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.

Roughness characterization and shearing dislocation failure for rock-backfill interface

Shearing dislocation is a common failure type for rock–backfill interfaces because of backfill sedimentation and rock strata movement in backfill mining goaf.This paper designed a test method for rock–backfill shearing dislocation.Using digital image techno-logy and three-dimensional(3D)laser morphology scanning techniques,a set of 3D models with rough joint surfaces was established.Further,the mechanical behavior of rock–backfill shearing dislocation was investigated using a direct shear test.The effects of interface roughness on the shear–displacement curve and failure characteristics of rock–backfill specimens were considered.The 3D fractal dimen-sion,profile line joint roughness coefficient(JRC),profile line two-dimensional fractal dimension,and the surface curvature of the frac-tures were obtained.The correlation characterization of surface roughness was then analyzed,and the shear strength could be measured and calculated using JRC.The results showed the following:there were three failure threshold value points in rock–backfill shearing dis-location:30%–50%displacement before the peak,70%–90%displacement before the peak,and 100%displacement before the peak to post-peak,which could be a sign for rock–backfill shearing dislocation failure.The surface JRC could be used to judge the rock–backfill shearing dislocation failure,including post-peak sliding,uniform variations,and gradient change,corresponding to rock–backfill disloca-tion failure on the field site.The research reveals the damage mechanism for rock–backfill complexes based on the free joint surface,fills the gap of existing shearing theoretical systems for isomerism complexes,and provides a theoretical basis for the prevention and control of possible disasters in backfill mining.

Using cemented paste backfill to tackle the phosphogypsum stockpile in China:A down-to-earth technology with new vitalities in pollutant retention and CO_(2) abatement

Phosphogypsum(PG),a hard-to-dissipate by-product of the phosphorus fertilizer production industry,places strain on the biogeochemical cycles and ecosystem functions of storage sites.This pervasive problem is already widespread worldwide and requires careful stewardship.In this study,we review the presence of potentially toxic elements(PTEs)in PG and describe their associations with soil properties,anthropogenic activities,and surrounding organisms.Then,we review different ex-/in-situ solutions for promoting the sustainable management of PG,with an emphasis on in-situ cemented paste backfill,which offers a cost-effective and highly scalable opportunity to advance the value-added recovery of PG.However,concerns related to the PTEs'retention capacity and long-term effectiveness limit the implementation of this strategy.Furthermore,given that the large-scale demand for ordinary Portland cement from this conventional option has resulted in significant CO_(2) emissions,the technology has recently undergone additional scrutiny to meet the climate mitigation ambition of the Paris Agreement and China's Carbon Neutrality Economy.Therefore,we discuss the ways by which we can integrate innovative strategies,including supplementary cementitious materials,alternative binder solutions,CO_(2) mineralization,CO_(2) curing,and optimization of the supply chain for the profitability and sustainability of PG remediation.However,to maximize the co-benefits in environmental,social,and economic,future research must bridge the gap between the feasibility of expanding these advanced pathways and the multidisciplinary needs.

Reinforcing effects of polypropylene on energy absorption and fracturing of cement-based tailings backfill under impact loading

Polypropylene(PP)fiber-reinforced cement-based tailings backfill(FRCTB)is a green compound material with superior crack resistance and has good prospects for application in underground mining.However,FRCTB exhibits susceptibility to dynamic events,such as impact ground pressure and blast vibrations.This paper investigates the energy and crack distribution behavior of FRCTB under dynamic impact,considering the height/diameter(H/D)effect.Split Hopkinson pressure bar,industrial computed tomography scan,and scanning electron microscopy(SEM)experiments were carried out on six types of FRCTB.Laboratory outcomes confirmed fiber aggregation at the bottom of specimens.When H/D was less than 0.8,the proportion of PP fibers distributed along theθangle direction of80°-90°increased.For the total energy,all samples presented similar energy absorption,reflectance,and transmittance.However,a rise in H/D may cause a rise in the energy absorption rate of FRCTB during the peak phase.A positive correlation existed between the average strain rate and absorbed energy per unit volume.The increase in H/D resulted in a decreased crack volume fraction of FRCTB.When the H/D was greater than or equal to 0.7,the maximum crack volume fraction of FRCTB was observed close to the incidence plane.Radial cracks were present only in the FRCTB with an H/D ratio of 0.5.Samples with H/D ratios of 0.5 and 0.6 showed similar distributions of weakly and heavily damaged areas.PP fibers can limit the emergence and expansion of cracks by influencing their path.SEM observations revealed considerable differences in the bonding strengths between fibers and the FRCTB.Fibers that adhered particularly well to the substrate were attracted together with the hydration products adhering to surfaces.These results show that FRCTB is promising as a sustainable and green backfill for determining the design properties of mining with backfill.

