Initial Stage Of Densification Research Articles

Modeling nonlinear stress-strain model for sulfate dry-wet cycle erosion of concrete: considerations for the initial compaction stage

Sulfate dry-wet cycle erosion significantly affects the mechanical properties of concrete. Investigating the uniaxial compressive stress-strain relationship under these conditions is essential for developing accurate constitutive models. This study analyzes the uniaxial stress-strain curves of concrete subjected to dry-wet cycles in 5% and 15% sulfate solutions. The results show that the initial compaction phase in the stress-strain relationship is particularly pronounced under increasing sulfate concentrations and cycle counts. The concrete experiences an extended compaction phase, which accounts for up to 35.71% of the total strain process. This finding challenge traditional constitutive models, which struggle to accurately describe this phase. To address this issue, the study develops a nonlinear stress-strain model for concrete, incorporating the initial damage caused by sulfate dry-wet cycle erosion, based on Weibull statistical damage mechanics principles. The research indicates that the effects of sulfate concentration and cycle count are predominantly reflected in the pronounced nonlinearity of the skeleton strain function’s opening size (a) and shape characteristics (b), modeled using a fourth-degree polynomial. The model demonstrates an excellent fit to experimental data with an R2 value of 0.99989, showing that the proposed nonlinear stress-strain relationship effectively captures the uniaxial mechanical behavior of concrete under sulfate dry-wet cycle erosion and provides a robust framework for developing constitutive models in such environments.

Влияние гранулометрических характеристик порошка на формирование контактной поверхности на начальной стадии уплотнения

Влияние гранулометрических характеристик порошка на формирование контактной поверхности на начальной стадии уплотнения

Mechanical and instability analysis of asphalt overlay during construction on cement concrete pavement with a large longitudinal slope

ABSTRACT There are insufficient stability problems in overlay construction on tourist highways in mountainous and island areas, especially the pavement with a large longitudinal slope. To elaborate on the instability characteristics of overlay, the construction stage of overlay is divided into loose state, paving stage and initial compaction stage. The moulding methods of specimens during the paving and initial compaction stages were determined. The evolution law of contact-slip characteristics and shear strength of overlay was analysed. The mechanical models of the construction stability of the composite pavements were constructed, instability characterisation methods were proposed and instability characteristics of asphalt overlay during construction were discussed. The results show that the paving state of overlay can be simulated by changing the rolling frequency and the compaction number. There is a risk of slip instability of loose overlay and the longitudinal gradient should be less than 12% to reduce this risk. The critical instability equivalent area ratio and critical slip area proposed can quantitatively describe the shape of the instability body and the instability critical condition. The longitudinal gradient significantly affects the interlaminar instability, but the internal instability mainly depends on the shear strength of asphalt overlay.

Experimental and Meshless Numerical Simulations on the Crack Propagation of Semi-Circular Bending Specimens Containing X-Shaped Fissures Under Three-Point Bending.

Cracks in rock and concrete have a great adverse effect on the stability of engineering structures; however, there are few studies on X-shaped fissures which widely exist in rock and concrete structures. Based on this background, three-point bending fracture tests of SCB specimens containing X-shaped fissures are carried out. The momentum equations in the SPH method are improved, and the crack propagations of SCB specimens under three-point bending are simulated. The results show that cracks grow simply along the vertical direction in the sample with no X-shaped fissures, and the existence of an X-shaped fissure changes the crack growth path and final failure modes of the SCB samples. The crack propagation simulation results are consistent with the experimental results, which verifies the rationality of the improved SPH method. The load-displacement curves mainly present three typical stages: the initial compaction stage, linear elastic deformation stage, and failure stage. The peak load decreases first then increases with an increase in eccentricity. With an increase in X-shaped fissure length and decrease in X-shaped fissure angle, the peak load decreases. The damage counts remain at 0 at the initial loading stage, corresponding to the initial compaction stage and the linear elastic deformation stage, and increase sharply at the later loading stage, corresponding to the failure stage, which is consistent with the experimental results. The influence mechanisms of X-shaped fissures on the crack propagation paths are discussed; the existence of different X-shaped fissure morphologies aggravate the tensile stress concentration at specific positions, leading to different crack propagation modes in the experiments. The research results can provide a certain reference for understanding the failure mechanisms of engineering structures containing X-shaped fissures and promote the applications of the SPH method into the simulations of cross-fissure crack propagations.

