| With the further development of the western region and the implementation of “the Belt and Road” strategy,the focus of transportation and water conservancy and hydropower projects in China has been transferred to the western mountainous areas and karst areas with extremely complicated topographic and geological conditions.More and more high-risk deep karst tunnel projects with the characteristics of "large burial depth,high stress,strong karst,high water pressure and large flow" have been put into construction.During the drilling and blasting construction,the blasting load and the transient unloading effect of in-situ stress caused by blasting will exacerbate the risk of water and mud inrush in karst tunnels,posing a major threat to tunnel construction safety.In this paper,the combination of theoretical analysis and numerical calculation is used to study the stability of tunnel surrounding rock under static condition,blasting load and coupling effect of blasting load and in-situ stress transient unloading,the law of crack propagation under the coupling effect of blasting load and transient unloading of in-situ stress in rock mass with fracture containing high-pressure water,and the evolution mechanism of water inrush in the surrounding cavities of tunnel under the dual dynamic disturbance of blasting load and in-situ transient unloading.The following results are obtained:(1)When the deep buried karst tunnel is constructed by drilling and blasting method,the strong disturbance action of blasting load will cause the transient unloading effect of the in-situ stress of the surrounding rock at the excavation profile of the tunnel,which will reduce the stability of the surrounding rock of the tunnel.Through the discrete element numerical simulation of the stability of tunnel surrounding rock,it is further confirmed that the blasting load and in-situ stress transient unloading can degrade the tunnel surrounding rock stability to a certain extent;(2)The crack propagation theory of fractured rock mass with high pressure water is revised and improved.Considering the transient unloading of in-situ stress,when the lateral pressure coefficient is greater than 1/3,the stress intensity factor of branch crack obtained by using the modified calculation formula is greater than that before modification;It is the first time to reveal the dynamic response mechanism of the fractured surrounding rock in the process of tunnel blasting excavation,and to establish the calculation method of stress intensity factor at the crack tip of the surrounding rock under the dynamic action of blasting load and transient unloading of in-situ stress.It is theoretically found that blasting load and in-situ transient unloading promote crack and expansion of original fracture.The reason is that strong impact,tension and compression of blasting load on rock mass and the stress concentration effect caused by transient unloading of in-situ stress aggravate crack and expansion of original fracture;(3)Considering the coupling effect of blasting load and in-situ stress transient unloading,numerical simulation of the expansion law of the original fracture in rock mass with high pressure water is carried out.The results show that with the increase of fracture angle,the growth rate of branch fracture increase.When the fracture angles are 0° and90°,the fracture expands horizontally and vertically.When the fracture angles are 15°and 75°,the slip and dislocation of the fracture occur along the slope where the original crack is located.When the fracture angles are 30°,45° and 60°,the fracture expands along the direction of blasting load and stress unloading.Under the influence of in-situ stress transient unloading,the fracture expands along the radial direction,and at the same time,the fracture deflects and expands in the tangential direction.Finally,tangential branch fractures appear at the tips of the original fracture;(4)When cavity with high pressured water is located in the upper part of the tunnel,the discrete element numerical simulation shows with the increase of rock stratum angle,the stability and anti water inrush ability of water-resistant strata decrease gradually,and when rock stratum angle is 0°,the stability and anti water inrush ability are the strongest,and they are positively related to the thickness of water-resistant strata and the buried depth of tunnel.When the thickness of water-resistant strata is 4m and the buried depth is 800 m,the stability of the water-resistant strata is the best;When cavity is located at the bottom of tunnel,the numerical calculation results show the stability of water-resistant strata increases gradually with the decrease of rock stratum angle,the increase of the thickness of water-resistant strata or the increase of the buried depth of tunnel,and the ability to resist the damage of karst water gradually increases.When rock stratum angle is 0°,the thickness of water-resistant strata is 4m,and the buried depth is 800 m,the stability of water-resistant strata and its anti water inrush ability are the strongest;(5)When the cavity with high pressure water is located at the side of the tunnel,the numerical results show that,with the increase of the rock stratum angle,the stability of the water-resistant strata is gradually enhanced,and its resistance to water inrush is gradually improved.When the rock stratum angle is 30°,the water-resistant strata has good stability.In addition,the stability of the water-resistant strata is positively correlated with the thickness of the water-resistant strata.When the thickness of the water-resistant strata is 4m,its stability is better.There is no strict positive or negative correlation between the stability of the water-resistant strata and the buried depth of the tunnel.When the depth of the tunnel is 500 m,the stability of the water-resistant strata is better and its ability to resist water inrush is higher,while when the buried depth of the tunnel is 800 m,the stability of the water-resistant strata is the worst. |
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