Paraffin–CaCl_(2)·6H_(2)O dosage effects on the strength and heat transfer characteristics of cemented tailings backfill

The challenge of high temperatures in deep mining remains harmful to the health of workers and their production efficiency The addition of phase change materials (PCMs) to filling slurry and the use of the cold storage function of these materials to reduce downhole temperatures is an effective approach to alleviate the aforementioned problem.Paraffin–CaCl_(2)·6H_(2)O composite PCM was prepared in the laboratory.The composition,phase change latent heat,thermal conductivity,and cemented tailing backfill (CTB) compressive strength of the new material were studied.The heat transfer characteristics and endothermic effect of the PCM were simulated using Fluent software.The results showed the following:(1) The new paraffin–CaCl_(2)·6H_(2)O composite PCM improved the thermal conductivity of native paraffin while avoiding the water solubility of CaCl_(2)·6H_(2)O.(2) The calculation formula of the thermal conductivity of CaCl_(2)·6H_(2)O combined with paraffin was deduced,and the reasons were explained in principle.(3) The“enthalpy–mass scale model”was applied to calculate the phase change latent heat of nonreactive composite PCMs.(4)The addition of the paraffin–CaCl_(2)·6H_(2)O composite PCM reduced the CTB strength but increased its heat absorption capacity.This research can give a theoretical foundation for the use of heat storage backfill in green mines.

Estimating shear strength of high-level pillars supported with cemented backfilling using the HoekeBrown strength criterion

Deep metal mines are often mined using the high-level pillars with subsequent cementation backfilling(HLSCB)mining method.At the design stage,it is therefore important to have a reasonable method for determining the shear strength of the high-level pillars(i.e.cohesion and internal friction angle)when they are supported by cemented backfilling.In this study,a formula was derived for the upper limit of the confining pressure σ3max on a high-level pillar supported by cemented backfilling in a deep metal mine.A new method of estimating the shear strength of such pillars was then proposed based on the Hoek eBrown failure criterion.Our analysis indicates that the horizontal stress σhh acting on the cemented backfill pillar can be simplified by expressing it as a constant value.A reasonable and effective value for σ3max can then be determined.The value of s3max predicted using the proposed method is generally less than 3 MPa.Within this range,the shear strength of the high-level pillar is accurately calculated using the equivalent MohreCoulomb theory.The proposed method can effectively avoid the calculation of inaccurate shear strength values for the high-level pillars when the original HoekeBrown criterion is used in the presence of large confining pressures,i.e.the situation in which the cohesion value is too large and the friction angle is too small can effectively be avoided.The proposed method is applied to a deep metal mine in China that is being excavated using the HLSCB method.The shear strength parameters of the high-level pillars obtained using the proposed method were input in the numerical simulations.The numerical results show that the recommended level heights and sizes of the high-level pillars and rooms in the mine are rational.