Investigation of the bedding effect on coal rock under Brazilian splitting tests

This contribution focuses on understanding the bedding effect of coal rocks under the Brazilian splitting test. First, multiple Brazilian splitting tests were performed on coal rocks with various bedding angles to systematically investigate the influence of stratification. Subsequently, numerical models with stratified structures were constructed, and a continuous–discontinuous numerical analysis method based on the cohesive zone model (CZM) was employed to conduct the corresponding numerical investigations. Results indicate that the load–displacement curves of coal rock specimens with different bedding angles can be classified into four stages: initial compaction stage, elastic deformation stage, crack rapid coalescence stage, and final destruction stage. With increase in the bedding angle, the failure patterns of coal rock specimens can be categorized into three groups: 1) stretching damage along bedding planes; 2) mixed tension and shear failure along the bedding planes and the coal matrix; and 3) stretching failure passing through the coal matrix. Furthermore, the tensile strength and cumulative acoustic emission (AE) energy–displacement relations are significantly influenced by the bedding angle. The numerical model can effectively predict the mechanical responses and fracture behavior of coal rock specimens, providing empirical parameters for the simulation of similar rock engineering.

Experimental Research on Energy Evolution of Sandstone with Different Moisture Content under Uniaxial Compression

In order to investigate the impact of moisture content on energy evolution, three types of rock samples with varying moisture contents were subjected to uniaxial compression tests. The study aimed to analyze the reasons behind the differences in energy during the deformation process of rocks with different moisture contents. The findings indicate that with increasing moisture content, the peak strength and elastic modulus of the samples decrease. However, the ratio of crack initiation strength σci to peak strength σf shows little effect, primarily because the characteristic strength ratio σci/σf is determined by external loads. The growth rate of elastic energy reaches its maximum value in the early stage of yield, while the proportion of elastic energy reaches its peak value in the later stage of yield. In the deformation and failure process of rocks with varying moisture contents, the increment in elastic performance is smallest in the initial compaction stage for saturated rocks, whereas it is the largest in the yield stage for dry rocks. Additionally, a damage evolution equation based on energy dissipation was established and validated.

Study on triaxial compression performance and damage characteristics of fiber-reinforced ecological matrix cementing gangue gypsum fill material.

Polypropylene fiber was equally mixed into alkali-activated slag fly ash geopolymer in order to ensure the filling effect of mine goaf and improve the stability of cemented gangue paste filling material with ecological matrix. Triaxial compression tests were then conducted under various conditions. The mechanical properties and damage characteristics of composite paste filling materials are studied, and the damage evolution model of paste filling materials under triaxial compression is established, based on the deviatoric stress-strain curve generated by the progressive failure behavior of samples. Internal physical and chemical mechanisms of the evolution of structure and characteristics are elucidated and comprehended via the use of SEM-EDS and XRD micro-techniques. The results show that the fiber can effectively improve the ultimate strength and the corresponding effective stress strength index of the sample within the scope of the experimental study. The best strengthening effect is achieved when the amount of NaOH is 3% of the mass of the solid material, the amount of fiber is 5‰ of the mass of the solid material, and the length of the fiber is about 12 mm. The action mode of the fiber in the sample is mainly divided into single-grip anchoring and three-dimensional mesh traction. As the crack initiates and develops, connection occurs in the matrix, where the fiber has an obvious interference and retardation effect on the crack propagation, thereby transforming the brittle failure into a ductile failure and consequently improving the fracture properties of the ecological cementitious coal gangue matrix. The theoretical damage evolution model of a segmented filling body is constructed by taking the initial compaction stage end point as the critical point, and the curve of the damage evolution model of the specimen under different conditions is obtained. The theoretical model is verified by the results from the triaxial compression test. We concluded that the experimental curve is in good agreement with the theoretical curve. Therefore, the established theoretical model has a certain reference value for the analysis and evaluation of the mechanical properties of paste filling materials. The research results can improve the utilization rate of solid waste resources.

