Research On The Mechanical Behavior Of Unsymmetrical Loading Twin-arched Tunnels Located On The Shallow Weak Rock Mass

In the past decade, the twin-arched tunnels were widely applied in high-way engineering in China. As an innovative structure of tunnelling, there are many problems and chanllenges during the process of construction. Based on the investigation of 222 twin-arched tunnels projects, the main characteristics and the relevant parameters of the twin-arched tunnels are analyzed to study the main issues and the factors in the application. It is recognized that the weak surrounding rock, shallow depth and unsymmetrical loading are proved to the major objective factors for the construction accidents and the tunnel deteriorations. In this paper, the twin-arched tunnels of Si(MAO)-Xiao(Meng Yang) highway in Yunnan Province, China, are taken as background. A set of laboratory tests, model experiments, field investigations and numerical simulations are employed to discover the mechanical properties of mudstone and the mechanical properties, and the time effect stability of an unsymmetrical loading tunnel; meanwhile, a design method to calculate the load of an unsymmetrical loading tunnel is explored.The mudstone samples No.A and No.B which were drilled from the excavation site of the background engineering are represented as a Class IV rock mass and a Class V rock mass respectively. Through laboratory testing, physical and mechanical properties of mudstone and the parameter values are discussed. It is proved that mudstone with higher water content and easy weathering characteristics is prone to producing fissures or block spilling phenomenon in the air. The compressive strength of mudstone depends on the water content and the existence of weak joints. It is found in the experiments that the whole process of a stress-strain curve under the uniaxial compressive action for a mudstone specimen can be divided into five sections: Section I in which micro-fractures closure in mudstone, Section II in which elastic deformation begins, Section III in which dilatancy germination occurs, Section IV in which unstable cracks grow, and Section V which is after the peak. Section I of the stress-strain curve of a No.A specimen is more obvious than that of a No.B specimen. But the length of Section V of the stress-strain curve of a No.B specimen is greater than that of a No.A specimen, and the ratio of the residual strength to the peak value of a No.A specimen is greater than that of a No.A specimen. Differing from the specimens without weak joint, the stress-strain curve of a specimen with weak joints is consisted of the compression deformation, the shear deformation and the softening deformation of the weak joints, which idenfies the supposition that the weak joints determine the strength and the deformation of the rock mass.Mudstone with a high degree of creep deformation is proved by the unialxial compression creep tests. And the creep deformation increases with the more moisture content of the mudstone. The creep properties of a mudstone specimen are related to the drainage conditions. A Burgers model is suitable to analyze the creep properties of mudstone without drainage conditions, and a generalized Kelvin model is suitable to analyze the creep properties of mudstone with drainage conditions. During the creep in the drainage, the hardening of mudstone is observed from the creep tests.A series of the small proportion of 1:100 model experiments, the field investigations and the numerical simulations are carried out to study the mechanic behaviour of twin-arched tunnels without or with unsymmetrical loading. Without unsymmetrical loading, the stress and the deformation of twin-arched tunnels differe from the construction stage. During the excavation, the surrounding rock pressures are rapidly relaxed, and the large deformation is achieved which maybe resolve in instability. In the stage of invert construction, as weakness of the side wall supporting structure, primary support structure is declined, and the surrounding rock pressure of the arch is deceased, meanwhile that of the side wall is increased. The concrete lining, invert and the middle wall integrate a closed structure, and effectively limit the surrounding deformation, then enforce the stability of the tunnel supporting. Finally, the secondary support and the primary support share the relaxing stress and the creep deformation. Compared to the first tunnel, the deformation of the later tunnel such as the arch settlement, the wall displacement is smaller.With unsymmetrical loading, the twin-arched tunnels move to the shallow ground, which resolves in the passive pressure on the shallow side wall and the active pressure on the buried side. The tunnel structure is not balanced state in the force, for the ground pressure on the shallow side is greater than that on the buried side. The unbalance force is increased with the increasing slope angle. If the unbalance is great enough to cause the large deformation of the tunnel lining, the cracks and the water leakage would get in the tunnel. The arch settlement of the first tunnel is increased with the increasing of the unsymmetrical pressure. Otherwise, the unsymmetrical pressure decreases with the increasing of the buried depth of the twin-arched tunnels. It is said that the buried depth is 1.6 times the excavation width of twin-arched tunnels, which insolves less unsymmetrical loading.Without unsymmetrical loading, the surface subsidence of the twin-arched tunnels appears to be a non-symmetrical shape according to the midpoint of the middle wall, and the subsidence on the first tunnel is greater than the other. The main surface subsidence occurres during the excavation of the two tunnels, the others is about 10% that of the final value.The aging characteristics of the twin-arched tunnels are relevance to the unsymmetrical loading. Without unsymmetrical loading, the ground pressure, the load of the tunnel lining and the deformation are a function of logarithm of time, and reached to a fixed value in the final stage. As of unsymmetrical loading, the primary support displacement increases with the time, the increasing of the slope and the decreasing of the buried depth.Based on the results of the model experiments, the field investigations and the simulation analyses, a criterion of an unsymmetrical loading tunnel is put forward. Combination of China's standards "highway tunnel design specification" (JTGD70-2004), a calculation method for the ground pressure of an unsymmetrical loading tunnel and the load of the middle wall and its eccentricity is proposed in this paper.