Process and influencing factors of karst ground collapse in the water source area of Tai’an-Jiuxian

Tai’an City lies in the western margin of the Tailai basin in Shandong Province, where karst is pervasive and controlled by regional geological setting. The karst collapse in this area started in the 1960s, causing serious social-economic impacts. This geologic hazard is a complex and multi-factor process, occurring mainly in the central and southern Tai’an City with high intensity of groundwater exploitation. The affected regions include， (1) the urban subsidence area, centered at the triangle south of Tai’an railway station with east boundary to the Shanggao street office south boundary to Honggou village, where limestone is widely present in the subsurface. (2) The Jiuxian subsidence area, which is centered at the Jiuxian water source area southeast of the city. (3) The Yanglou subsidence area, with east and west Yanglou village as the center, actually the periphery of the Jiuxian subsidence area. These three regions are largely connected into one, forming a karst collapse belt from the Jiuxian village to Taishan railway station, with an total area of about 200 km2.The objective of this work is to reveal the historical process and influencing factors of karst collapse in the study area,so as to provide the scientific guidance for prevention and control of this geologic hazard.We firstly summarized the initiation and development process of karst collapse in the study area.Then, based on the previous research results and the latest surveys,we analyze the distribution of the hazard and its basic control factors, as well as the action laws and mechanisms of each factor.The results show that the influencing factors of karst collapse can be divided into natural and human factors.(1) Under natural factors,karst collapse mainly occurs in the areas with intense karst development, near fault zones, and soil layer thickness less than 30 m. The Quaternary soil structure is mainly a binary-layer structure, followed by a multi-layer structure; most of the collapses occurred in the flood season, especially after heavy rainfall.(2) Under human factors,the large water level fluctuation caused by strong groundwater mining is the most important factor for the occurrence of collapse;and the fluctuation of karst groundwater level near the bedrock surface is the most likely to cause the collapse.Moreover,the karst collapse in the Jiuzhen railway interchange area is also closely related to the train vibration. Finally,some suggestions are put forward in this paper, the future research should be focused on the monitoring and simulation of collapse evolution, exploration of collapse forecasting methods , and the construction of database .