The construction of a large number of urban underground rail transit and urban renewal projects has intensified the urban karst geological disasters in our country. As one of the first demonstration areas of national urban construction, Shenzhen has developed rapidly in terms of underground rail transit and urban renewal projects over the years, hence leading to frequent karst collapse disasters due to its location in the karst area. Therefore, the summary of the experience and lessons from the development of karst underground space is of great significance for the engineering construction in karst areas. In this thesis, a preliminary study on the delimitation of the affected zone of geological environment for karst underground engineering is conducted based on three events of karst ground collapse in Shenzhen.

The affected zone of geological environment for karst underground engineering refers to the area where disasters are likely to happen due to the disturbance and damage of the rock and soil around the construction site during the construction process. The delimitation of the affected zone is not only conducive to the safe and smooth engineering construction, but also to the clear division of responsibility. The monitoring range of underground engineering construction in karst area usually reaches only tens of meters at the current stage. But when a disaster happens, the actual range influenced by engineering will often exceed hundreds of meters. Therefore, the current construction specification about the affected zone of geological environment for karst underground engineering is unreasonable and uncertain in some degree, and the relevant provisions are greatly challenged. For instance, it is stipulated that the engineering monitoring should be conducted within the plane range that is only 3 times as deep as the foundation pit during construction , and the description of the expansion of the monitoring range in the karst development area is not detailed. Therefore, the further research on the affected zone of geological environment for karst underground engineering is very necessary, so the actual affected range of the project can be effectively judged, and then the corresponding prevention and control measures can be taken.

On the basis of fully mastering the regional geological background and the geological conditions of site engineering, a preliminary study is conducted in this thesis. Firstly, the actual affected range of the project is monitored and analyzed by the high-frequency monitoring technology of karst groundwater pressure. The monitoring scheme should be formulated according to local conditions. The monitoring frequency should capture the disturbance changes of regional karst groundwater with more than 3-month monitoring cycle. The results of monitoring and data measurement of water levels indicate that the affected range of the project is closely connected with the karst groundwater drawdown funnel. The obvious anisotropy of karst aquifer medium at each site is indicated in the groundwater flow field, which is mainly controlled by karst development and structure. The maximum affected ranges of the three projects are 560 m, 820 m and 850 m respectively. Then, the analysis on the formation mechanism of karst collapse is conducted. Results indicate the collapse mechanism. The excavation and precipitation of foundation pit leads to the change of groundwater hydrodynamic internally caused of strong karst development, the change of hydrodynamic conditions disturbs karst groundwater or gas, and the force generated by karst water disturbance acts on the overburden floor through karst pipeline or crack. As a result, the overburden soil mass collapses and loses gradually, until the roof becomes unstable and damaged for the insufficient collapse resistance. Finally, the Gehart’s empirical formula of influence radius of confined water pumping hole is used for reference, combined with engineering construction parameters, main factors of karst collapse and hydrogeological test parameters. The mixed research method of qualitative analysis and quantitative calculation is adopted to theoretically deduce the empirical calculation formula of the affected zone of geological environment for karst underground engineering. Research results indicate that the affected range of karst underground engineering is mainly in direct proportion to the permeability coefficient and engineering depth. However, it is inversely proportional to the thickness of soil layer. The deduced semi-quantitative theoretical formula is suitable for the calculation of a relatively thick aquifer between the Quaternary and the karst aquifer. If the depth of foundation pit is greater than that of rock surface and of confined karst water in underground engineering, this fomular can be used to quickly provide the basis for safety construction of underground engineering as well as urban disaster prevention and reduction in the karst area lacking groundwater monitoring data.