Optimal Mix Design and Mechanical Properties of Rapid-Hardening Foam Concrete

This paper conducts compressive strength tests on foam concrete prepared under four factors and three levels through the design of orthogonal experiments. It delves into the phase change rules of the load–displacement curves obtained under various mix proportions. Furthermore, based on the 1-day and 3-day compressive strength values, the study explores different mix proportion results using range analysis and variance analysis methods, thereby determining the optimal mix proportion that can satisfy the maximum 1-day and 3-day compressive strength values. The results indicate that the compression process of rapid-hardening foam concrete includes four stages: initial compaction stage, elastic stage, yielding stage, and plateau stage, with each stage having different causes. Additionally, the sensitivity sequence of factors affecting the 1-day and 3-day compressive strength of rapid-hardening foam concrete is respectively rapid sulfoaluminate cement (α) > water-reducing agent content (δ) > foam content (β) > water-cement ratio (γ) and rapid sulfoaluminate cement (α) > water-cement ratio (γ) > foam content (β) > water-reducing agent content (δ). With 100% sulfoaluminate cement content, the 1-day and 3-day compressive strength values can reach 1.7054 and 2.5471 MPa, respectively, which are 13 times and 7 times the minimum values of 1-day and 3-day compressive strength under other admixtures. The analysis shows that the content of rapid sulfoaluminate cement has the most significant effect on the 1-day and 3-day compressive strength of rapid-hardening foam concrete, with foam content having the least impact on 1-day compressive strength and water-reducing agent content having the least impact on 3-day compressive strength. By integrating range analysis and variance analysis, the optimal mix proportion that simultaneously satisfies the maximum 1-day and 3-day compressive strength is determined to be 100% content of rapid-hardening sulfoaluminate cement, 4% foam content, 0.55% cement ratio, and 0.12% admixture content. Overall, this study provides theoretical support for the research and development of new rapid-hardening foam concrete materials and has significant practical implications for the emergency repair and construction of infrastructure projects.

Damage evolution analysis of concrete based on multi-feature acoustic emission and Gaussian mixture model clustering

In this work, a novel method for damage evolution analysis of concrete under uniaxial compression is proposed based on the multi-feature acoustic emission (AE) and the Gaussian mixture model (GMM) clustering. The hierarchical clustering algorithm is adopted to select optimal AE parameters, while multiple features of these parameters are generated through the principal component analysis (PCA). Then the concrete damage signals are separated by using the GMM clustering with multiple features. Meanwhile, the waveform signals associated with concrete damage in each cluster are validated using the Fast Fourier transform (FFT) and the continuous wavelet transform (CWT). Finally, the damage evolution process of concrete under uniaxial compression is divided based on the variation of AE characteristics. The results show that the frequency ranges of signals for micro-cracks, mixed cracks and friction of concrete under uniaxial compression are 20–30 kHz, 20–65 kHz, and 100–120 kHz, respectively. The damage process of concrete under uniaxial compression is divided into three stages according to the inflection points of the cumulative energy curve, namely the initial compaction stage, the crack formation stage and the failure stage.

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-and-fill 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.

Study on mechanical properties and acoustic emission characteristics of deep diorite under uniaxial compression

