Study On Hydro-mechanical Behaviour Of Rock Joints And Computation Method Of Bolted Jointed Rockmass

The presence of discontinuities significantly affects the strength, deformability and permeability of rock masses. Many failures of underground caverns during excavation and in operation are reported to closely relate to the occurrence nearby. Examples of such works are repositories for radioactive waste, dam foundations, excavation of tunnels and caverns, geothermal energy plants, oil and gas production, etc.The performance and safety of underground facilities mostly depend on the knowledge of permeability of rock masses, which varies with in situ and disturbed stresses around the repositories and the hydro-mechanical behaviors of rock fractures. The first step in understanding the rock mass conductivity is the analysis of single rock joint conductivity. When rock fractures experience a relative displacement process, the void spaces between their opposite surfaces, namely their apertures, may increase (dilation) with relative shear or decrease (closure) with normal loads, respectively. By coupling the mechanical aperture changes to the hydraulic aperture changes, a hydro-mechanical coupling is achieved. Most research concerning hydro-mechanical coupling in rock joints has been focused on the connection between normal loading and unloading and their effect on joint conductivity. The fact that shearing of rock joints can give an increasing or decrease of joint conductivity was highlighted in recent years. The studies considering both normal and shear stresses on fractures with fluid flow, the so-called coupled shear-flow tests, have attracted much attention.In laboratory shear testing for single rock joint, the constant normal loading (CNL) condition corresponds to the cases such as non-reinforced rock slopes, in concept. For deep underground opening or rock anchor-reinforced slopes, more representative behavior of rock fractures would correspond to a boundary condition of constant normal stiffness (CNS). Laboratory testing of rock fractures involve a number technical issues that may have significant impacts on the reliability and applicability of the testing results, chiefly among them are the quantitative estimation of the evolutions of hydraulic transmissivity fields of fractures during shear under different normal constraint conditions, and the sealing techniques when fluid flow during shear is involved. In the present study, a new shear-flow testing apparatus with specially designed fluid sealing techniques for rock fractures were developed, under constant normal load (CNL) or constant normal stiffness (CNS) boundary conditions. The topographical data of all fracture specimens were measured before testing to constitute the geometrical models for simulating the change of mechanical aperture distributions during shearing. A number of shear-flow coupled tests were carried out on three kinds of created rock fracture specimens to evaluate the influence of morphological properties of rock fractures on their hydro-mechanical behaviour. Some significative conclusions are drawed. During shearing, CCD camera is used to visualize the flow state of dyeing water in the rock joint. Numerical models using the measured topographical data of fracture surfaces were conducted to simulate the change of void spaces and fluid flow during shearing.The hydraulic conductivity and mechanical behavior of the joint depend on its surface morphology as well as aperture distribution. If the joints are rough, deformations will also change the joint aperture and fluid flow. The joint roughness governs the mechanical response of rock discontinuities, either in terms of stresses or displacements, as well as its hydro-mechanical behavior. Indeed, an increased (or decreased) void space due to dilation (or contraction) will lead to an augmentation (or reduction) of the hydraulic conductivity. The roughness evolves significantly during a shear test and the interface asperity degradation has a significant impact on the hydro-mechanical response and on the alteration in hydraulic conductivity. If the fracture conductivity increases when rock wall asperities are worn off, it can also decrease when sheared rock particles close the flow path, which is known as the gouge material effect.Many efforts have also been made to visualize the fluid flow in rock fractures using different visualization technique. It was found that fluid flows in rock fractures through connected and tortuous channels that bypass the contacts areas. Flow simulations in rough fractures are often performed considering effects of only normal stress or shear displacements without normal stress or with only very small normal stresses. The Reynolds equation is commonly applied to simulate such tests for simplicity. How to measure or calculate the fracture aperture under different normal stresses and shear displacements during the coupled stress-flow tests and numerical simulations are the most essential points to understand the process, interpret the testing and simulation results and quantify the hydraulic properties. In this study, a series of laboratory coupled shear-flow tests for fracture replicas under normal stresses was performed with visualization of fluid flow using a newly developed coupled shear-flow-tracer testing equipment and these laboratory tests were simulated by using FEM, considering evolutions of aperture and transmissivity with large shear displacements. The distributions of fracture aperture and its evolution during shearing and the flow rate were calculated from the initial aperture and shear dilations and compared with results measured in the laboratory coupled shear-flow-tracer tests using transparent fracture replicas and a CCD camera that provides continuous images of contact area and flow path evolution in real-time of the shearing tests, with reasonable agreements for the validation of the numerical model. The contact areas in the fractures were treated correctly with zero aperture values with a special algorism so that more realistic flow velocity fields and potential particle paths were captured, which is important for continued works on more realistic simulations of particle transport to be reported separately later.In other ways, as a flexible method, rock bolts have been widely used for rock reinforcement in civil and mining engineering for a long time. Bolts reinforce rock masses through restraining the deformation within the rock masses. Much field monitoring work carried out on the rock bolts installed in various rock types has shown that bolt reinforce has an huge effect on the rock masses deformation and the stability of underground structure. However, the interaction mechanism of the rock bolt and the rock mass, especially the jointed rock mass, is not well understood. In order to improve bolting design, it is necessary to have a good understanding of the behavior of rock bolts in deformed jointed rock masses.The design of bolts for stabilizing jointed rocks has been largely based on the tensile strength of the steel bars. The lateral shearing action of bolts is usually not taken into account in the design. It has been observed that the localized lateral deformation of bolt at its intersection with a rock joint is usually large. These investigations helped us to understand the tensile-shearing behavior of rock bolts qualitatively and quantitatively. This paper proposes the theoretical expressions of global resistance of bolted rock joints under the tensile-shearing or compression-shearing loads. The computational method of Finite Element Method for bolted rock joint is brought forward.Establishment of an analysis method for rock masses containing discontinuities and bolt reinforce is one of the most important issues in rock mechanics. The presence of discontinuities and bolts makes the mechanical behavior of the rock mass and underground structure complicated. It is well known that the existence and behavior of joints in a rock mass governs the mechanical behaviors of the jointed rock mass, and the bolt reinforce restrains the damage of joints to the rock mass and improves the stability of the underground structure. The number of joints and bolts is, however, so large that dealing with each joint and bolt individually is nearly impossible. Thus, the bolted jointed rock mass needs to be replaced with an equivalent continuum to conduct analysis that reflects the behavior of the joints. The adjacent rock mass of underground caverns is mainly under the state of tension-shearing stress and compression-shearing stress.According to the theories of damage mechanics, the constitutive model and fracture damage mechanism of jointed rockmass are systematically studied under the state of complex stresses. Firstly, with the equivalent strain energy, the constitutive relation of anchored jointed rockmass is derived under compression-shearing stresses. The constitutive relation under tension-shearing is also developed according to the theory of self-consistence. Based on the above constitutive models, the three-dimensional finite element procedures, considering the coupling of damage and elasto-plastic deformation by dint of the half solution coupling method, have been developed to model the ground movements that occur when underground power-houses of Pumped-storage Power Station are installed in jointed rockmass. Some useful conclusions are obtained and they have great significances to the project.