Study On Parameters Optimization Of Supporting Structure For Small-diameter Loess Railway Tunnel

In the loess tunnel project,it is the key and difficult point in the engineering design and construction to determine the stability of surrounding rock.Loess as a special surrounding rock,has its special mechanical properties,rock instability occurred from time to time,resulting in significant casualties and economic losses.In all kinds of underground construction,to maintain the stability of the surrounding rock and supporting the design and construction of the basic starting point.Therefore,it is of practical significance to study the stability of surrounding rock and support of loess tunnel in view of the characteristics of loess tunnel surrounding rock conditions.According to the New Austrian law,the surrounding rock and supporting structures need to bear the load,so the transfer of the wall rock pressure and the initial support is the real role.Before the tunnel excavation,the natural rock mass has certain original rock stress.After the initial supporting structure was applied,the elastic deformation and plastic deformation of the surrounding rock mass gradually produced the deformation pressure,which directly acted on the initial supporting structure.In this paper,the physical and mechanical properties of undisturbed loess are studied in laboratory,and the effects of different thicknesses of shotcrete thickness(17mm,22 mm,27mm),secondary lining thickness(30mm inversion arch thickness 35mm;Thickness 45mm;thickness of the two lining arch wall 50 mm and thickness of the arch 55mm),the excavation of the small radius loess railway tunnels with different initial periods of support(50,100,200 steps)after excavation was carried out.The stress and displacement of the surrounding rock and support,the stress cloud,and the displacement and stress of the corresponding monitoring points,the stability of the tunnel surrounding rock is studied under different supporting conditions.The main conclusions are as follows:1)The law of displacement and stress variation of surrounding rock under different thickness of shotcrete tunnel shows that with the increase of the thickness of shotcrete,the displacements of tunnel measuring points aredecreasing,Deformation of surrounding rock.Under the condition of different shotcrete thickness,the basic distribution law of surrounding rock and ground displacement is the same.The greater the thickness of shotcrete,the smaller the displacement of each monitoring point.For different thickness of shotcrete,with the increase of thickness,the stress of surrounding rock is higher,but it is not obvious.2)According to the law of displacement and stress of surrounding rock excavated with different secondary lining thickness,we can see that along with the increase of lining thickness,the displacements of measuring points of the tunnel are decreasing,Deformation of surrounding rock.Under the condition of different shotcrete thickness,the basic distribution law of surrounding rock and ground displacement is the same.The greater the thickness of shotcrete,the smaller the displacement of each monitoring point.For different thickness of the second lining,with its thickness increases,the greater the stress of the surrounding rock monitoring points,but not obvious.The maximum value decreases with the increase of the lining thickness,which indicates that the lining thickness increases,which is helpful to alleviate the stress concentration phenomenon.3)According to the law of displacement and stress of surrounding rock excavated under different support times,the displacements of tunnel measuring points are increasing with the increase of supporting time,so the longer the supporting time,the larger the displacement of surrounding rock.Under the condition of different support steps,the longer the support step,the larger the displacement release rate,the greater the displacement of each measuring point.Under different supporting times,the distribution of stress monitoring in the surrounding rock is basically the same,which is the largest in the left and right sides of the surrounding rock.The stress of the surrounding rock is smaller and the maximum value is distributed in the surrounding rock of a certain range.The longer the support step after excavation,the smaller the stress of each monitoring point.