Study On Stability Of Jointed Rock Mass Affected By Random Fracture And Its Application On Sub Sea Tunnel

Associating with fracture mechanics, damage mechanics and damage mechanics of rock mass, many experts studied the stability of jointed rock mass by using discrete medium model and continuum model. But there are a few researches to study the effect of random fracture to the stability of jointed rock mass, because of the limits of the 3D fracture network simulation technique. Because shape and distributing fashion of joints, as well as the mechanics character of joints, restrict and control directly the strength, displacement and destructive mode, it is more important for the rock engineering to study the stability effect of jointed rock mass by random fractures. With the development of Monte-Carlo simulation technique, it is possible to restructure the three dimension fracture network on basis of the fracture data in site. So it is to be true to research the stability effect of rock mass by random fractures. On based of simulation technique of fracture network, damage mechanics of rock mass and idea of the finite element method, this paper studies the stability effect of jointed rock mass by variable factors of random fractures by means of the finite element method with two-order damage tensor and finite difference method-Flac3D. These factors include the geometry character, damage parameter and filling condition of fracture. And this paper analyzes how to choose the minimum rock cover of profile (ZK9+750) of XIANG'AN sub sea tunnel in XIAMEN considering the effect of random fractures. The detailed research idea and result are as following.(1) This paper analyzes the stability effect of the sub sea tunnels in jointed rock masses by random fractures by means of the finite element method and finite element program considering a two-order damage tensor. It is compiled based on the simulation technique of fracture network, damage mechanics of rock mass and the finite element method. According to the geometry character of fractures in site, the 3D fracture network is restructured by the Monte-Carlo method by using the mathematic model. So the initial damage tensor of rock masses is rewarded by the three-dimensional fracture network. Global damage tensor based on the principle of energy equivalence can be obtained by the program of this paper. The global damage tensor is not the sum of the local damage tensor. So it can describe the anisotropic mechanical property of the rock masses. This program can research the stability of jointed rock masses which are effect by the influenced factors, such as dip direction angle, dip angle and the filling of the fractures and damage parameter. The engineering example is studied and some conclusions are as following:When the fracture dip angle is not great than 30 degree, the damage displacement of tunnel vaults and motherboard is great than that of the other place of tunnel. And the direction of this displacement is the same as that of the equivalent undamaged rock masses. While the fracture dip angel is great than 30 degree, the damage displacement of tunnel vaults and motherboard is great too. But this displacement direction is opposite to that of the equivalent undamaged rock masses with the increase of the damage parameter. The damage displacement around the tunnel increases with the increase of the damage parameter. The effect to the damage displacement around the tunnel by the parameter C_nis great than that by the parameter C_t. These analytic results can be the data to monitor and to reinforce the tunnel.(2)Since the first sub-sea tunnel of the world has been constructed in 1940s in Japan, other developed country began to construct the sub-sea tunnel. Because of their topographty, Japan and Norway have the most sub-sea tunnels in the world recently. With the development of the country, lots of sub-sea tunnels are planning to built, others are constructing. XIANG'AN sub-sea tunnel in XIAMEN and JIAOZHOU bay sub-sea tunnel in QINGDAO are constructing now, both of them are built by drill and blast method. There are some crucial problems for sub-sea tunnel constructed by drill and blast method. One of the most important problems is how to choose optimal rock cover of the sub-sea tunnel. The minimum rock cover is one of the key-parameters in the design of hard rock sub sea tunnels. If the rock cover is too small, more sever stability problems and large water inflow may be the result, and in worst case a total collapse of the tunnel. On the other hand, if the rock cover is too conservative, considerable extra costs due to extra tunnel length will be the result, and the water pressure on the lining structure increased will be the result too. So how to choose the rock cover of the sub sea tunnel is not only to consider safety but also to consider case. But it is obvious to know that the sub sea tunnel can be built no matter how the rock cover is. When the rock cover is too small to excavate the sub sea tunnel, the advanced excavation and support technique and more case will be the result. It is obviously dangerous for sub sea tunnel to be excavated.At present there are two methods, engineering analogism and numerical simulation, to design the minimum rock cover of sub sea tunnel. The main idea to confirm the minimum rock cover is as following. Firstly, the initial rock cover is chose by means of engineering analogism. Then it is to be reference rock cover, and it is to be checked by numerical simulation to ensure if the rock cover is rational. Final, the rock cover of the sub sea tunnel is optimized. The idea to choose the minimum rock cover is feasible presently but there are some shortcomings. Firstly, engineering analogism is the experience method which is gained by summarizing the sub sea tunneling experience all over the world. This method depends on the engineering geological and hydrological conditions of the sub sea tunnels. Because of the engineering geological and hydrological conditions of the sub sea tunnels are different, so the method has obvious limitations. Secondly, the idea of numerical simulation is to check the rock stability by means of the finite element method. This method has the defect to deal with the faults, weakness zones or joints. When the rock conditions are bad, they are simulated by degrading parameter based on the experience. So the result of the numerical simulation has its limitations. According to the limitations of the engineering analogism and numerical simulation to research the minimum rock cover of the sub sea tunnels in jointed rock masses. By means of definite differential method-Flac3D, the finite element method considering a two-order damage tensor can be used to discuss the minimum rock cover of profile ZkK9+750 of the sub sea tunnel in XIAMEN under random fractures. According to the results, the effects to minimum rock cover of jointed rock cover by random fractures are analyzed. The analysis shows that the displacement of symmetrical location around the tunnel is unsymmetrical. And the displacement of key points around tunnel under damage conditions is larger obviously than that under undamaged conditions, especially the horizontal displacement. According to the stability of the rock masses, the results rewarded show that the minimum rock cover of profile ZK9+750 of the sub sea tunnel in XIAMEN is about 8-meter thicker on damaged conditions than that on undamaged conditions.