Enhancing fly ash utilization in backfill materials treated with CO_(2)carbonation under ambient conditions

The environmental concerns resulting from coal-fired power generation that produces large amounts of CO_(2)and fly ash are of great interest.To mitigate,this study aims to develop a novel carbonated CO_(2)-fly ash-based backfill(CFBF)material under ambient conditions.The performance of CFBF was investigated for different fly ash-cement ratios and compared with non-CO_(2)reacted samples.The fresh CFBF slurry conformed to the Herschel-Bulkley model with shear thinning characteristics.After carbonation,the yield stress of the fresh slurry increased significantly by lowering fly ash ratio due to gel formation.The setting times were accelerated,resulting in approximately 40.6%of increased early strength.The final strength decreased when incorporating a lower fly ash ratio(50%and 60%),which was related to the existing heterogeneous pores caused by rapid fluid loss.The strength increased with fly ash content above 70%because additional C-S(A)-H and silica gels were characterized to precipitate on the grain surface,so the binding between particles increased.The C-S(A)-H gel was developed through the pozzolanic reaction,where CaCO_(3)was the prerequisite calcium source obtained in the CO_(2)-fly ash reaction.Furthermore,the maximum CO_(2)uptake efficiency was 1.39 mg-CO_(2)/g-CFBF.The CFBF material is feasible to co-dispose CO_(2)and fly ash in the mine goaf as negative carbon backfill materials,and simultaneously mitigates the strata movement and water lost in post-subsurface mining.

Experimental research and numerical simulation of the multi-field performance of cemented paste backfill:Review and future perspectives

Cemented paste backfill(CPB)technology is a green mining method used to control underground goaves and tailings ponds.The curing process of CPB in the stope is the product of a thermo-hydro-mechanical-chemical multi-field performance interaction.At present,research on the multi-field performance of CPB mainly includes indoor similar simulation experiments,in-situ multi-field performance monitoring experiments,multi-field performance coupling model construction of CPB,and numerical simulation of the multi-field performance of CPB.Because it is hard to study the in-situ multi-field performance of CPB in the real stope,most current research on in-situ multi-field performance adopts the numerical simulation method.By simulating the conditions of CPB in the real stope(e.g.,maintenance environment,stope geometry,drainage conditions,and barricade and backfilling rates),the multi-field performance of CPB is further studied.This paper summarizes the mathematical models employed in the numerical simulation and lists the engineering application cases of numerical simulation in the in-situ multi-field performance of CPB.Finally,it proposes that the multi-field performance of CPB needs to strengthen the theoretical study of multi-field performance,form the strength design criterion based on the multi-field performance of CPB,perform a full-range numerical simulation of the multi-field performance of CPB,develop a pre-warning technology for the CPB safety of CPB,develop automatic and wireless sensors for the multi-field performance monitoring of CPB,and realize the application and popularization of CPB monitoring technology.

Numerical investigation of the mechanical behavior of the backfill–rock composite structure under triaxial compression

To ensure safe and economical backfill mining,the mechanical response of the backfill–rock interaction system needs to be understood.The numerical investigation of the mechanical behavior of backfill–rock composite structure(BRCS)under triaxial compression,which includes deformation,failure patterns,strength characteristics,and acoustic emission(AE)evolution,was proposed.The models used in the tests have one rough interface,two cement–iron tailings ratios(CTRs),four interface angles(IAs),and three confining pressures(CPs).Results showed that the deformation,strength characteristics,and failure patterns of BRCS under triaxial compression depend on IA,CP,and CTR.The stress–strain curves of BRCS under triaxial compression could be divided into five stages,namely,compaction,elasticity,yield,strain softening,and residual stress.The relevant AE counts have corresponding relationships with different stages.The triaxial compressive strengths of composites increase linearly with the increase of the CP.Furthermore,the CP stress strengthening effect occurs.When the IAs are45°and 60°,the failure areas of composites appear in the interface and backfill.When the IAs are 75°and 90°,the failure areas of composites appear in the backfill,interface,and rock.Moreover,the corresponding failure modes yield the combined shear failure.The research results provide the basis for further understanding of the stability of the BRCS.

