Damage evolution of rock-encased-backfill structure under stepwise cyclic triaxial loading

Rock-encased-backfill(RB)structures are common in underground mining,for example in the cut-andfill and stoping methods.To understand the effects of cyclic excavation and blasting activities on the damage of these RB structures,a series of triaxial stepwise-increasing-amplitude cyclic loading experiments was conducted with cylindrical RB specimens(rock on outside,backfill on inside)with different volume fractions of rock(VF=0.48,0.61,0.73,and 0.84),confining pressures(0,6,9,and 12 MPa),and cyclic loading rates(200,300,400,and 500 N/s).The damage evolution and meso-crack formation during the cyclic tests were analyzed with results from stress-strain hysteresis loops,acoustic emission events,and post-failure X-ray 3D fracture morphology.The results showed significant differences between cyclic and monotonic loadings of RB specimens,particularly with regard to the generation of shear microcracks,the development of stress memory and strain hardening,and the contact forces and associated friction that develops along the rock-backfill interface.One important finding is that as a function of the number of cycles,the elastic strain increases linearly and the dissipated energy increases exponentially.Also,compared with monotonic loading,the cyclic strain hardening characteristics are more sensitive to rising confining pressures during the initial compaction stage.Another finding is that compared with monotonic loading,more shear microcracks are generated during every reloading stage,but these microcracks tend to be dispersed and lessen the likelihood of large shear fracture formation.The transition from elastic to plastic behavior varies depending on the parameters of each test(confinement,volume fraction,and cyclic rate),and an interesting finding was that the transformation to plastic behavior is significantly lower under the conditions of 0.73 rock volume fraction,400 N/s cyclic loading rate,and 9 MPa confinement.All the findings have important practical implications on the ability of backfill to support underground excavations.

Analysis of the Hydraulic and Heat Transfer Evolution Mechanism of a Single Rock Fracture

Mineral dissolution and mechanical deformation of granite are two main mechanisms that affect permeability evolution of rock fracture.In this study,two water flow-through experiments with large granite fractures were conducted at 200 0C with a constant flow rate for 24 h,under confining pressures of 5 and 10 MPa,respectively.Water pressure and temperature were measured,fracture aperture and permeability were calculated,and chemical element concentrations in effluent water were tested for mechanism analysis.The permeability fluctuates up and down between 2.62 × 10^(-12)and 3.16 ×10^(-12)m^(2)at confining pressure of 5 MPa;while it decreased monotonously by 24% from 1.92 × 10^(-12)to1.45 × 10^(-12)m^(2)at a confining pressure of 10 MPa.The heat transfer rates at both experiments stay stable at about 0.25 J/s.The mass concentration of Ca,Na,K,and Si in effluent water are between 5 to 23mg/L,indicating slight dissolution of Ca-plagioclase,Na-plagioclase,and K-feldspar,as well as possible precipitation of minor amount of kaolinite or quartz.The total amount of free-face dissolution and pressure dissolution are similar at 5 and 10 MPa.The geochemical reaction counts for only small part of the aperture change,while the mechanical deformation counts the major part of the aperture change.