查看更多>>摘要：This paper investigates mechanical behaviours of sandstone during post-peak cyclic loading and unload-ing subjected to hydromechanical coupling effect,confirming the peak and residual strengths reduction laws of sandstone with water pressure,and revealing the influence of water pressure on the upper limit stress and deformation characteristics of sandstone during post-peak cyclic loading and unloading.Regarding the rock strength,the experimental study confirms that the peak strength σp and residual strength σr decrease as water pressure P increases.Especially,the normalized strength parameters σp/σpk and σr/σre was negatively and linearly correlated with the P/σ3.Moreover,the Hoek-Brown strength criterion can be applied to describe the relationship between effective peak strength and effective con-fining stress.During post-peak cyclic loading and unloading,both the upper limit stress σp(i)and crack damage threshold stress σcd(i)of each cycle tend to decrease with the increasing cycle number.A hystere-sis loop exists among the loading and unloading stress-strain curves,indicating the unloading deforma-tion modulus Eunload is larger than the loading deformation modulus Eload.Based on experimental results,a post-peak strength prediction model related to water pressure and plastic shear strain is established.
查看更多>>摘要：This study investigated the correlations between mechanical properties and mineralogy of granite using the digital image processing(DIP)and discrete element method(DEM).The results showed that the X-ray diffraction(XRD)-based DIP method effectively analyzed the mineral composition contents and spatial distributions of granite.During the particle flow code(PFC2D)model calibration phase,the numerical simulation exhibited that the uniaxial compressive strength(UCS)value,elastic modulus(E),and failure pattern of the granite specimen in the UCS test were comparable to the experiment.By establishing 351 sets of numerical models and exploring the impacts of mineral composition on the mechanical properties of granite,it indicated that there was no negative correlation between quartz and feldspar for UCS,tensile strength(σt),and E.In contrast,mica had a significant negative correlation for UCS,σt,and E.The pres-ence of quartz increased the brittleness of granite,whereas the presence of mica and feldspar increased its ductility in UCS and direct tensile strength(DTS)tests.Varying contents of major mineral composi-tions in granite showed minor influence on the number of cracks in both UCS and DTS tests.
查看更多>>摘要：The face stability problem is a major concern for tunnels excavated in rock masses governed by the Hoek-Brown strength criterion.To provide an accurate prediction for the theoretical solution of the critical face pressure,this study adopts the piecewise linear method(PLM)to account for the nonlinearity of the strength envelope and proposes a new multi-horn rotational mechanism based on the Hoek-Brown strength criterion and the associative flow rule.The analytical solution of critical support pressure is derived from the energy-work balance equation in the framework of the plastic limit theorem;it is for-mulated as a multivariable nonlinear optimization problem relying on 2m dependent variables(m is the number of segments).Meanwhile,two classic linearized measures,the generalized tangential technique(GTT)and equivalent Mohr-Coulomb parameters method(EMM),are incorporated into the analysis for comparison.Surprisingly,the parametric study indicates a significant improvement in support pressure by up to 13％compared with the GTT,and as expected,the stability of the tunnel face is greatly influenced by the rock strength parameters.The stress distribution on the rupture surface is calculated to gain an intuitive understanding of the failure at the limit state.Although the limit analysis is incapable of calcu-lating the true stress distribution in rock masses,a rough approximation of the stress vector on the rup-ture surface is permitted.In the end,sets of normalized face pressure are provided in the form of charts for a quick assessment of face stability in rock masses.
查看更多>>摘要：This paper aims to determine the load bearing capacity of pre-stressed expandable props with different geometries and load eccentricities for flexible support in underground mining or excavation.It is deduced that the expandable device could have much higher strength(＞89 MPa)by laboratory tests,and the load bearing capacity of the expandable prop may depend on the stability of the supporting steel pipe struc-ture.A good agreement was found between the laboratory test and numerical results in terms of the load bearing capacity and the final macro-bending failure pattern for expandable props with heights of 1.5 and 2.7 m,and the theoretical calculation for the strength of traditional steel structures is not directly suitable for the expandable props.Moreover,additional numerical simulations were performed for the expand-able props with different normalized slenderness ratios λn and loading eccentric distances e.The variation of stability coefficient of the expandable prop is in line with the Perry-Robertson equation and its corre-lation coefficients are fitted as a of 0.979 and b of 0.314.For estimating the load bearing capacity of the expandable props,the strength equation for traditional steel structures is improved by introducing a bending magnification factor and by modifying the normalized slenderness ratio to a converted slender-ness ratio.Based on the underground field monitoring for the strength of expandable props with different heights,the empirical eccentric distances were back calculated,and a safety factor is introduced to obtain the designed strength of the expandable prop.In addition,a four-step design procedure is proposed for the expandable prop.