The research object is diorite in the Lingbei TBM section of the Hanjiang-To-Weihe River Qinling tunnel, with a buried depth of over 1 km. Using MTS-2000 microcomputer-controlled electro-hydraulic servo universal testing machine and DS5-16b acoustic emission (AE) monitoring system, uniaxial compression and acoustic emission monitoring tests were carried out on rock samples, to study the uniaxial compression mechanical properties and acoustic emission characteristics of the deep diorite. The results of the study indicate that: (1) During uniaxial compression, diorite undergoes four stages: initial compaction, elasticity, yield and failure, in which the curve of the initial compaction stage is significantly smoother. The uniaxial compressive strength is 41.95 MPã102.42 MPa, with an average of 74.07 Mpa; The axial peak strain ranges from 1 % to 1.4 %, and the failure mode belongs to brittle ductile splitting failure. (2) The cumulative ringing count and energy showed a very slow increase trend during the calm period; After entering a surge period (with the appearance of Kaiser points), both show a significant transition state; During the slow increase period, the overall growth rate of the two slowed down and remained almost silent. (3) On the basis of the analysis of RA-AF values during the deformation and rupture process of diorite, it can be seen that the damage type of diorite is tensile damage by the significant low RA value and high AF value characteristics, which coincides with the actual damage fracture characteristics of the rocks in the sample. (4) During the compaction stage, there are few acoustic emission location points, which correspond to low energy and are mainly distributed at the higher and lower ends of the sample; After entering the elasticity stage, the number of positioning points significantly increases and gradually expands towards the middle; Near Kaiser point, the number of location points and corresponding energy are both in a sharp increase state, and this trend is in good agreement with the changes in the ringing count-time and energy-time curves. (5) The damage time mainly starts at the end of the calm period, and the pattern of change in the damage curve coincides with the localization point and energy evolution. The results of the research can be used as a referential basis for the development of the excavation, protection and other construction plans for the Lingbei TBM section of the Hanjiang-To-Weihe River Qinling tunnel or similar surrounding rock tunnels, as well as for further conducting triaxial unloading tests on diorite.

Effect of interlayer mixed zone and effective stress on permeability anisotropy of NGH turbidite reservoir

Resulting from the bedding structure of turbidite reservoir, permeability anisotropy is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the evolution of permeability anisotropy under increasing effective stress are investigated in this study. Taking the turbidite sediments in northern Cascadia as an example, a series of experimental tests and co-simulations of discrete element method (DEM) and computational fluid dynamics method (CFD) are conducted. For layered turbidite sediments, an interlayer mixed zone will be formed in the interface between fine-grained layer and coarse-grained layer after loading. Fine particles will migrate to the coarse layer, blocking the horizontal preponderance flow paths and further reduce the permeability anisotropy. Moreover, the development of interlayer mixed zone mainly occurs in initial compaction stage and slows down in higher stress level, for which the permeability anisotropy shows a staged reduction with increasing effective stress. Considering the effect of interlayer mixed zone, a predictive model of permeability anisotropy is proposed and verified by the experimental results, which has higher accuracy than traditional “layered cake” model. The finding of this research also confirms the mixing effect in deep sediments, which may affect the geological history records in target sedimentary sequence.

Mechanical response of stress damaged cemented tailings backfill based on pore structure and acoustic emission characteristics

The mechanical response of cemented tailings backfill (CTB) is essential for its achievement of underground mine ground pressure management and productivity improvement. However, long-term, continuous, high-intensity mining causes CTB subject to mining stress disturbance, which inevitably affects its mechanical properties and stability when serving as a load-bearing structure. Therefore, the stress-damaged CTBs (SDCTB) were prepared to take 20%, 40%, 60%, and 80% uniaxial compressive strength (UCS) as the stress-disturbed levels (SDL) in this study. The effects of SDLs and curing ages (3, 7, 14, and 28d) on the mechanical properties and damage characteristics of STCTB were also investigated. The results showed that as SDL enhanced, the UCS and elastic modulus of the backfill (except SDCTB with 60%–80% SDL) increased and then decreased, and 20% SDL was the turning point. Meanwhile, the external load disturbance significantly affected the initial compaction stage of the stress–strain curve of the backfill. Proper SDL reduced the porosity of SDCTB and induced the transformation of macropores to mesopores and micropore. In addition, the AE signals of CTB and SDCTB with 20% SDL showed an active-calm-active stage during compression. The AE signals of SDCTB with 40%–80% SDL remained active stage. The higher SDL aggravated the crack sprouting and expansion rate of the backfill. Also, the correlation dimension (D) of AE ringing counts and amplitudes fully reflected the rupture characteristics of the backfill. The failure mode of SDCTB changed from mixed tensile-shear to tensile failure as the curing age increased. Comprehensive macro-mesoscopic experimental outcomes, the effect of compaction enhancement and cracking damage of the backfill under the external load were proposed, and the two were competing. The results of this study provide a theoretical basis for the stability assessment of the backfill under mining stress disturbance.

