Characteristics Of Surrounding Rock And Calculation Method Of Relaxation Pressures Of Underground Excavations In Up-soft/low-hard Jointed Rock Stratum

ABSTRACT:In recent years, with more and more underground excavations such as expressways, railways and metros being built at home and abroad, the construction environment becomes increasingly complex. The widely used designing methods which based on the continuous medium mechanics and homogeneous materials are difficult to meet the construction requirements. Among those problems that need be solved, the calculation methods of the relaxation pressures was seen as the most important and basic problem. Because the currently methods can not reflect the influence of distribution characteristics and mechanical properties of the rock joints on the shape and the size of the loosening zone, errors occur when they were used to calculate the loosening pressures of underground excavations located in rock mass strata. Hence, it is crucial to study the loosening and failure mechanism of the surrounding rock which could reflect the true characteristics of the jointed rock mass and develop a reasonable calculation method of the relaxation pressures.Relying on the project of the National Natural Science Foundation of China, the characteristics of the loosening zone and the estimation of the relaxation pressures were analyzed. For underground excavations in up-soft/low-hard rock stratum, a systematic research of the loosening mechanism, the loosening mode, the failure mechanism, the quantitative evaluation and qualitative analysis of the loosening zone, the scope and expressions of the boundaries of the loosening zone, and the relaxation pressures were analyzed by using combining methods of field tests, numerical simulations and theoretical analyses. The research results of this paper shed light on the influencing of the rock joints and provided a valid and efficient approach for the determination of relaxation pressures; it is also a useful auxiliary tool for stability problems of other similar projects. The main research conclusions are as follows:(1) Based on the rock mass sub-classification system, a comprehensive numerical analysis is carried out utilizing2D UDEC code, a software following distinct element method, total384numerical models were conducted, and12influencing factors (the depth, the span, the ratio of height and span, the lateral pressure coefficient, the thickness of the soft rock stratum, the physical and mechanical properties of the strata, the states of the joints, et al.) including4sets independent joints were considered in each model. Through analysis,4phases of the joints’loosening progress were obtained as follows:the crack propagation phase, the increasing loosening phase, the decelerating loosening phase, and the completion loosening phase. It is also pointed that, the slipping and opening of the joints were the main reasons of the occurrence of the instability of the excavations.(2) The failure modes of the surrounding rock with different joints dip angles were studied. The location and the reason of the failure were discussed, and the reasonable corresponding construction and supporting methods were given. It is concluded that, the prediction of the exact failure location is unlikely to be successful in practice since the collapse location is highly sensitive to the joint patterns and there could be more than one failure mechanism taking place at the same time. Note that, it is difficult to give a quantitative analysis of the range and the boundary of the surrounding rock collapse region.(3) The influence of the slipping and opening of the joints on the loosening zone were analyzed, and the range of the loose zone can be determined by identifying the slipping zone of the joints. Three loosening patterns were obtained:the ringent trumpet-shaped loosening zone, the closing trend trumpet-shaped loosening zone, and the arch-shaped loosening zone. By analyzing the relationship of the depths, the span and the angles of the excavation, conclusion obtained that the closing trend trumpet-shaped loosening zone is a transitional form.(4) A new method to determine the critical depth of excavations in rock strata was given, the minimum corresponding depth with which the loosening zone share the arch-shaped were seen as the critical depth.726distinct element models in total were conducted, and a critical depth table was given for excavations of which the span various5m to26m in Ⅳ class and Ⅴ class rock mass.(5) Quantitative analysis on the weight of each factor was conducted by using the data envelopment analysis. The efficiency of each of the influencing factors was analyzed, and the controlled key factors were selected.(6) Regression analysis is conducted to the boundary shape of loosening zone and results in two empirical expressions:one for the ringent trumpet-shaped loosening zone and another for arch-shaped loosening zone. These empirical expressions can be used to predict the shape of loosening zone for tunnel construed in layered rock strata. With this basis, the qualitative analysis of the influencing factors on the lateral length of loosening zone on the ground surface and the height of the loosening zone were conducted. (7) The calculation principle, the applied range, and the limitations of several classical calculation methods of relaxation pressures were discussed. Based on the origin calculation principles of the classical methods and combined the research results of loosening zone characteristics, the modified calculation formula were given.(8) Given the shape of loosening zone, the relaxation pressure can be analytically solved. Four series of formulas are derivate respectively according to the loosening pattern, i.e. ringent trumpet shape and arch shape, and excavation condition which is divided into two classes that the depth of the tunnel is greater or smaller than the thickness of the soft stratum.(9) In order to verify the two newly proposed methods, i.e., the proposed method based on the principles of stress transformation and the proposed method based on the upper bound limit theory, three engineering cases were introduced. The difference, the rationality and the applicability of the proposed methods and the classical methods were discussed, and the recommend method for calculating the relaxation pressures was given, i.e., the proposed method based on the principles of stress transformation. Note that, in order to ensure the accuracy of the results, it is suggested that, for shallow buried excavations, the vertical pressure can also be calculated by using the upper bound limit theory method, and the horizontal pressures can also be calculated by using the revised Xiejiaxiao’s method; for deep buried excavations, the vertical pressure can also be calculated by using the upper bound limit theory method.