| Hidden karst caves are distributed outside the tunnel excavation contour line,which brings greater safety risks to tunnel construction.Existing research on the influence of concealed karst caves on tunnel construction mostly focuses on single karst caves,which is different from the actual situation of karst cave development in groups.Taking Chengkai super-long tunnel as the engineering background,firstly analyze the distribution of karst caves in stratum.Then use the finite element software Midas/GTS to analyze the deformation characteristics of surrounding rocks and the mechanical characteristics of the supporting structure when the tunnel passes through different spatially distributed karst caves,and find out the spatial distribution of karst caves that are detrimental to tunnel construction.Then,according to the spatial distribution of karst caves which is detrimental to tunnel construction safety,analyze the influence of different tunnel excavation methods on the stress of the tunnel supporting structure,and analyze the tunnel’s grouting effect.Considering the influence of underground karst water on tunnel construction,combined with the Chengkai tunnel engineering background,finding the engineering geological characteristics of the water-inrush and mud-bursting-prone area and the change of the geological environment when the tunnel is close to the waterbursting and mud-bursting area as a basis to guide the tunnel construction.The main conclusions are as follows:(1)There are three types of karst caves: vertical development type,horizontal development type and oblique development type.Vertically developed karst caves are usually distributed near the surface.Obliquely developed karst caves are usually developed near the fault,rock layer or large joint surface of the soluble rock above the water level line.Horizontally developed karst caves usually appear in the soluble rock joint near the groundwater level.The planned location of Chengkai Tunnel is mainly developed horizontal karst caves,but there will be diagonally developed karst caves near the fault.(2)When the horizontally developed karst caves are located in the upper and lower positions of the tunnel,they have little effect on the deformation of surrounding rock and the supporting structure stress during tunnel construction.When the tunnel passes through the horizontal karst caves,the deformation values of surrounding rock and the stress value of the supporting structure have a large growth.When the tunnel passes through the fault oblique karst caves and the fault oblique karst caves located at the side of the tunnel,under the effect of the fault and the karst caves,the surrounding rock deformation and the supporting structure stress growth are greater.Therefore,the three conditions of tunnel crossing horizontal karst caves,tunnel crossing fault oblique karst caves and oblique karst caves on the side of the tunnel are disadvantageous to tunnel construction.(3)When the tunnel crosses the horizontal karst caves,the stress value of the supporting structure excavated by the step method is small,so it is more reasonable to choose this method.Considering the obliquely developed karst caves on the side of the tunnel and the tunnel passes through the obliquely karst caves in the fault.If there is no karst cave under the tunnel,it is safer to use the CD method for construction.If there are some karst caves under the tunnel,it is safer to use step method excavation.(4)When the tunnel passes through the horizontal karst caves,the stress value at the arch foot of the supporting structure decreases about 45%,and the grouting effect is better.When the tunnel passes through the fault and the oblique karst caves are located on the side of the tunnel,the stress at the arch foot position after grouting decreases 17%,the stress value at other positions in supporting structure does not decrease significantly,and the grouting effect is poor.So it is necessary to add steel arch frame and increase the thickness of the lining to guarantee the tunnel construction safety. |
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