Long-term strength and deformation size effect of gangue cemented backfill in acid mine water

The mine water environment can seriously affect the long-term strength and deformation of the backfill body, thereby affecting the stability of the goaf. In this study, the gangue cemented backfill (GCB) of different sizes were placed in different environments (air, water, single H2SO4 solution, and H2SO4 solution coupled with load), the deformation and ultrasonic pulse velocity (UPV) of specimens were recorded with age. The compressive strength, acoustic emission (AE), and resistivity of GCB during the uniaxial compression at 360 days were monitored. In addition, the microscopic morphology of GCB was observed by scanning electron microscopy (SEM), and the damage model of GCB in the process of uniaxial compression was established. The research showed that: the deformation of GCB was fast in the first 90 days and gradually slowed down. The bigger the size was, the greater the instantaneous creep of the GCB was, and the faster the increase rate of the initial deformation was. The UPV of eroded GCB experienced three stages of growth-stability-decrease. The UPV of the smaller GCB increased rapidly in the early stage and decreased rapidly in the later stage. At 360 days, the order of compressive strength of GCB in different media is: water > air > single H2SO4 solution > H2SO4 solution coupled with load, and the compressive strength decreases with the increase of specimen size. In the process of uniaxial compression, the larger the size of the GCB, the larger the number of AE rings detected. The smaller the size of GCB, the stronger the AE characteristics appeared in the initial compaction stage. During the loading process, the resistivity curve of GCB changed approximately in a “U” shape. The smaller the size of GCB, the slower the increasing rate of resistivity in the post-peak failure stage. The abrupt change in the AE ring and the stable resistivity stage could be used to predict the failure of the GCB. The research could provide a reference for the design of the erosion resistance of GCB.

Mechanical and seepage characteristics of polyvinyl alcohol fiber concrete under stress-seepage coupling

This study presents an experimental investigation of the coupled stress and seepage water pressure loading test conducted on polyvinyl alcohol (PVA) fiber concrete, which had undergone salt attack freeze-thaw (F-T) cycles. This study delves into the influence of the number of salt attack F-T cycles, as well as the magnitude of seepage water pressure, on the triaxial compression seepage mechanical properties and permeability evolution of the specimens. Additionally, the nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and X-ray diffraction (XRD) test results were analyzed, revealing that sodium sulfate solution penetrates the interior of PVA fiber concrete and reacts with cement stone to form ettringite and gypsum, which initially fills and compacts the pore structure. The increased production of such compounds causes pore structure to expand, with the maximum number of mesopores and macropores increasing by 115.5%. Macroscopically, it was observed that the mass and peak strength of the specimens increased at the beginning of the cycle, and the initial permeability decreased. However, the opposite phenomenon occurred after 15 cycles. When seepage water pressure was increased from 1 MPa to 3 MPa, the crack initiation stress, crack damage stress, and peak stress of the specimens decreased by 13.1%, 16.9%, and 15.6% on average, respectively, and the initial permeability increased by 69.3% on average. The deformation process of PVA fiber concrete under stress-seepage coupling loading was divided into five stages, including the initial compaction stage, elastic deformation stage, crack initiation and stable propagation stage, crack unstable propagation stage, and post-peak deformation stage. Correspondingly, the permeability exhibited a rapid decline, followed by a relatively stable phase, a slow rise, a rapid rise, and a continuous increase. Besides, the relationship between permeability and volume strain in compression and expansion stages was clarified.

A new hydro-mechanical coupling constitutive model for brittle rocks considering initial compaction, hardening and softening behaviors under complex stress states

After the excavation of underground engineering, the failure and instability of surrounding rock under hydro-mechanical coupling conditions is a common type of engineering disaster. However, the hydro-mechanical coupling mechanical characteristics of rock have not been fully revealed, and suitable models for the stability analysis of surrounding rock under hydro-mechanical coupling conditions are very scarce. Therefore, a series of triaxial compression and cyclic loading and unloading hydro-mechanical coupling tests were carried out to study the mechanical characteristics, deformation and mechanical parameters of rock under different confining pressures and pore pressures. Then, based on Biot’s effective stress principle, a hydro-mechanical coupling damage constitutive model within the framework of irreversible thermodynamics was proposed to describe the initial compaction effect, pre-peak hardening and post-peak softening behaviors. The functional relationships between the proposed model key parameters (η and ζ) and the effective stress were established to characterize the pre- and post-peak nonlinear behaviors of rock. A compaction function Ck for the evolution of the undamaged Young’s modulus in initial compaction stage was introduced to characterize the pre-peak compaction effect. A user-defined material subroutine (UMAT) was compiled in ABAQUS to numerically implemented the proposed model. The numerical simulation results are highly consistent with the test results, the proposed model can also predict the hydro-mechanical coupling characteristics of rock under untested stress levels. In addition, the yield function of the proposed model considers the influence of intermediate principal stress, which is also suitable for the simulation of hydro-mechanical coupling characteristics under true triaxial stress states.Graphical abstract

