| Abstrakt: |
If the tunnel construction work is done in the karst area, the tunnel shall keep a certain distance from the karst cave to ensure the stability of the surrounding rock and support. If the distance between the tunnel and the filled karst cave with high water pressure is short, it is easy to cause the instability and damage of tunnel face or the circumferential surrounding rock, thus resulting in karst water inrush, mud inrush, collapse and other disasters. Especially, the karst cave concealing around the tunnel is often difficult to be accurately predicted. Therefore, the study of the critical safety distance between the karst cave and the tunnel plays a vital role in the evaluation and prevention of disasters caused by concealing filled karst cave. Studies on the critical safety distance between tunnel and karst cave can be divided into qualitative research, semi-quantitative research and quantitative research. Qualitative research, such as judging the influence degree of karst cave on tunnel through numerical analysis, is generally conducted to comprehensively analyze the factors affecting the stability of structure for water burst prevention and obtain the empirical critical safety value. Semi-quantitative research mainly focuses on theoretical calculation, which can be roughly sub-divided into three types: simplified beam slab model based on strength theory, catastrophe theory model and the model of crack tension-compression shear based on fracture mechanics. Mainly focusing on numerical simulation, quantitative research is conducted to set up multiple groups of numerical tests for calculation, and to obtain the calculation model of safety distance through linear regression. The safety distance between the tunnel and the filled karst cave can be explored by theoretical calculation and numerical simulation test. Based on the bending and shear strength theory, the mechanical calculation models of the filled karst cave at the top, bottom and side of the tunnel are established by using the fixed beam model at both ends. Meanwhile, the self-weight of the internal filler and the pore water pressure are taken into account. Given the influence of the surrounding rock pressure above the karst cave on the waterproof rock stratum, the critical safety distance between the tunnel and the karst cave can be explored, and the calculation formula of the distance in different circumferential positions can be deduced. However, this formula does not consider the excavation effect of the tunnel and the process of support, nor can it reflect the changes of the displacement, stress and plastic zone of the surrounding rock. Therefore, it is difficult to obtain a universal formula to express relationship. It is necessary to supplement the predicted safety distance with the help of numerical simulation. In this study, the numerical model of the critical safety distance between the filled karst cave and the tunnel at different circumferential positions was established by FLAC 3D software. Based on the method of orthogonal experimental design (a total of 48 numerical test schemes), the stability of surrounding rock has been evaluated by the distribution range of plastic zone of surrounding rock caused by tunnel excavation. If the plastic zone connects the tunnel with the karst cave, the distance between these two indicates that the surrounding rock is unstable, and thereby the critical safety distance can be calculated. The results of critical safety distance under different working conditions were also analyzed by range analysis and variance analysis. Besides, the law and significance of three influencing factors--the level of surrounding rock, the water pressure in karst cave and the karst cave size--on the critical safety distance were explored. Through the nonlinear multiple regression analysis of the orthogonal test results, the prediction formula of the critical safety distance between the filled karst cave and the tunnel at different circumferential positions was established respectively. The results show that the critical safety distance increases with the increase of surrounding rock level, pressure of karst cave water and the karst cave size. The comprehensive influence degree of the three factors can be ranked from the strongest to the weakest as follows: the surrounding rock level, the pressure of karst cave water and karst cave size. Finally, the research results were applied to Yangzong tunnel project to verify the rationality and applicability of the prediction model of critical safety distance. The results show that the safety distance predicted based on the strength theory is relatively conservative, and the predicted results based on the numerical test are close to the reserved distance. The predicted results have a certain reference for the project of karst tunnel with water abundance. [ABSTRACT FROM AUTHOR] |
