Experimental And Modeling Study On Non-linear Seepage Characteristics Of Filling Fracture Networks

Structural surface fracture is ubiquitous in the practices of geotechnical engineering,mining engineering,nuclear waste management,as well as water conservancy and hydropower plant construction.Both primary and secondary fractures are major permeable channels of rock masses.In the process of project construction,continuous development of fractured zones under the actions of perturbation and unloading,etc facilitates the hydraulic connection from the nearby aquifer and the working face.Consequently,non-linear seepage of the fracture network may be caused,giving rise to a series of geological disasters such as water gushing,as well as water and sediment bursting.For this reason,the motion pattern of the fluids and the water-sand two-phase interaction mechanism in fracture networks are problems demanding prompt solutions in the field of geotechnical engineering.The motion of fluid in fracture networks is significantly different from their movement in porous media or a single fracture.Considering the complexity and particularity of seepage flow in the fracture network,effective measures were adopted for relevant investigations,including laboratory tests and numerical calculation model development.In this thesis,the following research is performed primarily.(1)Through extensive practice and testing,a set of planar experimental system for hydraulic sediment transport that satisfied the experiment requirements of high concentration sand-water mixture flow motion at higher pressure and greater flow rate is designed.The supporting individual phase measurement device could be used to accurately and individually measure the split-phase velocities of sand-water mixture flow in real time.Moreover,similar experimental investigations on piping,variable mass flows,water-sand inrush,and sediment incipient motion,etc.may all become significant sources of reference.(2)A non-linear seepage calculation model for cross fractures was established by taking local pressure loss into consideration,to not only explore the local pressure loss and flow distribution pattern of crucifix-shaped cross fractures,but also analyze the influence of Reynolds numbers,aperture ratios,and intersection angles on seepage characteristics of the crucifix-shaped cross fractures.In order to supplement and further explain the relevant mechanism,COMSOL was employed to solve the Navier-Stokes equation for incompressible fluid.Finally,a pressure differential simplification and correction method for actual fracture networks were put forward.(3)Systematic experimental studies on fracture networks with various apertures and intersection angles revealed the sophisticated changes in geometrical features and hydraulic characteristics of such fracture networks in the course of seepage flow.During these studies,the influences of fracture aperture and intersection angle on the non-linear seepage parameters(i.e.,permeability and non-Darcy factors)as well as the critical parameters of seepage transition,etc.(4)Experimental investigations were conducted on 10 groups of particle fillers with different grain gradation.Besides,the coefficients of curvature and non-uniformity were introduced to systematically probe into the effects of grain gradation on the non-linear seepage characteristics of porous media.In this way,not only was the influence of porous media size compositions on the critical parameters of seepage transition grasped,but the transformation pattern between the inertia force and the viscous force was also obtained.The semi-empirical half-theory formulae for permeability and non-Darcy factors was also acquired through fitting based on the four parameters of effective grain size,porosity,coefficient of curvature,and coefficient of non-uniformity.These formulae can be applied during the estimation of the seepage parameters of porous media and the preparation of granular accumulation with particular requirements for permeability in engineering practices.(5)Through on a series of experimental investigations on the non-linear seepage characteristics of a filled fracture network,the critical flow velocity range for initial particle motion was obtained on the one hand;on the other hand,the functional relationship between its critical flow velocity and grain size was also established.Additionally,the influences of grain gradation,hydrodynamic force,and the geometric features of the fracture network,etc.on the particle loss features,distribution pattern,and water-sand two-phase migration characteristics were explored.As for the interactions of water and movable particles,as well as the skeletons and fracture asperities,they were qualitatively analyzed,so was the formation mechanism of leakage channels.The variation trend of mass loss features and porosity,etc.with time were also successfully tested for the specimens,while the filled fracture network permeability and non-Darcy factor calculation method were determined as well.(6)In this thesis,a roof water inrush accident of Jiangjiawan Coal Mine was taken as an example.A non-Darcy flow model for water inrushing in the water-flowing fracture zone was set up based on physical experimental results by giving considerations to the impact of seepage erosion.The entire flow path involving goaf ponding,water flowing fractured zone,and roadways was connected together organically.Furthermore,the relevant numerical model was solved using FELAC.Hence,the whole process of water inrushing from its initiation to development was dynamically reproduced to uncover the non-Darcy seepage mechanism for water inrushing in the water flowing fractured zone under the action of seepage erosion,which can be used as a reference for prediction and prevention of goaf water disasters.