Tunnel Rock Mass Stability And Control Of Fully Weathered Red Sandstone Stratum

Red sandstone,mainly Cretaceous,Jurassic and Paleogene,is widely distributed all over the world.In view of its relatively new generation age,poor cementation performance,easy weathering,softening and disintegration,it is mostly classified as the soft rock.The fully weathered red sandstone,which is viewed as the extremely soft rock,has the highest degree of weathering,the worst stability and mechanical properties.The construction of traffic tunnels crossing this stratum may face great challenges and difficulties.At present,there are few tunnel construction projects carried out in the fully weathered red sandstone stratum,and the research on its engineering characteristics and stability needs to be supplemented.Relevant specifications and regulations also lack specific concern of rock mass strength,stability evaluation and rock mass classification,tunnel supporting structures and stability control technology for this special stratum.With the development of tunnel construction,it is necessary to study the stability and its control technology of fully weathered red sandstone tunnels.Based on the engineering project of Yangcheng Tunnel of Haoji Railway,the tunnel rock mass stability and control of fully weathered red sandstone stratum are systematically studied by means of laboratory test,numerical simulation,theoretical analysis and field test.Firstly,the algorithm model and prediction system of rock mass strength of fully weathered red sandstone are developed and constructed.Then,the rock mass classification and its evaluation system of deep buried tunnel in fully weathered red sandstone stratum are proposed and constructed.Finally,the tunnel stability control technology including supporting structure,stratum reinforcement and stratum dewatering technology is explored and proposed.The main results are as follows:1.Based on the results of 300 groups of direct shear tests and 300 groups of consolidation tests of the combination of water content,fine content and relative compaction,a 10-layer decision tree algorithm model and prediction system for rock mass strength of fully weathered red sandstone using the combination of three physical property indices are proposed for the first time.The prediction accuracy of shear strength and compressive strength are 94.5%and 94.8%respectively.This method,revealing the coupling relationship between mechanical strength and physical properties of fully weathered red sandstone,overcomes the shortcomings of single physical property index and inaccurate prediction of rock mass strength in existing research.2.Based on the rock mass strength algorithm model and 3D discrete element numerical simulation,the distribution characteristics and evolution laws of pressure arch of deeply buried tunnels in fully weathered red sandstone of different rock mass grades are obtained.Then the calculation method of the load of deeply buried tunnel in fully weathered red sandstone is put forward and the calculation formula of rock mass stability evaluation criterion is revised.Furthermore,the rock mass classification of deeply buried tunnel with fully weathered red sandstone is proposed for the first time through discrete element numerical test,which fills the gap of tunnel rock mass classification of this stratum.The tunnel rock mass classification includes grade V(V_a&V_b)and grade VI(VI_a&VI_b)and IV,accounting for 72.77%(28.49%&44.28%),24.63%(13.11%&11.52%)and 2.60%respectively.The value range of physical property index,rock mass strength and rock mass stability evaluation criterion under different rock mass grades are also obtained.Based on the10-layer decision tree model of rock mass strength,the evaluation system for rock mass classification of fully weathered red sandstone tunnel considering the combination of three physical property indices is proposed for the first time.3.The steel fiber reinforced concrete(SFRC)is proposed as the adaptive supporting structure for tunnels in fully weathered red sandstone.Based on the test results of SFRC members,a three-dimensional discrete element numerical model is established for the first time to characterize the comprehensive mechanical properties of SFRC members in compression,tension,bending and shear tests.For the first time,the calculation method of strain energy of SFRC considering the difference of stress state,deformation stage,numbers of cracks and crack absorption energy index is proposed,after being verified with indoor test and three-dimensional discrete element numerical simulation.Based on the large-scale three-dimensional discrete element numerical test of rock mass and supporting structures in tunnel excavation,the adaptability of SFRC with different supporting thickness used in fully weathered red sandstone tunnel primary lining with different rock mass grade is examined.4.Through the similar model test of tunnel excavation,combined with digital image correlation and three-dimensional discrete element numerical simulation,the instability failure law of fully weathered red sandstone rock mass and evolution law of tunnel collapse in deep buried tunnel is revealed.Further,through field test and finite element numerical simulation,the adaptive control technology of the fully weathered red sandstone tunnel stability is proposed.It is obtained that the horizontal rotating-jet grout pile and curtain grouting reinforcement are given priority to the tunnel face reinforcement and the basement reinforcement respectively.The full section light well point drainage supplemented by the lower bench gravity deep well drainage can be effectively used for drainage.