Collapse Mechanism And Prevention Measures Of Karst Trough Zone Along Guiyang Metro Line 2

Guiyang metro line 2 is the backbone line connecting Baiyun District and Longdongbao District of Guiyang city.The laying form of the central urban area is mainly underground line,with a total length of 43.8km and a total investment of about 19.4 billion.Collapse is one of the main geological disasters that restrict the development of urban underground traffic construction in karst areas of China.Karst landforms account for 71.8%of the total area in Guiyang City,and there are concealed water-eroded groove,stone buds,underground fissures and karst caves.The design elevation of subway tunnel top is common in the contact zone between bedrock and overburden.Under the influence of karstification,the surface layer of bedrock is mostly developed with different scales of stone buds and solution channels,which are complex and variable in shape and difficult to predict accurately.As a concave cutting position,the filling body of the solution tank is usually in soft plastic or even flow plastic state.When the solution tank intrudes into the tunnel contour,it is easy to cause the collapse of the overlying soil layer of the tunnel.The covering soil layer in the tunnel area is mainly composed of red clay.Its unique water sensitivity and distribution characteristics of "hard on top and soft on the bottom" will have a further adverse impact on the stability of the surrounding rock of the tunnel,which will bring great difficulty to the tunneling construction of the subway.The mining method is the main construction method of Guiyang Metro Line 2.It is often used in advance with small conduit pre grouting and curtain grouting.Due to the complex development characteristics of the karst trough zone,the reinforcement effect of the support system of the leading small pipe in some sections is not ideal.Curtain grouting method is superior in supporting performance,but the technology is more complex and the price is more expensive.It is of great significance to study the collapse mechanism of subway when it passes through the solution channel zone,predict the collapse scope and the collapse shape of the section with potential collapse hazards,and reasonably adopt the tunnel support system and collapse prevention measures in the karst area.Based on the project of Bainan section of Guiyang Metro Line 2,this paper studies the collapse mechanism and prevention measures of karst trough zone.The main research contents are as follows:(1)through field investigation and laboratory test,the spatial distribution and physical and mechanical properties of the red clay overburden in the tunnel area,the development form of bedrock and the influence of water are studied,and the development characteristics of the karst trough zone are summarized.(2)According to the engineering geological conditions of Bainan section,combined with the collapse arch theory and PLAXIS Based on 3D numerical simulation,the transformation index of internal collapse to surface collapse induced by subway passing through the karst trough zone is proposed,and the calculation method of 3D collapse geometry boundary conditions under two collapse modes is deduced.In the study area,the burial depth of the subsidence soil cave is less than 2.5 times the calculated height of the subsidence soil cave as the criterion for converting the internal collapse model to the surface collapse model.(3)According to the development characteristics and collapse development mode of the karst trough zone,the design parameters and process characteristics of two kinds of pre support systems,advance small conduit pre grouting and curtain grouting,are summarized.Based on this,two kinds of support methods in karst trough zone are proposed: When the ground is in the internal collapse mode,the pre-grouting can be carried out with the advance small pipe;if it is the surface collapse mode,the curtain grouting method can be used for reinforcement.