| A large number of small and medium-sized reservoirs filled with local loess have been built in the northwest loess plateau,aiming at ecological storage and soil and water conservation.During the operation of the reservoir,there are many problems such as seepage erosion,deformation,serious leakage and even damage,and the fractures developed in the dam are one of the important channels for reservoir leakage.The evolution rules of soil erosion under the condition of multiple porous complex networks are not completely clear yet.Therefore,it is of great theoretical significance and application value to conduct research on the permeability characteristics of cracked soil.This paper focuses on the seepage erosion mechanism of earth dams in loess hilly areas in southern Ningxia,and studies the seepage erosion phenomenon of loess dam with fractures by combining theoretical calculation,experimental verification and actual engineering case analysis.Based on fractal theory,an equivalent permeability model of the intermingled fractal units(IFU)for the dual-porosity and bi-dispersed porous media(DBPM)was established in an unsaturated state.Then,a joint anti-seepage test was conducted on the "protected soil-filter "composite soil as the research object to study the fractal characteristics of the pore structure and changes in permeability characteristics during the internal erosion process of the cracked loess under saturation.Next,the transport of fluid from the bi-dispersed porous matrix to the fracture was considered,and the IFU equivalent permeability model of DBPM in saturated state is established by combining the fractal dimension of pore area of the bi-dispersed porous matrix with effective stress.Finally,taking the Nanping Reservoir in the loess hilly area of southern Ningxia as an engineering case,the seepage evolution mechanism and the differences in soil erosion between the calculation model that considers earth dam fractures proposed in this paper and the traditional calculation method that does not consider earth dam fractures are comparatively analyzed and discussed.The rationality of the calculation model in this paper is verified,and the preliminary evaluation index of seepage erosion damage is given.Through the above research,it is expected to provide theoretical basis and engineering design reference for the seepage and erosion mechanism and risk assessment of earth dams in Ningxia and even the northwest Loess Plateau.This article mainly achieved the following research results:(1)The equivalent permeability model of the dual-porosity and bi-dispersed porous media based on the intermingled fractal units in the unsaturated state was established.The most important novelty of this model is to quantify the number np of fractal units of matrix in IFU as the function of bi-dispersed porous matrix structural parameters.The established equivalent permeability model of DBPM is most sensitive to the applied pressure Pm,the maximum fracture length lmax and the maximum pore diameter λmax.(2)When comparing the individual contribution to equivalent permeability of a single fracture fractal unit and np fractal unit of bi-dispersed porous matrix in IFU under general conditions,the result indicates that the permeability of the porous matrix is much lower than that of the fracture network.The fracture network is the highly permeable system that provides preferential flow paths for fluid flow.However,the quantitative results of dimensionless permeability KIFU ++ show that the preferential flow phenomenon of fractures becomes less and less obvious with the increase of λmax.The result indicates that when fractures are embedded in the porous matrix,whether the permeability of DBPM can be controlled by the fractures largely depends on the relative size of the pore diameters formed between particle clusters.Therefore,the premise of preferential flow in fractures is that the fracture aperture is a multiple of the order of magnitude of the pore channel diameter.This result reveals the preferential flow mechanism to a certain extent.(3)A joint impermeability test was conducted on the "loess-filter" under three different axial stresses and three different forms of loess fractures.Through the changes in pore water pressure,porosity,local effective stress,flow rate and permeability coefficient,the characteristics of infiltration erosion failure of loess with fractures under the protection of filter were explored.The results indicate that the fractal dimension of the matrix and permeability coefficient increase after the internal erosion of the soil sample begins,and at the same time,the local effective stress also decreases after the internal erosion of the soil sample begins.However,the order of changes in the local effective stress and the fractal dimension of the pore area and permeability coefficient from high to low is not completely consistent.Therefore,this article suggests using a "σe-Dp-k" joint method to predict its local stability.(4)Considering the fluid transport from the bi-dispersed porous matrix to the fracture,an equivalent permeability model of DBPM based on IFU in the saturated state was established.Considering the actual situation,this model not only involves the fluid transport from the porous matrix to a single fracture and but the key is to consider the entire fractal unit of fracture cross-sectional area.And the above equivalent permeability model also takes into account the inclination angle of the fracture,the tortuosity of the fracture and the penetration angle of the capillary where the internal fluid flows into the fracture.The rationality of the model was then verified through laboratory experiments,comparison with existing literature,and comparison with Hejiagou Reservoir Dam design reports.The IFU equivalent permeability is a function of structural parameters of the bi-dispersed porous matrix and fracture network as well as the parameters related to the pressure difference at both ends.It is also a function of the pressure difference ΔPf at both ends of the fracture and the pressure differences Δptm and Δpbm at both ends of the capillary flowing into the fracture.(5)Based on the three components of the equivalent permeability expression,considering fluid transport from the bi-dispersed porous matrix to the fracture,the dimensionless permeability Km-f+ is defined to characterize the impact of fluid migration from the matrix to the fracture on the fracture seepage behavior.The results show Km-f + increases with the increase of the ratio ζ and pressure difference ratio Δpm/ΔPf,which means that fluid transport has an important influence on the fracture seepage behavior.The second dimensionless permeability Kf-np ++ is also defined to characterize the impact of fracture units caused by fluid migration on the IFU equivalent permeability within the cross-sectional area.Considering the fluid migration from the bi-dispersed porous matrix to the fracture lead to the dimensionless permeability difference ΔKf-np ++ is 5 orders of magnitude,further indicating that considering fluid transfer has an important impact on the entire intermingled fractal unit.(6)Based on the equivalent permeability models of DBPM in saturated and unsaturated states proposed in this paper,and according to the collected reservoir engineering data in the loess hilly region of southern Ningxia,the applicability of the models is compared and analyzed with and without considering the existence of fractures.In addition,the permeability model of dual-porous media in saturated state in the existing literature is compared with the equivalent permeability model in saturated state in this paper.The results show that without considering fractures,the permeability model in the saturated state is a function of the unique structural parameters(R,φp,d0,φi)of bi-dispersed porous media,so its permeability prediction value is better than that in the literatures.Therefore,our model is closer to the true permeability values of engineering soil.Considering the presence of fractures,the predicted permeability value of the model is 3-4 orders of magnitude lower than the predicted value of the existing fractal permeability model of the dual-porous media.Because the model in the existing literature overestimates the fracture aperture,which makes the predicted value much larger,the applicability of this model is better than the fractal permeability model in the existing literature. |
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