Rheological properties of a multiscale granular system during mixing of cemented paste backfill:A review

The technology of cemented paste backfill(CPB)is an effective method for green mining.In CPB,mixing is a vital process aiming to prepare a paste that meets the non-stratification,non-segregation,and non-bleeding requirements.As a multiscale granular system,homogenization is one of the challenges in the paste-mixing process.Due to the high shearing,high concentration,and multiscale characteristics,paste exhibits complex rheological properties in the mixing process.An overview of the mesomechanics and structural evolution is presented in this review.The effects of various influencing factors on the paste's rheological properties were investigated,and the rheological models of the paste were outlined from the macroscopic and mesoscopic levels.The results show that the mechanical effects and structural evolution are the fundamental factors affecting the rheological properties of the paste.Existing problems and future development trends are presented to change the practice where the CPB process comes first and the theory lags.

Single-factor analysis and interaction terms on the mechanical and microscopic properties of cemented aeolian sand backfill

The use of aeolian sand(AS)as an aggregate to prepare coal mine cemented filling materials can resolve the problems of gangue shortage and excessive AS deposits.Owing to the lack of research on the mechanism of cemented AS backfill(CASB),the response surface method(RSM)was adopted in this study to analyze the influence of ordinary Portland cement(PO)content(x_(1)),fly ash(FA)-AS(FA-AS)ratio(x_(2)),and concentration(x_(3))on the mechanical and microscopic properties of the CASB.The hydration characteristics and internal pore structure of the backfill were assessed through thermogravimetric/derivative thermogravimetric analysis,mercury intrusion porosimetry,and scanning electron microscopy.The RSM results show that the influence of each factor and interaction term on the response values is extremely significant(except x_(1)x_(3),which had no obvious effect on the 28 d strength).The uniaxial compressive strength(UCS)increased with the PO content,FA-AS ratio,and concentration.The interaction effects of x_(1)x_(2),x_(1)x_(3),and x_(2)x_(3) on the UCS at 3,7,and 28 d were analyzed.In terms of the influence of interaction items,an improvement in one factor promoted the strengthening effect of another factor.The enhancement mechanism of the curing time,PO content,and FA-AS ratio on the backfill was reflected in the increase in hydration products and pore structure optimization.By contrast,the enhancement mechanism of the concentration was mainly the pore structure optimization.The UCS was positively correlated with weight loss and micropore content but negatively correlated with the total porosity.The R^(2) value of the fitting function of the strength and weight loss,micropore content,and total porosity exceeded 0.9,which improved the characterization of the enhancement mechanism of the UCS based on the thermogravimetric analysis and pore structure.This work obtained that the influence rules and mechanisms of the PO,FA-AS,concentration,and interaction terms on the mechanical properties of the CASB provided a certain theoretical and engineering guidance for CASB filling.

A machine learning model to predict unconfined compressive strength of alkali-activated slag-based cemented paste backfill

The unconfined compressive strength(UCS)of alkali-activated slag(AAS)-based cemented paste backfill(CPB)is influenced by multiple design parameters.However,the experimental methods are limited to understanding the relationships between a single design parameter and the UCS,independently of each other.Although machine learning(ML)methods have proven efficient in understanding relationships between multiple parameters and the UCS of ordinary Portland cement(OPC)-based CPB,there is a lack of ML research on AAS-based CPB.In this study,two ensemble ML methods,comprising gradient boosting regression(GBR)and random forest(RF),were built on a dataset collected from literature alongside two other single ML methods,support vector regression(SVR)and artificial neural network(ANN).The results revealed that the ensemble learning methods outperformed the single learning methods in predicting the UCS of AAS-based CPB.Relative importance analysis based on the bestperforming model(GBR)indicated that curing time and water-to-binder ratio were the most critical input parameters in the model.Finally,the GBR model with the highest accuracy was proposed for the UCS predictions of AAS-based CPB.

Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl_(2)·6H_(2)O/expanded vermiculite shape stabilized phase change materials

CaCl_(2)·6H_(2)O/expanded vermiculite shape stabilized phase change materials(CEV)was prepared by atmospheric impregnation method.Using gold mine tailings as aggregate of cemented paste backfill(CPB)material,the CPB with CEV added was prepared,and the specific heat capacity,thermal conductivity,and uniaxial compressive strength(UCS)of CPB with different cement-tailing ratios and CEV addition ratios were tested,the influence of the above variables on the thermal and mechanical properties of CPB was analyzed.The results show that the maximum encapsulation capacity of expanded vermiculite for CaCl_(2)·6H_(2)O is about 60%,and the melting and solidification enthalpies of CEV can reach 98.87 J/g and 97.56 J/g,respectively.For the CPB without CEV,the specific heat capacity,thermal conductivity,and UCS decrease with the decrease of cement-tailing ratio.For the CPB with CEV added,with the increase of CEV addition ratio,the specific heat capacity increases significantly,and the sensible heat storage capacity and latent heat storage capacity can be increased by at least 10.74%and 218.97%respectively after adding 12%CEV.However,the addition of CEV leads to the increase of pores,and the thermal conductivity and UCS both decrease with the increase of CEV addition.When cement-tailing ratio is 1:8 and 6%,9%,and 12%of CEV are added,the 28-days UCS of CPB is less than 1 MPa.Considering the heat storage capacity and cost price of backfill,the recommended proportion scheme of CPB material presents cement-tailing ratio of 1:6 and 12%CEV,and the most recommended heat storage/release temperature cycle range of CPB with added CEV is from 20 to 40℃.This work can provide theoretical basis for the utilization of heat storage backfill in green mines.

Mechanical properties and damage characteristics of solidified body-coal combination in continuous driving and gangue backfilling

Recovery of the coal buried under buildings,railways and water bodies and the residual coal in irregularly arranged fully mechanized mining faces is a common engineering problem facing underground coal mining.In this study,a mining technology of continuous driving and gangue backfilling(CDGB)was proposed.The technology,which can not only alleviate ground subsidence and gangue discharge,but also release the above-mentioned coals,contributes to green and efficient sustainable development of mining.The stability of the system of the solidified body-reserved coal pillar combination(S-C combination)is crucial to the CDGB technology.Therefore,it is of great significance to explore the mechanical and damage characteristics of S-C combination in the synergistic bearing process.First,four sets of differentshaped S-C combination specimens were fabricated and a S-C combination bearing structure in CDGB was constructed to explore the differences in mechanical characteristics and damage modes of different-shaped S-C combination specimens during CDGB.Subsequently,their surface strain field evolutions and acoustic emission(AE)response characteristics in the load-bearing process were obtained with the aid of the digital image correlation technique and the AE signal monitoring system.Furthermore,a damage evolution model based on AE parameters and mechanical parameters was established to clarify the damage evolution law.The following results were obtained:(1)The free area of S-C combination can serve as a quantitative index to evaluate the stability of the overburden control system;(2)The concept of critical value k of the free area was first proposed.When the free area exceeds the critical value k(free area ratio greater than 1.13),the deformation resistance and the free area changes becomes negatively correlated;(3)As the free area expands,the failure of the S-C combination specimen evolves from tensile failure to shear failure.The distribution characteristics of the axial strain field also verified such a change in the failure mode;(4)When the free area expands,the peak AE count gradually changes from“double peaks”to“a single peak”.In this process,the expansion of free area shortens the time for accumulating and releasing energy during loading.Micro cracks generated in the specimen change from a phased steep growth to a continuous increase,and the process in which micro cracks develop,converge,intersect and connect to form macro cracks accelerates.The damage evolution law concluded based on AE parameters and mechanical parameters can well characterize the damage evolution process of S-C combination,providing certain reference for the study on the synergistic bearing of S-C combination during CDGB.

Carbonate-activated binder modified by supplementary materials for mine backfill and the associated heavy metal immobilization effects

Cemented paste backfill(CPB)is one of the effective methods for resource utilization of tailings,but the high cost of ordinary Portland cement(OPC)limits its utilization.Considering the poor performance of Na_(2)CO_(3)-activated binders,in this work,supplementary materials,including CaO,MgO,and calcined layered double hydroxide(CLDH),were used to modify their properties with the aim of finding an alternative binder to OPC.Isothermal calorimetry,X-ray diffraction,and thermogravimetric analyses were conducted to explore the reaction kinetics and phase assembles of the binder.The properties of the CPB samples,such as flowability,strength development,and heavy metal immobilization effects,were then investigated.The results show that the coupling utilization of MgO and CLDH showed good performance.The strength of the Mg_(2)-CLDH_(3) sample was approximately 2.94 MPa after curing for 56 d,which was higher than that of the OPC-based sample.Moreover the cost of the modified Na_(2)CO_(3)-activated binder was lower than that of the OPC-based binder.Modified sample showed satisfactory heavy metal immobilization effects.These findings demonstrate that carbonate-activated binder modified by supplementary materials can be suitable in CPB.

