Study On The Unsaturated Flow Properties And Frictional Behaviors Of The Fracture

The study on the unsaturated flow properties and frictional behaviors of the fracture is belonged to the basic research; meanwhile, the results of the research have broad applications in many areas like construction, oil/gas/heat explorations and earthquake warning. The key of the research lies in setting up precise experimental systems and lies in building up high-quality numerical models. The thesis mainly focuses on the roughed fracture, the unsaturated flow properties and frictional behaviors are studied through numerical models and experiments. The main jobs and results are as follows.1. The roughed fracture surface was produced by the fractal theory, and the fracture was formed by mating two roughed surfaces together with a certain offset (represents the displacement during geological time), then the numerical model on the unsaturated flow through the fracture was conducted. The simulated results show that there is a hysteresis between the drainage curve and the absorption curve, the drainage is lagged behind the absorption. The water exchange between the fracture and the matrix seem to significantly affect the fracture permeability when the quasi-water exchange is introduced into the numerical model. Parameters sensitivity analysis indicated that the matrix permeability shown the strongest influence on the fracture permeability whereas the matrix porosity shown the weakest impact, and the matrix initial saturation show the intermediate effect.2. An experimental apparatus for rainfall infiltration in a single fracture-matrix system was developed, and rainfall infiltration experiments were conducted to observe the rainfall infiltration and fracture-matrix interactions, and to analyze the effect of the interactions on the unsaturated hydraulic parameters of the matrix and fracture. The infiltration results show that, the matrix is the main channel for the infiltration, and the fracture acts as a barrier blocking the hydraulic connection between two adjacent matrices. While there are hydraulic exchange between fracture and matrix, the water acculmulates at the upper end of the fracture due to the capillary resistence, part of the accumulative water flow into the matrix from both sides of the fracture surface, and become an extra water source for the matrix, whose saturation process was then accelerated, while saturation process of the fracture was delayed. There are water exchange between the fracture and matrix as the rainfall infiltrating the matrix, the water flow from the wet matrix to the dry fracture. An increase in the fracture aperature will also accelerate the saturation process of the matrix. Finally, we use VG model to fit the soil-water characteristic curve of the matrix and fracture, fitting results are ideal.3. A standard triaxial apparatus was set up to perform sliding tests. Slide-hold-slide tests were conducted on dry and wet sample to study the evolution of fracture friction under holding and sliding processes. Limestone fracture shows time-dependent behaviors in SHS tests, friction drops (creep) gradually during hold periods, and restrengthens (healing) after re-slide. Frictional healing and creep increment are proportional with hold times, while are inversely proportional with effective confining pressures. Water impacts friction by two opposite ways, one is cementation (water-wear product reactions), which will increase healing, the other is flushing, where wear product is flushed out by water.4. Velocity steps tests were conducted under sliding velocities of1-6μm/s, the tests were used to study sliding stability of split limestone sample. Test results show that the friction increases with an increase in sliding velocity, the limestone sample exhibits velocity-strengthening behaviors, and the slid is stable. Flow-through test indicates permeability of the fracture not only decreases with an increment in effective confining pressure, but also continues to go down as sliding at each effective confining pressure, which mean that the fracture is closing during slid, and the rate of closure is calculated.5. Pressure solution and stress corrosion model is used to interpret the observed frictional healing in SHS tests, the processes of dissolution, diffusion, precipitation and the micro-crack propagation is incorporated to analyze the modification of fracture surface, and the corresponding fracture contact-area growth is computed. Pressure solution model underestimates frictional healing at short hold times (less than1000s) by comparing simulated contact-area growth with experiments observed frictional healing, the simulated results approache the experimental observation as hold times grow, but the simulated data didn’t fit the observed curve well. The simulated results were significantly improved by adding stress corrosion into the simulation, the short-time prediction increases, and the simulated curves agree well with the observed curves.