Modelling Rough Joint Network And Study On Hydro-Mechanical Behavior Of Fractured Rock Mass

It is known that fracture is the important factor which controls the Hydro-Mechanical behavior of rock mass. Research on deformation and seepage behavior of fracture is a key component of performance assessment for rock mass engineering. With the construction of rock mass engineering such as underground mining, slopes, excavations, nuclear waste disposal and so on, the requirement of research on Hydro-Mechanical behavior of fractured rock mass is more and more strong. But, the present studies on deformation and seepage behavior of fracture are limited to the smooth and parallel fracture and the joint papameters (such as trace, aperture, dip) are assumed to be independent with each other. And the joint is assumed to be under the constant normal load condition. On the application of Hydro-Mechanical model of fractured rock mass, the related prerequisite is ignored. Aiming at those shortage of present studies, combining the international coorperated project DECOVALEX, with a view of performance assessment for nuclear waste disposal, the discrete fracture network (DFN) model and the discrete element method are adopted to analyze the effect of joint roughness, non-persistent joint, correlations between joint parameters and stress boundary on Hydro-Mechanical characteristics of fractured rock mass. A methodology for Hydro-Mechanical analysis of fractured rock mass in nature is presented. The content is list as following:1) A new method for generation of rough fracture network is proposed which is named SAW method. The rough joint is regard as a self-avoiding work. The morphology of fracture is controlled by changing the basic parameters of SAW, such as bone length, the pile-up height and so on. The main idea, implementation program and the determination of basic parameters are presented in this paper. At last, a systematic method for generation of rough joint network is presented. Considering the effect of sampling length on morphology of rough joint, SAW method is applied to analyze the relationship between JRC and other statistic parameters on description of joint roughness.2) Considering the broad applicability of joint constitutive model and the convenience of determination on the related material parameters, the SA joint model is adopted for any kind of boundary condition and is implemented into UDEC as user defined joint model. The numerical uniaxial compression test and direct shear test are used to verify the user defined joint model. This is a foundation for Hydro-Mechanical calculation of fractured rock mass.3) Based on SAW method, a rough joint with variable aperture is generated and the numerical seepage test is done to analyze the flow rule. It is concluded that the relationship between equivalent hydraulic aperture and mean mechanical aperture is an exponential function and the relationship between equivalent hydraulic aperture and JRC is a negative exponential function. After fitting these three parameters, the result of flow rule is similar with that of Banton-Bandis model (BB model). The modified cubic law of BB model is verified from a numerical simulation. SA joint model is used, combining with this modified cubic law. And then the Hydro-Mechanical joint constitutive model is presented for rough joint which is the base of study on Hydro-Mechanical characteristics of fractured rock mass.4) To consider the weakened effect of non-persistent joint on mechanical strength of fractured rock mass, a damage variable D is defined as the ratio of total trace length of non-persistent joints in a block to the total trace length of fiction joints which are the extention of non-persistent joints to the block boundaries. The implementation program of damage variable for each block is presented and the Young's modulus of intact rock is deduced with the damage variable. Then, the effect of non-persistent joints on Hydro-Mechanical characteristics of fractured rock mass is analyzed. It is concluded that the non-persistent joints have a little effect on mechanical REV and a large weakened effect on the equivalent elasitic modulus of fractured rock mass. It is shown that the non-persistent joints have a little effect on Hydro-Mechanical REV and a certain weakened effect on the equivalent elasitic modulus of fractured rock mass. Especially, the maximum equivalent permeability reduces very significantly, while the minimum equivalent permeability and the main permeability direction change very little. In practical application, the weakened effect of non-persistent joint should be considered.5) The correlation between trace length and aperture with different distribution is studied. Especially, the correlation between trace length and aperture with power-law distribution is analyzed. And the effect of correlation on Hydro-Mechanical behavior of fractured rock mass is studied. Three cases are analyzed. The first one is the fracture with constant aperture. The second one is the fracture with variable aperture following a power-law distribution which is not correlated with trace length. The last one is the fracture with variable aperture which is correlated with trace length both following a power-law distribution. It is concluded that the correlation has a significant effect on hydraulic REV and permeability of fractured rock mass. The variable aperture has a little effect on hydraulic REV, but a significant effect on permeability. Thus, the correlation between trace and aperture should be analyzed based on the geological investigations and then the research should be done for different cases in practical engineering.6) based on the above research and combining with the international cooperated project DECOVALEXⅢBMT 2, the permeability of fractured rock mass for each zone in nuclear waste disposal in Sellafield is calculated. And the permeability under different stress boundaries is analyzed. It is concluded that the permeability of fractured rock mass for zone 1, zone 2 and the fault zone respectively. The maximum permeability for zone 1, zone 2 and the fault zone are 3.89×10-6 m/s,5.76×10-6 m/s and 5.75×10-6 m/s respectively. The minimum permeability for zone 1, zone 2 and the fault zone are 2.27×10-6 m/s, 1.06×10-6 m/s and 1.97×10-6 m/s respectively. The permeability direction for zone 1, zone 2 and the fault zone are -15.32°,-32.03°and -31.33°. The permeability compenents in zone 1 change with deep and the relationships are Kxx=8.209×10-6·d-0.536,Kxy=3.081×10-6·d-0.905 and Kyy=2.205×10-6·d-0.635 respectively.The permeability compenents in zone 2 change with deep and the relationships are Kxx=4.275×10-6·d-0.255,Kxy=5.299×10-6·d-0.325 and Kyy=5.971×10-6·d-0.115 respectively.The permeability compenents in fault zone change with deep and the relationships are Kxx=4.04×10-6·d-0.141, Kxy=3.048×10-6·d-0.246 and Kyy=5.99×10-6·d-0.114 respectively.Compared with the result of Blum,the calculation result in this paper is smaller.The reason for that difference is the joint roughness and the weakened effect of non-persistent joint which is considered in this paper.