Study On Interaction Mechanism Of Pre-arching And Soft Surrounding Rock

ABSTRACT:Tunnelling in the geological conditions of soft surrounding rock is one important issue of tunnel engineering for years. Due to lack of understanding in deformation mechanism and development law of soft surrounding rock, the construction methods and the deformation control technologies adopted are lack of pertinence. Therefore, deformation of tunnelling in soft surrounding rock is difficult to control. ADECO-RS gives a new way to control deformation of tunnelling in soft surrounding rock.This dissertation is supported by the projects of National Science Foundation of China named Reaserch on Interaction Mechanism of Pre-arching and Stability of Tunneling Face in Weakly Cemented Sandstone With Water (No.51178026).Based on ADECO-RS, the pre-support performance of pre-arching constructed by mechanical pre-cutting method is analyzed. A study of the reasonable parameters of the pre-archining is conducted by the finite element calculating in ANSYS. The mechanism and performance of pre-arching in controlling tunnelling deformation in soft surrounding rock is obtained through FLAC3D numerical simulation method. And the mechanism of interaction between pre-arching and soft surrounding rock and the characteristics of pre-arching support is derived by theoretical analysis. The main work and conclusions are described as follows.1. The reasonable parameters of pre-lining are analyzed according to load-structure model. Safety factor of pre-lining with different thickness, different steel arch spacings, and different concrete strengths are analyzed based on safety factors method. And recommendations are provided to select reasonable pre-lining parameters.2. Based on the numerical simulation of arching effect through FLAC3D, the ground deformation curves, earth pressure-depth curves of ground above tunnel roof, development and distribution of ground arching of ground above tunnel roof are obtained. The performances of pre-arching in controlling pre-convergence of the advance core, convergence of tunnel and stress release during tunnelling are analyzed. The development process and distribution of ground arching is calculated under the support of pre-arching during tunnelling. And therefore the mechanism of pre-arching in controlling deformation of soft surrounding rock is obtained. 3. A lot of numerical analysis about different buried depths of tunnel, different rock mass conditions, different thicknesses of pre-lining and different tunnel diameters are carried out by FLAC3D. And based on the results from numerical simulation, effect of pre-arching in controlling tunnel deformation, stress release and redistribution process and ground arching development process are obtained. By leading up normalizedratio of ground pressure above tunnel roof γ, the release and distribution of earth pressure of tunnel roof is visualised. And finally, the characteristics of pre-arching in controlling rock deformation are obtained.4. Based on convergence-confinement method, the ground convergence curves (groud characteristic curve) of elastic and elastic-plastic rock mass condition are derived according to elastic-plastic mechanics knowledge. The change of the ground response curve as ground is supported by pre-arching is also deduced. Considering space-time effect of face-advance core during tunnel excavating, rock mass stress release factor fr is introduced. And therefore the interaction between pre-arching and rock mass during tunnelling is obtained. Furthermore, the interaction loads between pre-arching and rock mass is derived. Relationship between concrete strength and domain of applicability of pre-arching is calculated. The development law about applicability depths of pre-arching varying with rock mass parameters is also derived The theoretical analysis has layed the foundation for the establishment of the pre-lining support design theory.