查看更多>>摘要：The stress concentration and failure at chamber intersections in coal mine are intense,especially in deep-buried,super-large section conditions.In this paper,the plastic radius of super-large section chamber under unequal pressure was corrected on the basis of the size effect.Then,stress and failure evolution of intersections under different crossing angles and equivalent angular bisectors were revealed.Furthermore,2 trajectory curves of failure and stress were analytically expressed,which divided the intersection into 5 influencing zones in the light of stress superposition degree.After determining insta-bility trigger point and instability path,instability energy criterion of intersection can be obtained as K＞1,which means that the external energy is greater than the sum of energy consumed by surrounding rock instability and supporting structure failure.Taking coal-gangue separation system of Longgu Coal Mine as example,it was found that there was instability risk under original parameters.For long-term stability,an optimization design method was proposed by considering safety factor,and optimal support scheme was obtained.Field monitoring showed intersections deformations were relatively small with the maximum of 125 mm,which verified the rationality of theoretical analysis.This study provides guidance for the stability control of the intersections under the same or similar conditions.
查看更多>>摘要：Foamed concrete as energy absorption material for high geo-stress soft rock tunnels has been proven to be feasible due to its high compressibility and lightweight.However,the lengthy curing and defoaming problems caused by the cast-in-place method of large-volume foamed concrete remain unsolved.In this study,we propose a novel energy absorber composed of foamed concrete-filled polyethylene(FC-PE)pipe and analyze its deformation and energy absorption capacity via quasi-static lateral compression experi-ments.Results show that FC-PE pipes exhibit typical three-stage deformation characteristics,comprising the elastic stage,the plastic plateau,and the densification stage.Furthermore,the plateau stress,energy absorption,and specific energy absorption of the specimens are 0.81-1.91 MPa,164-533 J,and 1.4-3.6 J/g,respectively.As the density of the foamed concrete increases,the plateau stress and energy absorption increase significantly.Conversely,the length of the plastic plateau and energy absorption efficiency decrease.Moreover,based on the vertical slice method,progressive compression of core mate-rial,and the 6 plastic hinges deformation mechanism of the pipe wall,a theoretical calculation method for effective energy absorption is established and achieves good agreement with experimental results,which is beneficial to the optimization of the composite structure.
查看更多>>摘要：Three-dimensional rock fracture induced by blasting is a highly complex problem and has received con-siderable attention in geotechnical engineering.The material point method is firstly applied to treat this challenging task.Some inherent weaknesses can be overcome by coupling the generalized interpolation material point(GIMP)and the convected particle domain interpolation technique(CPDI).For the media in the borehole,unchanged GIMP-type particles are used to guarantee a homogenous blast pressure.CPDI-Tetrahedron type particles are employed to avoid the fake numerical fracture near the borehole for the rock material.A blasting experiment using three-dimensional single-borehole rock was simulated to examine the applicability of the coupled model under realistic loading and boundary conditions.A good agreement was achieved between the simulation and experimental results.Moreover,the mechanism of three-dimensional rock fracture was analyzed.It was concluded that rock particle size and material parameters play an important role in rock damage.The reflected tensile waves cause severe damage in the lower part of the model.Rayleigh waves occur on the top face of the rock model to induce a hoop failure band.
查看更多>>摘要：CO2 drilling is a promising underbalance drilling technology with great advantages,such as lower cutting force,intense cooling and excellent lubrication.However,in the underbalance drilling,the mechanism of the coupling CO2 jet and polycrystalline-diamond-compact(PDC)cutter are still unclear.Whereby,we established a coupled smoothed particle hydrodynamics/finite element method(SPH/FEM)model to sim-ulate the composite rock-breaking of high-pressure CO2 jet&PDC cutter.Combined with the experimen-tal research results,the mechanism of composite rock-breaking is studied from the perspectives of rock stress field,cutting force and jet field.The results show that the composite rock-breaking can effectively relieve the influence of vibration and shock on PDC cutter.Meanwhile,the high-pressure CO2 jet has a positive effect on carrying rock debris,which can effectively reduce the temperature rising and the ther-mal wear of the PDC cutter.In addition,the effects of CO2 jet parameters on composite rock-breaking were studied,such as jet impact velocity,nozzle diameter,jet injection angle and impact distance.The studies show that when the impact velocity of the CO2 jet is greater than 250 m/s,the CO2 jet could quickly break the rock.It is found that the optimal range of nozzle diameter is 1.5-2.5 mm,the best injec-tion angle of CO2 jet is 60°,the optimal impact distance is 10 times the nozzle diameter.The above studies could provide theoretical supports and technical guidance for composite rock-breaking,which is useful for the CO2 underbalance drilling and drill bit design.
