Study On Influence Of Hydraulic Fracturing On Stability Of Crack In Pressure Tunnel Wall

The hydraulic fracturing is a major reason of damage of hydraulic water pressure tunnel. In this paper, by using the finite element method, the pressure tunnel numerical models considering cracking are established, and smeared crack model and discrete crack model are adopted respectively in them. The influence laws of the stability of crack in pressure tunnel wall with the changing of inner water pressure, the crustal stress, the lengths and positions of fracture are studied. The main tasks of this paper are as following:First, this paper chooses typical engineering cases and establishes a numerical analysis model of cracking in pressure tunnel applying smeared crack model. The cracking rules in the influences of inner water pressure and the crustal stress are studied. The results show that the water pressure and the crustal stress are key factors that affect the cracking in pressure tunnel and the stability of the crack in pressure tunnel wall. With the increase of water pressure, the crack area of pressure tunnel increases, the number and the length of cracks increase gradually. But when the water pressure reaches a certain value, new cracks are generated no longer, while the lengths of several cracks are increased. Compared with the condition without initial crack, the length of cracks of the pressure tunnel with initial crack increased rapidly, and may even to the surrounding rock, so the hydraulic fracturing effect is obviously. The smaller the angle with the maximum principal stress, the easier the cracking occurred. For the pressure tunnel without initial crack, the greater the lateral pressure coefficient, the more difficult the occurrence of cracking pressure tunnel; on the other hand, the smaller the coefficient of lateral pressure, the more prone to cracking, so the lower the safety of the structure.Second, this paper sets up a crack in the pressure tunnel wall artificially and establishes a numerical analysis model of crack in pressure tunnel wall applying discrete crack model. The influence of the stability of crack with the changing of water pressure, the crustal stress, the lengths and positions of fracture are studied. The study results shows that, in the same crustal stress and boundary conditions, the higher the water pressure in water filling process(or the lower the water pressure in water discharge process), the easier the crack propagation; in the same water pressure and boundary conditions, in the condition of the angle with the maximum principal stress is small, when the coefficient of lateral pressure is very small or large, the easier the crack propagation; in the same load and boundary conditions, the longer the crack, the easier the crack propagation. The value of crack propagation is not very large in one water filling (water discharge) process. But after several times, the value is quite considerable. So the hydraulic fracturing of hydraulic water pressure tunnel may occur. At the same time, as the water pressure in crack is a key factor, it must be considered in the study.