Study On Failure Mechanism And Control Measures Of Karst Tunnels Under Deep And Complicated Conditions

In recent years,with the rapid and steady development of economy,the demanding for the development and utilization of underground space is increasing.Hydraulic and Hydro-Power Engineering,South-to-North Water Diversion Projects,and Mining Projects,etc.are all inseparable from the construction of the tunnel,it will inevitably meet the bad geological problems such as Karst cave and the Karst water which is widely distributed in the southwestern region of China.With the increasing of tunnel's depth,the geological problems are more complicated encountered in deep Karst areas.Due to the increase of tunnel's depth,deep karsts are often accompanied by complicated geological problems such as high ground stress,high water pressure caverns and fissures.Since the uncertainty and complexity,the current research level of Karst tunnel is not enough to meet the needs of engineering construction,especially under the deep and complicated condition.Therefore,the in-depth research is particularly important to study Karst tunnels under deep and complicated conditions.In this paper,the Karst Section of Chaoyang tunnel is studied.By means of theoretical analysis,indoor test,numerical simulation and engineering application,the development trend of karst is studied and analyzed as well as the surrounding rock's stress distribution law and supporting mechanism of Karst tunnel under the deep complicated condition.The main conclusions are as follows:(1)The destroy mechanism of karst tunnels under deep and complicated conditions is analyzed,and the characteristics are revealed.(2)The characteristics of the surrounding rock under high-stress conditions and deep complicated conditions are analyzed,and a rigid-soft joint grouting support scheme is proposed for the deep karst tunnel.(3)By analyzing the properties of different grouting materials,the suitable rigid and flexible materials are founded for grouting tunnel under deep complex conditions.The coagulation time and strength of grouting materials were tested with different water-cement ratios and sodium silicate content,and the suitable ratio of rigid grouting materials for deep karst tunnels was obtained.The composition,coagulation mechanism,solidification characteristics and the engineering properties of the flexible grouting material have been analyzed,and the reasonable grouting material was obtained.(4)The supporting model of Karst tunnel under deep and complicated conditions is established,and the numerical analysis model of Karst tunnel is established by using FLAC3 D finite element difference software.The numerical simulation analysis of the original support scheme and the rigid flexible grouting support scheme is carried out respectively.From the simulation,it is concluded that the rigid-flexible joint grouting support scheme is obviously superior to the original scheme,and the displacement of the tunnel section can be controlled within the allowable range.(5)On the basis of numerical simulation,combining the theory of rigidflexible grouting,the characteristics of Karst tunnel under the deep complex condition and the test of grouting material,rigid-flexible joint grouting support is adopted in Chaoyang tunnel.The monitoring results show that the deformation at the top of the tunnel is significantly smaller,which proves that the rigid-flexible joint grouting support scheme has achieved a good support effect to ensure the safe and smooth construction of the tunnel.