Interaction And Stability Study On Shallow Part Surrounding Rock And Slope Of Double-Arch Tunnel

Double-arch tunnel is a kind of new tunnel form. Because of its more degrees of freedom in harmonizing route and choosing the position of entrance, and its good adaptability to terrain, etc, it got more and more application in recent years. But, owing to its big span, complicated structure and cockamamie working procedure, it was difficult to be constructed under unsymmetrical pressure and complex geological conditions. Especially, a series of engineering problems, such as lining cracking and tunnel slope deformation, often occur in the entrance region. In this paper, referring to the similar project of Fuxi tunnel, using both geological analysis and Numerical Simulation method, stress-strain processes of lining and rock mass were researched systematically during the construction of the double-arch tunnel under unsymmetrical pressure and complex geological conditions. Based on these results, deformation mechanism and stability of the system consisting of surrounding rock and slope were analyzed. In addition, Excavating Sequence of the double-arch tunnel under unsymmetrical pressure was studied specially as a key problem in construction. Research contents and achievements were represented as follows:(1) With the exploration of slope discontinuities, rock mass construct of tunnel entrance was comprehensively analyzed, including engineering geological property, spatial distribution and arrangement relationship. Results indicated that F₅ fault was the controlling discontinuity. Owing to its influence, rock masses and subgrade bearing of tunnel entrance got poor.(2) Based on the symptom of deformation and failure of tunnel and slope, considering the characteristics of rock mass structure, mechanism and mode of deformation were analyzed. Results indicated that main tunnel deformation mode was settlement and lateral deformation due to poor subgrade bearing capacity and unsymmetrical pressure, and main slope deformation mode was "creep sliding-fracturing" which resulted from surrounding rock deformation. (3) The whole construction process of the entrance region of the tunnel was studied with FLAC3D numerical simulation. As a result, stress-strain process of lining and the rock masses during constructing was obtained. So systematical understanding was established for the deformation mechanism and mode of the tunnel and the slope. On these grounds, stability of this system consisting of tunnel surrounding rock and slope was reasonably evaluated.(4) Based on the rock mass structure and deformation mode, a conclusion was drawn that surrounding rock deformation was the inducing factor of slope deformation. So control of the tunnel vault deformation is a key step to control the whole deformation of the entrance region. Additionally, secondary lining and inverted arch had to be constructed in time. At the same time, extension of the cut-and-cover tunnel and backfill soil on it had to be done timely.(5) Different excavating sequence of main tunnel and guide tunnel was simulated with FLAC3D. Comprehensive analysis showed that reasonable excavating sequence of guide tunnel was to excavate left guide tunnel firstly, and reasonable excavating sequence of main tunnel was to excavate right main tunnel firstly.Such research results above will make great impetuses improve the double-arch tunnel construction techniques under the complicated geological conditions of weak rock and unsymmetrical pressure, and they will be also helpful to direct the future similar project construction.