Research On Tunnel Face Stability Control And Pre-Reinforcement Technology In Soft Surrounding Rockmass

With the development and upgrade of infrastructure such as high speed railway, expressway, railway, water conveyance, urban subway and many other facilities, the NATM tunnels with soft surrounding rockmass construction are gradually increasing in China. Particular attention is focused on the design and construction of NATM tunnels with soft surrounding rockmass, which are regarded as the key issues so as to determine success of the whole transportation engineering. In view of this, the stability control of tunnel face and pre-reinforcement technology in soft surrounding rock is studied systematically and deeply in this thesis. The main works and research achievements as follows:(1) Based on the limit equilibrium analysis theory, a new model for tunnel face stability analysis is presented by use of horizontal slice method. This model can overcome the shortcomings that the intersections between the slices and the bolt by using vertical slice method to some extent. The equivalent pressure coefficient’ka’of tunnel face bolt reinforced is put forward. With this coefficient, the reinforcement effect of tunnel face bolt is simulated conveniently and reasonably by using the horizontal slice method. Furthermore, the influence of the soil parameters and the strength of tunnel face reinforce on the tunnel face limit supporting forces are studied.(2) According to the limit analysis method, the tunnel face failure model considering the factors of unsupported section and tunnel face bolt and pre-consolidation measures is established. In order to investigate the tunnel face limit support forces, the paper studies the factors such as soil parameters the length and support force of unsupported section, tunnel depth and tunnel face bolt reinforce strength. Thus, the failure modes of shallow and deep buried tunnels with longitudinal arching effect are obtained.(3) Based on the interaction between support structure and ground, a double-parameter elastic foundation beam model with variable coefficients of coefficient of subgrade reaction for pipe roof is presented and the solution of this model is given by finite difference method. The research of the mechanical behavior, deformation characteristics and load transferring effects of pipe roof are carried out. In addition, the influence of excavation length, tunnel face reinforce strength and tunnel depth on pipe roof action are studied.(4) Through the tests of bond behavior between GFRP rebar and concrete, the paper draw conclusions that the bond proportional coefficient of steel to GFRP reinforcement approximately equal to1.2～1.5and minimum anchorage length is20times diameter. The CMR model of bond-slip relationship of GFRP rebars in concrete parameters sr and P are modified. These parameters have not only high fitting degree in comparison with test results, but also have certain degree of reliability and safety. The formula for calculating shear stiffness of GFRP bolt with high degree of reliability is put forward.(5) By means of large-scale similarity model test, the mechanism of pipe roof pre-support is studied. It is found that the load can be transferred to the distance between1.25m and3.75m (i.e.0.1～0.3D) in front of tunnel face. In short, the mechanism of pipe roof pre-support is regarded as the action of load transferring with beam effect and dispersing load and decreasing stress concentration. (6) The pre-reinforcing mechanism of face bolt improving the stability of surrounding rook by controlling deformation of tunnel face is closely related to the effect of longitudinal pressure arch. In addition, the range of longitudinal pressure arch by face bolt reinforced is about0.6D, which is less than the results of small pipe reinforced and pipe roof reinforced. By face bolt promoting the formation of longitudinal pressure arch in short distance, the stability of tunnel face can be effectively improved and prevent surrounding rock collapse.(7) Though analysing of the stability of soft surrounding rockmass in unsupported section, the result shows that the deformation of the upper rock mass of tunnel is controlled by limiting the loosening ground deformation around tunnel in cases of supporting by pipe roof or small pipe, the effects of pipe roof or small pipe are regarded as the passive-control. Comparing the difference of the effects above, the deformation of surrounding rock mass of tunnel in case of face bolt supporting is effectively controlled by enhancing the rockmass strength of the core zone in front of tunnel face with forming longitudinal pressure arch, the effect is regarded as the active-control.(8) Base on the analysis of work characteristics of all kinds of the pre-reinforcement measures around face, we can benefit from the combination action of face bolt support together with small pipe, making use of face bolt advantage in controlling deformation of face extrusion and in limiting the deformation of rock mass in unsupported section around tunnel. In the combination supporting system, their controlling effects can not be counteracted. The face bolt pre-reinforcing measure is seen as the major load bearing component of the pre-support system, and the small pipe supporting measure is relatively seen as a partial load bearing component of the pre-support system.(9) By comprehensive analysing surrounding rockmass stress, deformation and plastic zone, the different tunnel construction methods in soft surrounding rockmass are studied. The result shows that the full face method with tunnel face bolt pre-reinforcing is much better than three-bench seven-step method, and the three-bench seven-step method has an advantage over the double side drifts method. Through enhancing the rockmass strength of the core zone in front of tunnel face by bolt, the surrounding rock deformation can be controlled effectively by the full face method with tunnel face bolt pre-reinforcing. In contrast with other methods, the full face method with tunnel face bolt pre-reinforcing is more competitive in terms of cost and excavation speed and construction safety, and allows the use of a large excavation area, using large construction machinery with deeper excavated length.