Finite Element Analysis Of Jointed Rock Mass In Tunnel By Blasting

The complexity of rock mass blasting is aggravated by Joints. At present, the determination of blasting influence scope is generally tested by afterwards means, and the test condition has many limitations and parochialisms. Study the mechanics of jointed rock under blasting and analysis the property of joint and rock affecting on blasting result to get rational blasting parameter and decrease overbreak and underbreak for fine profile of tunnel, which is paid attention in rock mechanics academic circle home and abroad, and is also the purpose and meaning of this paper.Because theory calculation needs lots of simplifies and the cost of experiment research is much too high, numerical analog method has become the main tool to resolve science and technology problems with the quick development and extensive application of computers. Numerical analog method also has been successfully applied in blasting area. It makes numerical analog of dynamic damage phenomenon of materials by mathematical physics that approaches actual conditions, in order to show the whole process of rock mass blasting and its effect. Therefore, it helps to deeply understand the mechanics of rock mass blasting and attack damage. Besides, it can be checked mutually with the theory calculation and experiment research, so as to provide better service for actual engineering.This paper studies the effect of geometric and mechanical characters of joint rock mass to blasting effect. It uses ANSYS/LS-DYNA dynamic finite element code to analog the cracking process of joint rock mass under blasting loads, and analyzes the blasting rule. It establishes blasting rock mass model, damage rule, explosion model and state equation by the character of rock mass blasting. The paper makes numerical analog of rock mass blasting on tunnel face that has many kinds of joint, gets effective stress and maximum shearing stress when the rock mass is blasting, and analyzes the influence rule of joints to blasting effect. The process of building model in ANSYS/LS-DYNA has been shown simply, and non-reflecting boundary has been applied in simulating endless rack mass. Good results have been gained by adding non-reflecting to the model of 3D multi-layer mesh.As for the study subject in this paper, there are two algorithms to be chosen, Lagrange and ALE. In order to determine which is better, two kinds of blasting hole axial direction numerical model in tunnel face has been built and the result indicated that the blasting effect with Lagrange method is more close to the practice and more convenient to build model. Therefore, Lagrange method has been used in the simulation and analysis in this paper.The failure character of rock mass under dynamical load is unclear,which is at a controversial stage, in the other words, there is a distance to the stage of accurate mathematics description. For avoiding the unfavorable factor and ensuring the reliability of modeling process and the conclusion, the constitutive property of material has been simplified, with assumption that rock mass belongs to elastic-plastic medium. Using Mises yield condition, the change of effective stress of elements has been used to describe the failure property of rock mass, and then the influence of joints on the rock mass by blasting can be analyzed.Builting 3D numerical model of intact rock mass by blasting and axial direction charge structure model, the dynamical process of rock mass by blasting has been gained and pave the way for further study. The models of decoupling charge and coupling charge in radial direction have been built respectively to analysis the blasting effect of the two models. The result shows that decoupling charge can produce sound simulation of blasting process. so that the models in this paper will adopt decoupling charge.Discuss the influence of joints direction on the blasting effects: 1)The position of Joints to the tunnel excavation boundary has affect on the blasting effects that overbreak will appear when the joint is between the blasting hole and tunnel excavation boundary, while underbreak would appear when the joint is out of the excavation boundary.2)Build the model of jointed rock mass by blasting with single joint which has intersection angle 45°and 60°with the horizontal line。The results show that there appear overbreak and underbreak. Advise to increase the number of blasting hole to solve the problem of underbreak and to drill empty hole to solve the problem of overbreak.3)Build the tunnel rock mass blasting models with 0°,45°,90°and 135°joints, and by the method of effective stress analysis, the barrier of joints to the stress wave results in the problem of overbreak and underbreak in tunnel rock mass.Through adopting the curve the change of the effective stress as time in several elements, study how the effective stress decays before and after improve the strength of joints. The result shows that the strength of joints is a very important factor to the stress wave decay.Three rock mass blasting models have been built, with joint width 0.5cm, 1cm and 2cm, and the influence of the width of joints on the rock mass blasting effective has been discussed. The results show that the width of joints has not very much influence to the propagation of the stress wave.