Experimental study of gas tracer simulation of karst collapse development process: An example of Jinshazhou, Guangzhou

Karst collapse is one of main geological hazards in karst areas in China， which can be generally divided into two types： sinkhole and karst collapse. Sinkhole is widely distributed and numerous， and how to monitor them and make an early warning is a focused issue. The primary characteristic of sinkhole development is that under the action of groundwater and other factors， the internal structure of soil is deformed and damaged and then produces cracks and caves. The common monitoring and early warning methods include physical exploration， groundwater dynamic condition monitoring and other nondestructive detection tools. A relatively new technical method in this aspect is to use gas tracer. Its principle is to analyze the relationship between gas characteristic values and soil deformation and damage based on observation to gas transport in soil pores， cracks and holes of different shapes and sizes， and then indirectly judge the development of sinkhole to achieve karst collapse monitoring and early warning. In this work， taking the karst collapse area of Jinshazhou， Guangzhou as an example， the whole process from sinkhole development to ground collapse was reproduced by indoor physical model tests， and then the relationship between gas concentration， gas accumulation time and other parameter indexes and different development stages of karst collapse was studied by using gas tracing technique in five processes of a sinkhole： formation， development， blockage， expansion and collapse. The results show that （1） there is a correspondence between the development of sinkhole and tracer gas concentration， and the increase of gas concentration reflects the further development of soil deformation and damage. （2） There is a positive correlation between the soil deformation and damage and tracer gas concentration in general， and the closer to the ground collapse， relatively higher the concentration. （3） There is a correspondence between the soil deformation and damage and the time of tracer gas concentration reaching peak concentration， and the closer to the ground collapse， the faster the gas concentration reaching the peak. These experimental results demonstrate that it is feasible to use gas tracer technology to conduct monitoring and early warning of karst collapse. Later on we will conduct further research on early warning thresholds and carry out practical verification in the field.