查看更多>>摘要：To investigate the failure process and characteristics of D-shaped tunnels under different maximum principal stress directions θ,true-triaxial tests were conducted on cubic sandstone samples with a through D-shaped hole.The test results show that the failure process can be divided into 4 periods:calm,buckling deformation,gradual buckling and exfoliation of rock fragment,and formation of a V-shaped notch.With an increase in θ from 0° to 90°,the size of the rock fragments first decreases and then increases,whereas the fractal dimension of the rock fragments first increases and then decreases.Meanwhile,the failure position at the left side shifts from the sidewall to the corner and finally to the floor,whereas the failure position at the right side moves from the sidewall to the span-drel and finally to the roof,which is consistent with the failure position in underground engineering.In addition,the initial vertical failure stress first decreases and then increases.By comparing the results,the failure severities at different maximum principal stress directions can be ranked from high to low in the following order:90°＞60°＞30°＞45°＞0°.
查看更多>>摘要：During high-intensity,fully mechanized mining of extra-thick coal seam,the top coal would cave to a cer-tain 3D form.Based on the data collected during drilling,a 3D model of top coal caving surface space was established to determine the relationship between the location of the stope roof and the caving surface,enabling the mathematical computation of the top caving angle(ψ).The drilling method was employed to measure the top caving angle on two extra-thick fully mechanized coal caving faces under the conditions of three geological structures,namely,no geological structure,igneous rock structure,and fault structure.The results show that the value of top caving angle could be accurately estimated on-site with the 9-parameter 3D top coal caving surface model built with the drilling method.This method is a novel on-site measurement that can be easily applied.Our findings reveal that the characteristics of the coal-rock in the two mining faces are different;yet their caving angles follow the rule ψigneous rock structure＜ψno geological structure＜ψfault structure.Finally,through the data fitting with two indexes(the top coal uniaxial compressive strength and the top caving angle),it is found that the relationship between the two indexes satisfies an exponential decay function.
查看更多>>摘要：In this paper,the cyclic constitutive equations were proposed to describe the constitutive behavior of cyclic loading and unloading.Firstly,a coupled damage variable was derived,which contains two parts,i.e.,the compaction-induced damage and the cracking-induced damage.The compaction-induced dam-age variable was derived from a nonlinear stress-strain relation of the initial compaction stage,and the cracking-induced damage variable was established based on the statistical damage theory.Secondly,based on the total damage variable,a damage constitutive equation was proposed to describe the constitutive relation of rock under the monotonic uniaxial compression conditions,whereafter,the application of this model is extended to cyclic loading and unloading conditions.To validate the proposed monotonic and cyclic constitutive equations,a series of mechanical tests for marble specimens were car-ried out,which contained the monotonic uniaxial compression(MUC)experiment,cyclic uniaxial com-pression experiments under the variable amplitude(CUC-VA)and constant amplitude(CUC-CA)conditions.The results show that the proposed total damage variable comprehensively reflects the dam-age evolution characteristic,i.e.,the damage variable firstly decreases,then increases no matter under the conditions of MUC,CUC-VA or CUC-CA.Then a reasonable consistency is observed between the experi-mental and theoretical curves.The proposed cyclic constitutive equations can simulate the whole cyclic loading and unloading behaviors,such as the initial compaction,the strain hardening and the strain soft-ening.Furthermore,the shapes of the theoretical curves are controlled by the modified coefficient,com-paction sensitivity coefficient and two Weibull distributed parameters.
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