Multiphysics processes in the interfacial transition zone of fiber-reinforced cementitious composites under induced curing pressure and implications for mine backfill materials: A critical review

The mesoscale fiber-matrix interfacial transition zone(FM-ITZ) under induced curing pressure plays a key role in the effectiveness of fiber reinforcement and the engineering application of fiber-reinforced cementitious composites(FRCCs). This critical review establishes the link among induced curing pressure(i.e., external loading condition), multiphysics processes(i.e., internal governing mechanism), and interface behavior(i.e., material behavior) for FRCC materials through analysis of the state-of-the-art research findings on the FM-ITZ of FRCC materials. The following results are obtained. For the mechanical process, the induced curing pressure changes the stress state and enhances multicracking behavior, which can strengthen the FM-ITZ. For the hydraulic process, the strengthened seepage of the FM-ITZ under induced curing pressure weakens the effective stress and exaggerates the deficiency in water retention capacity between the bulk matrix and the FMITZ. For the thermal process, the induced curing pressure causes a steep temperature gradient in the FM-ITZ and thus influences the temperature evolution and thermally-induced microcracks in the FM-ITZ. For the chemical process, the induced curing pressure enhances hydration kinetics and results in the formation of additional hydration products in the FM-ITZ. Moreover, recommendations are proposed on the basis of findings from this review to facilitate the implementation of fiber reinforcement in cemented paste backfill technology.

Physical model investigation on effects of drainage condition and cement addition on consolidation behavior of tailings slurry within backfilled stopes

Estimation of stressses within the tailings slurry during self-weight consolidation is a critical issue for cost-effective barricade design and efficient backfill planning in underground mine stopes.This process requires a good understanding of self-weight consolidation behaviors of the tailings slurry within practical stopes,where many factors can have significant effects on the consolidation,including drainage condition and cement addition.In this paper,the prepared tailings slurry with different cement contents(0,4.76wt%,and 6.25wt%)was poured into1.2 m-high columns,which allowed three drainage scenarios(undrained,partial lateral drainage near the bottom part,and full lateral drainage boundaries)to investigate the effects of drainage condition and cement addition on the consolidation behavior of the tailings slurry.The consolidation behavior was analyzed in terms of pore water pressure(PWP),settlement,volume of drainage water,and residual water content.The results indicate that increasing the length of the drainage boundary or cement content aids in PWP dissipation.In addition,constructing an efficient drainage boundary was more favorable to PWP dissipation than increasing cement addition.The final stable PWP on the column floor was not sensitive to cement addition.The final settlement of uncemented tailings slurry was independent of drainage conditions,and that of cemented tailings slurry decreased with the increase in cement addition.Notably,more pore water can drain out from the cemented tailings slurry than the uncemented tailings slurry during consolidation.

Molecular mechanism of fly ash affecting the performance of cemented backfill material

