Mechanism And Analysis Of Rock Mass Excavation Damage And Relaxation

Rock mass excavation damage and relaxation has great effect on the engineering safety and economic rationality. Probing the relaxation mechanism and establishing the mechanical model and analysis method of relaxed rock mass are of great importance for reasonably reflecting the relaxation effect on the project, which also have very important practical significance and significant social benefits.In this paper, the relaxation criterion, analysis method and damage constitutive model of relaxed rock mass are studied through combining the relaxation problem in Xiaowan arch dam and relying on on-site monitoring data, the present research basis of research group and domestic and international literature. The thesis mainly contains following contributions:(1) The research achievements of domestic and oversea literature about rock mass excavation damage and relaxation are reviewed in detail, some critical scientific and technical issues to be explored in the research are put forward, and the thesis research contents are introduced.(2) Aiming at the deficiencies in current back analysis methods of initial geostress field, the method combining first-stage back analysis and second-stage sub-model analysis has been proposed for improving the back analysis precision. Then the section in the direction perpendicular to the river at an assumed hydropower dam site has been taken as an example for preliminary judging the effectiveness of the new method. The research suggests that the soft geological structures such as faults have much effect on the results of initial geostress field, especially in the vicinity of discontinuity during the first-stage back analysis. And the precision of the initial geostress field in the first-stage back analysis has been greatly improved in the second-stage calculation. The geostress of most measuring points are much closer to measured values and the relative error has also been reduced gradually.(3) Based on the relaxation mechanism of rock excavation and results from a large number of rock mechanics testings, a criterion of maximum principal strain for pinpointing the relaxation region is put forward, and the principles for determining ultimate tensile strain and the method for calculating the maximum principal tensile strain are explained clearly, and also a practical finite element algorithm for the evaluation of the relaxation effect is established, which considers the changes of both elastic indexes and strength indexes. At last, the relaxation criterion and practical finite element algorithm have been applied into the analysis of the section in the direction perpendicular to the river at an assumed hydropower dam site and the results of calculation have quantitatively determined the extent of relaxation region and the phenomenon of stress redistribution.(4) Aiming at the compaction phenomenon of unloading fractures in relaxed rock mass at dam foundation during the period of dam pouring and based on the basic principles in the rheology model of jointed rock mass, the fractures in rock mass are simulated by the filled medium model and the relationship between the stiffness coefficient on fracture surface and normal stress is deduced. Then, the formula of equivalent elastic modulus for relaxed rock mass is conducted and also the implementation details of the algorithm are explained clearly. At last, the reasonability of the algorithm is verified by the analysis of a square section of rock mass which contains the circular hole cell, concrete sprayed on the wall layer and joints. The results of the example suggest that the new algorithm can simulate the reverse slow transformation process very well and it also has important potentiality for reasonably reflecting the relaxation effect during the later construction period.(5) In order to establish the damage variable and corresponding evolution equation for each rock mechanical parameter, the accumulation equivalent viscoplastic deviatoric strain in the classical theory of elastio-viscoplastic potential has been used as the internal variable. Then, through continuously renewing the relaxed mechanical parameters and calling the practical relaxation algorithm, the elasto-viscoplastic damage constitutive relation of relaxed rock mass could be derived and its reasonability and effectively are proved by a simple example. Meanwhile, aiming at the difficulty in determining the accumulation equivalent viscoplastic deviatoric strain for practical application, a new numerical test system(NTS) has been established which can calibrate its specific value and correct the damage constitutive model. Finally, combining with the test curves of Baishan marble which comes from access tunnel in Jinping hydropower station, the accumulation equivalent viscoplastic deviatoric strain has been determined and a basis for correcting the damage constitutive model of relaxation effect has been provided, which verifies the rationality of NTS.(6) The new method for analyzing the relaxation effect of relaxed rock mass proposed in this thesis has been applied into Xiaowan project. The initial geostress field at dam site zone and the relaxation effect of rock mass during the period of foundation excavation and dam pouring are both studied, the simulation results and field monitoring datas are compared in detail. The research does not only reasonablely reflect the conventional stress-strain laws of the bedrock during the period of foundation excavation, but also highlights the stress redistribution phenomenon in a certain region; not only truthfully reflect the relaxation rebound of relaxed rock mass, but also accurately determine the impact time that the relaxation effects to the dam-foundation system. The reliability and practicality of the new method is verified further by the comparative analysis between numerical simulation and monitoring data.Finally, some researches to be explored in future are put forward after summarizing research achievements of the thesis.