Study On Stress Calculation And Construction Mechanics Of Surrounding Rock Of Shallow-buried Bias Tunnel Under Rainfall

In recent years,extreme weather has occurred frequently,especially rainfall and flood,which often occurs once in a few decades or a hundred years.Because the landforms in Southwest China are mostly hills and mountains,geological disasters（landslides,collapses,etc.）caused by rainfall emerge in endlessly.At the same time,with the rapid development of various road network constructions,many complicated and changeable geological features have appeared,and some shallow tunnels with unsymmetrical pressure have emerged.The geological structure of the tunnel is an important condition to be considered in the tunnel construction and scheme formulation,especially under the strong surrounding rock weathering,the surrounding rock of the tunnel is often weak and broken,resulting in construction difficulties.Southwest China is rich in rainfall.After rainfall,the infiltration of surface water can easily cause large deformation and local collapse of the surrounding rocks in the entrance and exit sections of tunnels and the shallow-buried unsymmetrical pressure sections.Therefore,the stability analysis of the shallow tunnel under the rainfall condition has both theoretical practical value and great engineering significance.In this paper,theoretical research,finite element numerical simulation and other methods are used scientifically to study the theoretical calculation method of surrounding rock stress of shallow-buried unsymmetrical pressure tunnel under rainfall conditions,and the construction mechanical characteristics,excavation and supporting results are optimized.（1）Based on the existing theoretical calculation methods of surrounding rock stress,considering the influence of rainfall factors,combined with unsaturated seepage theory,the correction methods of internal friction angle and cohesion (8 of rock and soil strength parameters under different soil saturation are discussed,and the influence of rainfall infiltration on internal friction angle and cohesion (8 of rock and soil strength parameters is analyzed,thus the calculation formula of surrounding rock stress of shallow-buried unsymmetrical pressure tunnel under rainfall conditions is deduced,and the numerical simulation results and theoretical results are compared and analyzed to verify the rationality of the formula.（2）Based on the theory of stress,seepage and their coupling,combined with MIDASGTS NX finite element software,the tunnel construction process with and without rainfall is simulated,and the surrounding rock stress and deformation characteristics of the tunnel with and without rainfall are analyzed.In addition,considering the influence of rainfall intensity and rainfall time,firstly,the theoretical value and simulation value of surrounding rock stress under different rainfall time conditions are compared and analyzed,and the reliability of theoretical calculation formula and numerical model of surrounding rock stress of shallowburied unsymmetrical pressure tunnel under rainfall conditions deduced above is further demonstrated.Then,the influence of rainfall time and rainfall intensity on the displacement and deformation of shallow-buried unsymmetrical pressure tunnel is analyzed.（3）According to the theory of unsaturated seepage under rainfall infiltration,combined with engineering examples,a three-dimensional model of tunnel with or without rainfall boundary is established by using numerical simulation software,and the variation laws of stress and displacement during tunnel construction under different rainfall boundary conditions are emphatically analyzed.（4）Taking the actual construction condition of shallow-buried unsymmetrical pressure tunnel as the research object,the deformation characteristics of tunnel surrounding rock under different excavation sequences and different supporting opportunities of secondary lining structures are discussed,and the construction scheme of shallow-buried unsymmetrical pressure tunnel under rainfall conditions is optimized,which provides certain reference value for tunnel construction under similar topography and structure conditions.