The great challenge of cemented tailings backfill(CTB)is difficult simultaneously maintaining its excellent mechanical properties and low cost.Fly ash(FA)can potentially address this problem and further replace cement in favor of low carbon development.However,its mechanism on CTB with low cement dosage and low Ca system remains unclear.Consequently,this study conducted uniaxial compression,Xray diffraction(XRD),and scanning electron microscopy(SEM)-energy dispersive spectrometer(EDS)tests to investigate the effect of FA dosage on the mechanical property and microstructure of CTB.A molecular model of FA-CSH was constructed to reproduce the molecular structure evolution of CTB with FA based on the test results.The influences of FA dosage and calcium/silica molar ratio(Ca/Si ratio)on the matrix strength and failure model were analyzed to reveal the mechanism of FA on calcium silicate hydrated(C-S-H).The results show that the strength of CTB increases initially and then decreases with FA dosage,and the FA supplement leads to a decrease in Ca(OH)_(2) diffraction intensity and Ca/Si ratio around the FA particles.XRD and SEM-EDS findings show that the Ca/Si ratio of C-S-H decreases with the progression of hydration.The FA-CSH model indicates that FA can reinforce the silica chain of C-S-H to increase the matrix strength.However,this enhancement is weakened by supplementing excessive FA dosage.In addition,the hydrogen bonds among water molecules deteriorate,reducing the matrix strength.A low Ca/Si ratio results in an increase in water molecules and a decrease in the ionic bonds combined with Ca^(2+).The hydrogen bonds among water molecules cannot withstand high stresses,resulting in a reduction in strength.The water absorption of the FA-CSH model is negatively correlated with the FA dosage and Ca/Si ratio.The use of optimal FA dosage and Ca/Si ratio leads to suitable water absorption,which further affects the failure mode of FA-CSH.

Detecting the backfill pipeline blockage and leakage through an LSTM-based deep learning model

Detecting a pipeline's abnormal status,which is typically a blockage and leakage accident,is important for the continuity and safety of mine backfill.The pipeline system for gravity-transport high-density backfill(GHB)is complex.Specifically designed,efficient,and accurate abnormal pipeline detection methods for GHB are rare.This work presents a long short-term memory-based deep learning(LSTM-DL)model for GHB pipeline blockage and leakage diagnosis.First,an industrial pipeline monitoring system was introduced using pressure and flow sensors.Second,blockage and leakage field experiments were designed to solve the problem of negative sample deficiency.The pipeline's statistical characteristics with different working statuses were analyzed to show their complexity.Third,the architecture of the LSTM-DL model was elaborated on and evaluated.Finally,the LSTM-DL model was compared with state-of-the-art(SOTA)learning algorithms.The results show that the backfilling cycle comprises multiple working phases and is intermittent.Although pressure and flow signals fluctuate stably in a normal cycle,their values are diverse in different cycles.Plugging causes a sudden change in interval signal features;leakage results in long variation duration and a wide fluctuation range.Among the SOTA models,the LSTM-DL model has the highest detection accuracy of98.31%for all states and the lowest misjudgment or false positive rate of 3.21%for blockage and leakage states.The proposed model can accurately recognize various pipeline statuses of complex GHB systems.

Experiment on acoustic emission response and damage evolution characteristics of polymer-modified cemented paste backfill under uniaxial compression

The mechanical properties of cemented paste backfill(CPB)determine its control effect on the goaf roof.In this study,the mechanical strength of polymer-modified cemented paste backfill(PCPB)samples was tested by uniaxial compression tests,and the failure characteristics of PCPB under the compression were analyzed.Besides,acoustic emission(AE)technology was used to monitor and record the cracking process of the PCPB sample with a curing age of 28 d,and two AE indexes(rise angle and average frequency)were used to classify the failure modes of samples under different loading processes.The results show that waterborne epoxy resin can significantly enhance the mechanical strength of PCPB samples(when the mass ratio of polymer to powder material is 0.30,the strength of PCPB samples with a curing age of 28 d is increased by 102.6%);with the increase of polymer content,the mechanical strength of PCPB samples is improved significantly in the early and middle period of curing.Under uniaxial load,the macro cracks of PCPB samples are mostly generated along the axial direction,the main crack runs through the sample,and a large number of small cracks are distributed around the main crack.The AE response of PCPB samples during the whole loading process can be divided into four periods:quiet period,slow growth period,rapid growth period,and remission period,corresponding to the micro-pore compaction stage,elastic deformation stage,plastic deformation stage,and failure instability stage of the stress-strain curve.The AE events are mainly concentrated in the plastic deformation stage;both shear failure and tensile failure occur in the above four stages,while tensile failure is dominant for PCPB samples.This study provides a reference for the safety of coal pillar recovery in pillar goaf.