Analysis And Application For The Middle Pillar Of A Tunnel With Super-large-span And Small-spacing During Excavation Process

Based on previous theory research and engineering practice of tunnel with super-large-span and small-spacing, the result shows that whether the middle rock pillar is stable concerns success or failure of such tunnel in design and construction. The stablity of middle surrounding rock pillar is considered mainly how to let stress and deformation of surrounding rock in control.To achieve this goal, it is very necessary to further study from theory and in-situ monitoring for mechanics theory characteristics and deformation law and reinforcing optimization method of the middle rock pillar during construction.This paper based on the results of previous studies and I have done a lot in-situ monitorings, regarding Qilin Mountain Tunnel as background, choosing typical sections (section of the tower), using the large-scale finite element software MIDAS/GTS to simulate by a two-dimensional model for many stages tunnel division excavation. Sorting out and analyzing in-situ monitoring results, find out the deformation regularity of middle rock (soil) pillar. The changing relation between stress and strain of the middle pillar during many stages tunnel division excavation process can be analyzed and calculated by Finite Characteristic Ratio Theory (FCRT) constitutive model, analyzing comparatively with the results between in-situ monitoring and the traditional plastic theory.Through the research mainly get the following points:(1) According to two-dimensional numerical simulation, the effect of vertical displacement, surface settlement, horizontal displacement and stress all in connection with the distance between excavation site and the middle pillar, the closer, the more influential. For the middle rock pillar, the maximum horizontal displacement appears in middle part of its right; vertical displacement appears "both ends is small, among big "parabolic law between left and right tunnel midline; vertical stress appears in the corner; maximum and minimum shear stress appears in hance and arch foot; the maximum tensile stress appears in surface rock; the maximum compressive stress appears in lower rock. The mainly monitoring should focus on hance and arch foot and "middle, and lower rock" of the middle pillar. (2) Under V level rock,for "upper, middle, and lower rock" reinforced and "middle, and lower rock" reinforced, comparatively analysis for the control effect of vault and surface settlement, the surrounding rock displacement, max and min principal stress etc, showing "upper, middle, and lower rock" reinforced is better than "middle, and lower rock" reinforced. But forâ…¢,â…£level rock, these improvements are not obvious.(3) The control point in middle rock pillar surface subsidence process have four stages: small deformation stage, deformation rapidly growing stage, slow deformation stage, gradually tend to stable stage.(4) The surrounding rock displacement numerical size of the middle pillar is related to the measuring points distance from tunnel periphery, the closer the bigger, the farther the smaller. When a multipoint-displacement measuring point exceeds 3 and radial depth measuring greater than 0, the relationship between displacement of surrounding rock and radial depth can be fitted by quadratic functions y= ax2+6x+c, results appear better, can accurately backstep surrounding rock loose area.(5) Considering height-width ratio of the middle pillar for a tunnel with super-large-span and small-spacing is greater than 2, without losing its general, assume for one-dimensional problems are analyzed. Analysis proves that the FCRT theoretical model result is consistency, the D-P model (the classic plastic theory) result has obvious difference, comparing with in-situ monitoring result. This shows deformation characteristics of the middle pillar during tunnel excavation process can be reasonably described by the FCRT model, reflecting applicability and uniqueness of the model.