Mechanical Properties Analysis Of Soft Surrounding Rock Tunnel In Construction Stage

With the development of our country traffic routes,there will be meted all kinds of geological conditions in the excavation of tunnel engineering construction process. Of course, there would be inevitably encountered soft rock area.Because of the low intensity,loosen structure,easily disturbed and high hydrophilic of soft rock compared with the hard rock. These characteristics determine its deformation and mechanical properties in the process of tunnel excavation is also distinct from hard rock.Firstly,this paper discusses the pressure of surrounding rock,stress distribution and deformation of surrounding rock after excavation in the theory and analyze the causes and specific forms and related formulas which produces. Taking the soft rock section of a tunnel in Shanxi for example,simulate the construction process of the excavation with the software of MIDAS/GTS.Using whole section excavation method and heading excavation method to simulate the construction process,obtained during the construction phase of the surrounding rock displacement and stress-strain.(1)When using full-face tunnel construction method for rock excavation process disturbances fewer constraints surrounding rock lifted relatively quickly,although the final value of the displacement is not too large,but the pace of change is faster, is not conducive to control the deformation of the surrounding rock.During the excavation will be significant crown settlement,invert muster up, and both sides of the side wall to the inside of the tunnel squeeze contraction trend. Surrounding inwardly extrusion shrinkage in the x direction maximum displacement is 1.794 mm,as the side wall presses inward displacement value;down wardly along the z-axis direction of the maximum displacement value settlement settlement 13.029 mm, for the dome uplift the displacement value.(2)When using full-face tunnel construction method,because it is a whole section of the excavation,and the excavation of circular cross section,strong load bearing capacity of the structure.The first principal stress and third principal stresses in the side wall of the greater curvature of the arch in the foot a large stress concentration occurs.From the side walls to the dome, along the tunnel contour,from sidewall to the vault,from the bottom of the arch of the foot arch,the main stress becomes smaller. In the dome and vaults at minimum there will be stress.The first is the maximum principal stress-707.521 k Pa,appeared in the side wall at a minimum of-165.885 k Pa, appears in the bottom of the arch.The third is the maximum principal stress-1852.85 k G,appeared in the side wall at a minimum of-514.439 k Pa,appears at the bottom of the arch.(3)When using full-face tunnel construction method, shotcrete layers of internal forces and bolt axial force are relatively uniform.Because the sidewall and the arch of the foot inward squeeze maximum shrinkage deformation, so in two parts, the shaft bolt greatest force.Axial force shotcrete layers in the transverse shrinkage sidewall and spandrel at large,at a minimum spandrel;up and down the stairs at the interface,because it is distributed up and down the stairs excavation,stress concentration occurs,so the sidewall out the maximum shear;maximum bending moment also appears in sidewall place.(4)When using bench cut tunnel construction method,x-axis lateral displacement than the whole section excavation large,the maximum displacement occurs in the sidewall at as 3.304 mm. z-axis direction than in the whole vault sinking large section excavation,the maximum settlement value of 11.107 mm,dome uplift smaller than the full-face, the maximum uplift is 12.805 mm.(5) When using bench cut tunnel construction method,while the upper half of the excavation, the lower the level of stress concentration will be very easy,very detrimental to withstand external loads,so timely,prompt appropriate facilities for the supporting structure,while on the upper and lower level connections for reinforcement.The first principal stress and the value of the third principal stresses are small when compared with the whole section excavation,the maximum value of-665.456 k Pa,the third principal stress is-1820.9k Pa,appeared at the stress concentration at the sidewall.(6) When using bench cut tunnel construction method,lateral displacement happening at the side wall section is larger than the whole law,and therefore the axial force shotcrete layer is larger than the full-face method,the maximum is-1398.92 k N,maximum shear force values and maximum bending moment values are Min is larger than when the whole paragraph.The maximum shear force values appear in the sidewall at for 36.899 k N,the maximum bending moment is-25.085 k N ? m,appears in the spandrel.