查看更多>>摘要：Twenty-seven specimens were tested to investigate the uniaxial compression behaviour of an innovative standing support for underground space applications.The innovative standing support consisted of an external fibre reinforced polymer(FRP)jacket and the infill column made of cementitious grout,tailings and coal wash rejects.Effects of the FRP layers number and water to the cementitious grout(w/g)ratio were evaluated.Test results indicated that lower w/g ratios produced stronger infill columns.With FRP confinement,the standing support demonstrated strain-hardening loading characteristics with a signif-icant improvement in both strength and ductility.The highest strength and strain of the specimens achieved was 58.4 MPa and 11.8％respectively.Compared with the unconfined specimens,the confine-ment with four FRP layers increased the specimen strength and associated strain up to 3.6 and 27.0 times respectively.A correlation between the compressive strength of the infill material and the ultrasonic pulse velocity was also investigated.Furthermore,a simple design-oriented model was proposed to pre-dict the peak strength and the corresponding strain of the innovative standing support.
查看更多>>摘要：The study of the dynamic disaster mechanism of coal and gas outburst two-phase flow is crucial for improving disaster reduction and rescue ability of coal mine outburst accidents.An outburst test in a T-shaped roadway was conducted using a self-developed large-scale outburst dynamic disaster test sys-tem.We investigated the release characteristics of main energy sources in coal seam,and obtained the dynamic characteristics of outburst two-phase flow in a roadway.Additionally,we established a forma-tion model for outburst impact flow and a model for its flow in a bifurcated structure.The results indicate that the outburst process exhibits pulse characteristics,and the rapid destruction process of coal seam and the blocking state of gas flow are the main causes of the pulse phenomenon.The outburst energy is released in stages,and the elastic potential energy is released in the vertical direction before the hor-izontal direction.In a straight roadway,the impact force oscillates along the roadway.With an increase in the solid-gas ratio,the two-phase flow impact force gradually increases,and the disaster range extends from the middle of the roadway to the coal seam.In the area near the coal seam,the disaster caused by the two-phase flow impact is characterized by intermittent recovery.In a bifurcated roadway,the effect of impact airflow on impact dynamic disaster is much higher than that of two-phase flow,and the impact force tends to weaken with increasing solid-gas ratio.The impact force is asymmetrically distributed;it is higher on the left of the bifurcated roadway.With an increase in the solid-gas ratio,the static pressure rapidly decreases,and the bifurcated structure accelerates the attenuation of static pressure.Moreover,secondary acceleration is observed when the shock wave moves along the T-shaped roadway,indicating that the bifurcated structure increases the shock wave velocity.
查看更多>>摘要：Hard rock pillar is one of the important structures in engineering design and excavation in underground mines.Accurate and convenient prediction of pillar stability is of great significance for underground space safety.This paper aims to develop hybrid support vector machine(SVM)models improved by three metaheuristic algorithms known as grey wolf optimizer(GWO)),whale optimization algorithm(WOA)and sparrow search algorithm(SSA)for predicting the hard rock pillar stability.An integrated dataset containing 306 hard rock pillars was established to generate hybrid SVM models.Five parameters includ-ing pillar height,pillar width,ratio of pillar width to height,uniaxial compressive strength and pillar stress were set as input parameters.Two global indices,three local indices and the receiver operating characteristic(ROC)curve with the area under the ROC curve(AUC)were utilized to evaluate all hybrid models'performance.The results confirmed that the SSA-SVM model is the best prediction model with the highest values of all global indices and local indices.Nevertheless,the performance of the SSA-SVM model for predicting the unstable pillar(AUC:0.899)is not as good as those for stable(AUC:0.975)and failed pillars(AUC:0.990).To verify the effectiveness of the proposed models,5 field cases were investigated in a metal mine and other 5 cases were collected from several published works.The validation results indicated that the SSA-SVM model obtained a considerable accuracy,which means that the combination of SVM and metaheuristic algorithms is a feasible approach to predict the pillar stability.
