Investigation On The Deformation Feature And Crack Restricting Design Of Circular Lining Of Hydraulic Pressure Tunnel

The current crack confining design for lining structure of hydraulic tunnel is basically emerged from the working mechanism of ordinary reinforced concrete elements, which differs a lot from actual variation of lining structure.Because the lining structure of hydraulic tunnel engineering is subjected to the effects of restriction of water pressure and surrounding rocks, its stress variation, characteristics of deformation, stress and cracking situations are essentially different from the ordinary concrete element of hydraulic engineering.Thus, an urgent study on the working performance of the lining structure and the load ratio of the lining to the surrounding rock must be taken. In recently years,with the vast exploration into the large diameter and deeply buried high-pressure diversion tunnel, a new understanding has been developed on the bearing role of lining structure.That is,to meet the Norwegian code and minimum principal stress criterion, the surrounding rock can perform as a primary bearing body,yet the bearing capacity of lining structure begin to reduce after cracking,In this paper, an indoor modeling test and a non-linear finite element analysis on the large-scaled hydraulic tunnel based on crack limit design were carried out. The objective is to investigate the regularity of stress and deformation during loading, obtain the initial cracking load and crack propagation process under water pressure and seepage field, analyze the influence of cracks and surrounding rocks on the lining structure as well as evaluate its bearing ability,and develop a feasible crack restricting design method of the lining structure for a safe and reasonable tunnel design in the future.The detailed contents of this thesis are as follows:(1)An indoor large-scaled(1:10) hydraulic tunnel experiment was carried out, as well as the systematic simulations on concrete lining, surrounding rocks,consolidated grouting region, cracks in surrounding rocks, etc,were taken in this thesis.During the experiment, actual water pressure was applied directly on the lining structure,and some advanced data collection devices like optical fiber were employed.The stress and deformation of the lining structure under internal water pressure, external water pressure,and the combined effect of internal and external water pressure were completely measured. It is well known that the joint work of the lining structure with the surrounding rocks and its seepage are its inherent characteristics of lining.The joint work improved the load carrying ability of the tunnel obviously,while the seepage decreased the load carrying ability of the lining.It was observed that the initial cracking strength of the lining structure was about1.1MPa, moreover, during the experiment, only a small number of cracks with clear width were produced, with an obvious feature about their developing order. Under the effect of seepage,The joint work of lining and surrounding rock was weakened, load capacity of lining led to the decrease.And the plane force applied on the lining structure was distributed to the lining and the surrounding rocks according to their stiffness before the lining cracking, while distributed on the basis of their relative osmotic coefficients after cracking.The analyzing results showed that20%～40%internal water pressure was born by the double reinforcement lining before cracking, while less than30%,even less than5%,after cracking. Furthermore, the bearing pressure for the single reinforcement lining was even smaller. Under the effect of seepage,the stress of the lining reinforcement after cracking increased to a threshold instead of increased continually with the growth of water pressure, while tended to decrease when the water pressure reduced or kept stable.A comparison on the crack width of single and double reinforcement linings was made.The results indicated that the reinforcement played an apparent role on restricting the propagation of cracks at initial loading, but it had little effect on restricting cracks under the long-term loading and improving load carrying capacity.As a result, the constructional reinforcement would satisfy the bearing requirements of lining when a good environment of surrounding rocks and consolidated grouting existed.(2) Initial failure state under internal water pressure and reinforced stress changes under external water pressure of lining structure had been expounded with analysis and verification using numerical simulation methods.The result indicated that the cause of crack and failure mode of lining structure are greatly influenced by the joints composition of surrounding rock and the defects of juncture between lining and grouting ring.A coupling model on the seepage-stress and strain relations of fractured rock mass was built. In addition, the corresponding finite element calculation program was complied. Then, according to a numerical analysis on the model under internal water pressure was taken, it could simulate the seepage after the lining cracking exactly.The numerical analysis results showed that the75%of total head lose in lining and grouting ring,and the load-bearing ratio of grouting ring was larger than that of lining.Maximum crack width was linearly correlated with reinforced stress. The inhibition role of reinforcement to crack decreased with the increasing internal water pressure.The values of calculated reinforcement stress were similar to that of the experimental results,which could exactly simulate the seepage phenomenon after the lining cracking.And coupled stress and fluid flow analysis results indicated that it is not necessary to design the crack limitation calculation of lining under favourable surrounding rocks condition or good consolidated grouting quality which has good oconstruction quality and small seepage quantity.Though the crack width increases, the reinforcement stress cannot exceed the ultimate.tensile strength and the crack cannot influence stability of surrounding rock.(3)Based on the large-scale indoor model test, analysis results and principle of statistics, two empirical formulae on average reinforced stress,σs, and average crack width,co, of tunnel lining under both internal and outer water pressure were presented. Comparing the calculated values to the experimental values and the recording data from the engineering prototype, the result indicated that the average error was about10%,which was up to the engineering standard. Therefore, the two empirical formulae may have good potential for application for tunnel lining.