| The penetration of fractured rock mass involves a lot of people's livelihood and national defense projects,including deep foundation pits and tunnel excavation,oil and gas mining,stability of rock slope and dam foundation,etc.Rough fracture permeability will have a greater impact on the safety,stability and service life of these geotechnical projects.Therefore,it is of great significance to deeply study the law of fluid flow in rock fractures.Accurately characterizing the geometric characteristics of rough fractures is the prerequisite for exploring the fluid flow characteristics of rough fractures.Many studies have shown that the rock fracture surface has good fractal characteristics.Therefore,this paper uses fractal geometry theory to describe rough fracture surfaces,and combined with 3D printing technology to prepare a high-precision combined rough fracture sample for rough fracture fluid flow test.Investigate the effects of fracture roughness,degree of mismatch and anisotropy on the fluid flow of rough fractures.The main research results of this article are as follows:(1)A new method for the development of rough fracture samples is proposed.Based on the theory of fractal geometry,program software can be developed to generate rough fracture surfaces with fractal geometry.By setting different fractal dimension D,standard deviation?,mismatch length and anisotropy coefficient in the program software,different types of rough fracture surfaces are formed.The formed fracture surface with fractal geometric characteristics is introduced into Rhinoceros to establish a combined fracture seepage model.The high-precision combined rough crack sample was prepared by 3D printing technology.The rough fracture samples of any fracture roughness prepared by the method can be adjusted at the same time to accurately adjust the fracture opening and adapt to the seepage samples with different fracture aperture.(2)According to the test requirements,a seepage device capable of adjusting the height of the head is developed independently.The seepage device mainly includes a seepage water tank that can adjust the height of the head.The seepage water tank is made of plexiglass,and a water outlet is provided every 15 cm on the side of the water tank.Different seepage water pressures are obtained by controlling the height of the water outlet.sample are connected by a flexible(3)Through grey correlation analysis method,comparative analysis of hydraulic fracture opening degree,fractal dimension,standard deviation,anisotropy coefficient and mismatch length affect the degree of influence of fracture permeability.After calculation,it is found that the hydraulic fracture opening has the greatest influence on the fracture permeability,followed by standard deviation,anisotropy coefficient,fractal dimension and mismatch length.(4)Under the conditions of linear seepage,matched,unmatched and anisotropic rough fracture fluid flow tests were conducted.The study found that:the fracture permeability is negatively correlated with the fracture roughness,unmatched length and anisotropy coefficient,and positively correlated with the fracture aperture;Under the same fracture roughness,the permeability of the mismatched rough fracture and anisotropic rough fracture is lower than that of the matched rough fracture.According to the test results,a correction equation for rough fracture permeability is proposed.The equation can well describe the relationship between the fracture permeability and the fracture roughness and the fracture aperture.And it can meet the prediction of fracture permeability of different rough types,and has wide applicability.(5)Under different hydraulic gradients,conduct fluid flow tests on multiple sets of matched and unmatched rough fracture samples.Through experimental research,it is found that under the condition of high-speed seepage,the rough fractures with different matching degrees show nonlinear characteristics,and the Forchheimer equation can be used to describe the seepage flow and hydraulic gradient.The study found that the hydraulic fracture aperture eh has a good correlation with the physical fracture apertures em and Z2.The empirical equation proposed can predict the hydraulic fracture opening of rough fractures.At the same time,according to the test results,an empirical equation between the nonlinear influence factor?and the physical fracture opening or average fracture opening and Z2 is established.The empirical formula proposed can meet the prediction of nonlinear fluid flow of rough fractures with different matching degrees. |
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