collapses and strata, precipitation, exploitation and karst water level, respectively. The analytical results clearly showed that the karst groundwater level depressed along with increasing resource exploitation in karst area. As a result, when the cavern water level is lower than bedrock surface, the karst collapse begin to occur. Moreover, with the decrease of the water level and the increase of the fluctuation range, the number of the collapse is obviously increased.