| With the development of underground engineering,more and more geotechnical engineering are damaged and destroyed under the action of hydraulic-mechanical coupling.The problem of hydraulic-mechanical coupling involves the evolution law of seepage characteristics and the initiation,deformation,and expansion of micro-cracks in the rock mass.To date,there is still a research gap in the mechanical and seepage characteristics and their coupling mechanism for the progressive failure process of the fractured rock mass.Taking the water inrush disaster in the coal seam floor as the research background,this research attempts to investigate the non-linear seepage law for rock specimens with different flaws through theoretical analyses,experiments and numerical simulations.Acoustic Emission(AE)techniques were employed to monitor the real-time progressive damage processes of fractured rock under the coupling effect of hydraulic-mechanics.Numerical modeling of coal seam floor failure induced by mining disturbance was carried out by the finite element method.Accordingly,improvement methods were proposed to prevent water inrush and coal seam floor failure.The main conclusions can be given as follows:(1)Sandstone specimens were made with a single flaw,T-shape flaws and Y-shape flows,respectively.Based on the Forchheimer equation,the non-linear characteristics of pressure gradient and flow rate for the specimens with flaws were obtained by the hydraulic-mechanical coupling experimental tests.It has been found that,the non-linear curve for fractured sandstone is convex to the axis of the pressure gradient.The confining pressure and flaw shape significantly affect the linear coefficient a and non-linear coefficient b in the Forchheimer equation.(2)The relationship between the inertial resistance coefficient β and intrinsic permeability k was analyzed.A non-linear inertial parameter equation for fluid flow in the fractured sandstone was developed.Further,the critical pressure gradients of linear Darcy law and non-linear Forchheimer law were determined.Moreover,the contours of pressure gradient ratio and volumetric flow ratio under different fracture occurrences were obtained.Finally,the effective stress coefficient and coupling coefficient of the fractured sandstone sample were determined from the relationship between confining pressure and permeability.In order to more conveniently obtain the critical thresholds(crack closure stress and crack initiation stress)of the progressive failure of pore-fracture sandstone,the advantages of volumetric strain method,lateral strain method,crack volumetric strain method and lateral strain difference method are absorbed,The volumetric strain difference method is proved to be reasonable.Through the hydraulic coupling test of pore-fracture sandstone samples,the strength characteristics,crack initiation laws and failure modes of single-crack,T-shaped fracture and Y-shaped pore-fracture sandstone samples are analyzed.With the help of RFPA2D-FLOW software,the whole process of sample failure was numerically simulated from a meso-level perspective,real-time distribution characteristics of AE events inside the rock were obtained,the whole process of crack failure was recorded in real time,and the internal crack development characteristics of samples under different working conditions were deeply analyzed.(3)The hydraulic-mechanical coupling experiments were carried out.The strength,deformation characteristics,crack initiation law and failure mode for the sandstone specimens with a single flaw,T-shape flaws and Y-shapes flaws were obtained.RFPA2D-FLOW software was employed to reproduce the crack development during different hydraulic-mechanical loads.It has been found that,the peak strength for intact sandstone without hydraulic pressure is higher than that for specimens with flaws and intact specimens with hydraulic pressure.The strength weakening for sandstone induced by existing flaws to the strength is more significant than that induced by hydraulic pressure.The failure mode for the intact specimen and specimen with a single flaw is shearing failure and the crack initials with good directionality.The failure of the specimens with T-shape and Y-shape flaws are caused by shearing and tensile-shearing fracture and more secondary cracks are produced.(4)Through AE monitoring techniques,the experimental data including the AE ring counts,RA-AF value,b-value and peak frequency were collected and analyzed.Experimental results show that,the sudden increases of AE ring counts and density of AE peak frequency are the precursor indicator for rock failure.When the specimens completely fail,the b value of AE reaches the peak value.And the RA-AF values indicate that the failure of rock is dominated by shearing.(5)Based on the stress-seepage-time curve,the developments of permeability for intact specimens and the specimens with a single flaw and T-shape flaws and Y-shape flaws were obtained,respectively.It has been found that,the flaws and fluid flowing reduce the process of specimen compacted to crack propagation.The permeability reaches the peak value when the stress suddenly drops in the post-peak stage.Therefore,the sudden dropping coefficient for permeability can be determined and used as a key parameter for engineering scale hydraulic-mechanical coupling modelling.(6)Taking Yangchangwan coal mine as background,a hydraulic-mechanical coupling model was developed by RFPA2D-FLOW.The effective stress coefficient,coupling coefficient and sudden drop coefficient were introduced in the numerical model.The failure process of fractured rock in coal seam floor from meso-scale damage development to macro-scale water inrush channel formation induced by mining disturbance and hydraulic pressure was reproduced.The hydraulic-mechanical coupling mechanism for the water inrush in the coal seam floor was revealed.Finally,the corresponding measurements for preventing and control water inrush disasters were proposed to ensure mining safety. |
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