查看更多>>摘要：In underground coal mines,fibre reinforced polymer(FRP)bolt is ideal for mined rib reinforcements as it can prevent gas explosions caused by shearer frictional spark.With increasing mining depth,small diam-eter FRP bolts used in shallow underground mining cannot fulfil the rib support requirements.Under the engineering background of deep underground shortwall mining in Wudong coal mine,this paper system-atically studies Φ27 mm FRP bolt support for large deformation coal rib.Specimens with a fan-shaped cross-section were used to enable the tensile testing of the bolt rod,the measured average tensile strength of the studied FRP bolt was(486.1±9.6)MPa with a maximum elongation of 5.7％±0.6％.The shear strength of the bolt was measured as approximately 258 MPa using a self-made double shear testing apparatus.Based on the equivalent radial stiffness principle,a laboratory short encapsulation pullout test(SEPT)method for rib bolting has been developed undertaken consideration of the mechan-ical properties of the coal seam.Results showed that the average peak anchorage forces of the Φ27 mm FRP bolt and Φ20 mm steel rebar bolt were 108.4 and 66.4 kN,respectively,which were agreed with the theoretical calculations and field measurements.Based on theoretical analysis of the loading states of the bolt under site conditions,bolting method of full-length resin grouting was adopted to offset the weak-nesses of the FRP bolt.Numerical method was employed to compare the bolting effect using Φ27 mm FRP bolts and steel rebar bolts.Large diameter FRP bolting was determined as the optimum rib support scheme to increase the productivity of the coal mine and to enhance the ground control capability for+425 level mining roadways.This study provides the laboratory testing design and theoretical prediction of large diameter FRP bolts used for rib support in large deformation roadways.
查看更多>>摘要：Surfactants were proposed to be added into magnesium sulfate solution to improve the leaching process of weathered crust elution-deposited rare earth ores(WREOs).Effects of surfactants and their concentra-tion on the seepage of leaching solutions and the leaching efficiency of rare earth(RE)and aluminum(Al)were investigated,and the leaching kinetics,the mass transfer process,the adhesion work and the adhe-sion work reduction factor were analyzed to reveal its strengthening leaching mechanism.The results show that cetyltrimethylammonium bromide(CTAB)has a better strengthening effect on the leaching process than dodecyl trimethyl ammonium bromide(DTAB),sodium dodecyl sulfate(SDS),sodium oleate and oleic acid.In the presence of 0.04％CTAB in 0.2 mol/L solution,the permeability coefficient of WREOs increases from 0.945×10-5 to 1.640×10-5 cm.s-1,and the leaching efficiency of RE increases from 80％to 90％,confirming the promotion of surfactants on the leaching process of WREOs.Kinetic analysis shows that the leaching process conforms to the inner diffusion control model,and the leaching kinetics equa-tions of RE and Al related to CTAB content are obtained.Mass transfer discussion shows a smaller height equivalent to theoretical plate(HETP)of RE and Al at CTAB content of 0.04％,suggesting the higher mass transfer efficiency here.According to the interfacial properties of leaching solutions,the calculated adhe-sion work and the adhesion work reduction factor further demonstrate the strengthening leaching effect of CTAB on the leaching process of WREOs.
查看更多>>摘要：Chalcopyrite is a common copper-bearing mineral with antiferromagnetic properties.However,this property has rarely been considered in previous studies for detailed adsorption behaviors of molecules on chalcopyrite.Based on density functional theory(DFT),new adsorption pathways by H2O and O2 on the chalcopyrite metal terminated(112)surface((112)-M)is found in this work.First,through simulating the adsorption of an isolated water molecule and monolayer water molecules,it is confirmed that H2O molecules tend to adsorb on the surface Fe atoms more than on the surface Cu atoms.Then,we studied various adsorption behaviors of the O2 molecule.It is found that the adsorption on the hollow Fe—Fe site is the most stable case;however,O2 is undissociated.Two adsorption cases will happen when H2O—O2 adsorb simultaneously on the surface.For the S site,the H2O molecule thoroughly dissociated and formed S—O species,and the other case is H2O undissociated adsorbing at the Cu site.For the former case,it is interesting that H2O is dissociated before O2.
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