| Geotechnical stability issues and environment and energy issues such as carbon dioxide capture and sequestration,nuclear waste disposal,shale-gas exploitation,et al.,which are significant to national interest and people's livelihood,have received much attention.These projects are all involved with coupling between fluid and solid,therefore,it is of great practical significance to understand the mechanism of fluid-solid coupling.In experimental studies,many actual fluid-solid coupling conditions cannot be reproduced due to the limitations of sample size,laboratory apparatus,operation process,and et al.,in addition,it is hard to implement real-time measuring of the structural evolution in meso-scale,which is beneficial to understand the mechanism of fluid-solid coupling.With the improving of computer technique,numerical modeling is getting more popular.Comparing to laboratory experiment,numerical modeling is economic,efficient and flexible,and it is a useful supplementary measure to experiments.Discrete element method(DEM)is widely used in geotechnical problems for its applicability to large deformation problems.In DEM,mechanical mechanism in meso-scale,which controls the material responses in macro-scale,can be monitored,and it is highly significant to the study of fluid-solid coupling mechanisms.Optimizing and revision of the fluid-solid coupling algorithm in DEM is important to achieve better numerical results,and it is a vital subject worth studying.This paper centered on the optimizing and revision of fluidsolid coupling algorithm,and conducted supporting,verification and application of the revised algorithm by experimental study,theoretical deducing and numerical modeling.Followings are the major advances and conclusions:(1)Based on the parrellel bond model in Partical Flow Code,the fluid-solid coupling algorithm has been written,the permeation mechanism has been studied,impacts of the pore structure and crack structure to overall permeability have been revealed,and the fluid-solid coupling algorithm has been verified by comparing to former conclusions.(2)Aiming on the shortages of the original fluid-solid coupling algorithm that the flow net would be destroy under large deformation and the flow advantage of fracture cannot be embodied,the fluid-solid coupling algorithm has been revised.By building bond system and crack system respectively,and defining the flow net according to the characters of each system,the destruction of flow net under large deformation can be avoided.Different constitutive equations are assigned to the bond system and crack system respectively to distinguish different flow behaviors of the rock matrix and fracture.(3)According to the triaxial compression-permeability test,a steady-flow testing model has been build and the original and revised fluid-solid coupling algorithm have been used for numerical modeling.By comparing to the experimental results,the revised algorithm has been found better reflecting the percolation effect in the triaxial compression-permeability test.By monitoring the flow rate and pore pressure fields in the samples of different triaxial loading stages,the fluid transformation law inside the rock matrix and fracture has been revealed.(4)Circumferential deformation and permeability of an intact sample and a sample with a single fracture under different confining pressure have been measured,and the apertureconfining pressure curve of fracture has obtained by processing the circumferential deformation data.Based on the definition of joint stiffness,an aperture-effective stress relation model of joint has been deduced,and by means of the cubic law,a permeability-effective stress relation model has been obtained.Using the joint stiffness-based aperture-effective stress model and permeability-effective stress model,the experimental curves have been fitted well.(5)Applying the aperture-effective stress model,which base on the joint stiffness,to the fluid-solid constitution law and utilizing the revised fluid-solid coupling algorithm,numerical modeling of hydraulic fracturing has been conducted.By a large number of modeling results on hydraulic fracturing in jointed rock masses,four kinds of interaction modes between hydraulic fracture and joint are concluded.Through the force-field monitoring in meso-scale,the effects of joint,hydraulic fracture and principal stress difference on force-field have been analyzed.The results show that the superposition of the force-fields under these three factors controls the behavior of the propagation of hydraulic fracture in jointed rock masses. |
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