Permeability Of Fractured Rock Masses And Its Variations Induced By Engineering Disturbances

The permeability of fractured rock masses is determined by its fracture network and connection, and is also influenced by its lithology and stress environment. The stress environment and structural characteristic of fractured rock masses vary with engineering disturbances, such as excavation, anchoring and grouting. It leads to a strong evolution of the permeability of fractured rock masses, which evidently leads to change in distribution of the seepage field and performance of the seepage control system. The permeability of fractured rock masses and its variation are not only the hotspots in the field of international rock mechanics in recent decades, but also the key scientific issues in water resources and hydropower engineering, deep underground engineering, mineral, oil and gas mining engineering and nuclear waste disposal engineering. Particular concerns are paid on the permeability of fractured rock mass in JinPing-I and Kala hydropower stations. By using the methods of theoretical analysis, numerical simulation and monitor feedback, the permeability and its evolution of fractured rock masses under saturated and unsaturated condition are studied. The main contents of the paper are as follows:(1) The multi-scale effects of permeability of fractured rock masses and its representation are stated. The method which uses the results of packer tests in boreholes and the statistical information of fractures to determine the permeability tensor of fractured rock masses is improved. The variation mechanism of permeability of fractured rock masses induced by engineering disturbances, such as excavation, anchoring and grouting, is stated. The disturbance effects of the excavation of large scale rock masses in Kala underground powerhouse and the excavation-induced permeability variation are studied based on the strain-dependent permeability tensor model. The influences of the shear dilatancy and normal deformation on the permeability of fractured rock masses are revealed. The significant influence of the excavation-induced permeability variation on the seepage field in the surrounding rocks and the overall behaviors of the seepage control system are evaluated. This work provides a theoretical support for the optimal design of the seepage control system.(2) A multi-objective back analysis method of permeability of rock masses which is based on NSGA-II is proposed. Firstly, the time series data of pressure head and seepage flow rate are used for constructing the multi-objective functions. Secondly, the training and testing samples are determined by orthogonal design and Latin hypercube sampling, respectively. The neural network is used to map the relation between the inputted hydraulic conductivities and outputted time series values of piezometer head and flow rate. Thirdly, the Pareto optimal fronts of seepage parameters are finally determined by multi-objective optimization method NSAG-Ⅱ. The method not only considers the variation of the seepage monitoring data with time, but also overcomes the shortages of traditionnal single objective method. The uniqueness and reliability of the solution are preferably ensured by using this method.(3) Based on the analysis of the geological condition and structural characteristics of the high left-bank slope of JinPing-Ⅰ hydropower station, the variation of permeability of fractured rock masses of the slope is studied using the strain-dependent permeability tensor model. The variation of permeability of rock masses of the slope is determined during the process of excavation, dam filling and reservoir impounding. It can be helpful to the feedback of seepage field and the performance assessment of seepage control in the process of impounding. The multi-objective back analysis method is used to analyze the seepage field of the left-bank slope of JingPing-Ⅰ hydropower station in the impounding period based on the analysis of the time series data of pressure head and flow rate. The permeabilities of the rock masses are determined subsequently. The seepage distribution and seepage control performance of the slope in the progress of reservoir impounding are analyzed. Based on the above analysis, the reason of water inrush in EL.1595m drainage gallery of this power station is finally found out. These studies show that the method can solve the seepage back analysis problem well, even if the geological condition is complicated and seepage control system is complex.(4) A methodology for predicting the unsaturated permeabilities of a rock fracture under deformation condition is presented. The morphology properties of a rock fracture are obtained by three-dimensional laser scanning. The relationship between capillary pressure and saturation and relative permeability under deformation condition are built by analyzing the variation of the distribution of fracture aperture. Subsequently, the models are verified by test results. Base on the analysis of the micromorphology and the equation of force balance among solid phase, liquid phase and vapor phase in rock fracture, the effective stress principle for unsaturated rock fracture is studied. The effective stress formula in smooth parallel fracture and rough fracture with any aperture distribution are then derived. The relationship between the effective stress parameter and saturation, and the influence of the area contact between two faces of the fracture are discussed. This work is helpful to study the mechanism of hydromechanical coupling of fractured rock mass under unsaturated condition.