Study on Compaction Characteristics and Compaction Process of an Unsaturated Silt Based on PFC3D

In order to reduce the construction cost of excessively wet silt roadbeds and improve the compaction quality, this paper studies the compaction characteristics and compaction technology of excessively wet silts through theoretical derivation, indoor experiments, numerical simulation, engineering verification, and other research methods. Based on the pendulum-type liquid bridge structure, this paper uses the Hertz–Mindlin theory, particle motion equation of state, and other theories to modify the force between particles of an unsaturated soil. Through the contact angle, which is a medium, the matrix suction is linked to the water content and is used to approximate the water content state of soil samples. Using PFC3D (particle flow code 3D) numerical simulation software, an unsaturated silt model was established based on Hill contact, and the model parameters were calibrated through basic geotechnical tests and triaxial compression tests. The correctness of the theory is verified by comparing the simulated triaxial compression test of wet silts with the indoor test. By simulating subgrade rolling, considering factors such as the distribution of contact force chain and the variation of compaction degree, the effects of rolling times, strong vibration, and weak vibration on the compaction effect were compared and analyzed. Finally, the optimal rolling and compaction process under the optimal water content is obtained through on-site engineering tests. The results show that: (1) The Hill contact model is reasonable for simulating the wet silt. (2) During the simulation of the roadbed compaction process using PFC3D software, it was found that the compaction degree change during the entire compaction process can be roughly divided into the initial compaction stage and the re-compaction stage. The reasonable number of compaction times was determined to be five through discrete element simulation. (3) It is found from the numerical simulation results that compared to static compaction, both strong and weak vibration compaction can effectively improve the compaction effect of subgrade compaction. The larger the vibration amplitude, the more obvious the improvement of the compaction effect. The compaction effect of strong vibration followed by weak vibration is stronger than that of weak vibration followed by strong vibration. (4) The optimal compaction process obtained under the optimal moisture content of 15% is static pressure once + strong vibration twice + weak vibration twice.

Research on the Response Mechanism of Coal Rock Mass under Stress and Pressure.

In this paper, the strength and deformation failure characteristics of bearing coal rock mass are related to the confining pressure, and the SAS-2000 experimental system is used to carry out uniaxial and 3, 6, and 9 MPa triaxial tests on coal rock to assess the strength and deformation failure characteristics of coal rock under different confining pressure conditions. The results show that the stress-strain curve of coal rock undergoes four evolutionary stages after fracture: compaction, elasticity, plasticity, and rupture. With confining pressure, the peak strength of coal rock increases, and the elastic modulus increases nonlinearly. The coal sample changes more with confining pressure, and the elastic modulus is generally smaller than that of fine sandstone. The stage of evolution under confining pressure constitutes the failure process of coal rock, with the stress of different evolution stages causing various degrees of damage to coal rock. In the initial compaction stage, the unique pore structure of the coal sample makes the confining pressure effect more apparent; the confining pressure makes the bearing capacity of the coal rock plastic stage stronger, the residual strength of the coal sample has a linear relationship with the confining pressure, and the residual strength of the fine sandstone has a nonlinear relationship with the confining pressure. Changing the confining pressure state will cause the two kinds of coal rock samples to change from brittle failure to plastic failure. Different coal rocks under uniaxial compression experience more brittle failure, and the overall degree of crushing is higher. The coal sample in the triaxial state experiences predominantly ductile fracture. The whole is relatively complete after failure as a shear failure occurs. The fine sandstone specimen experiences brittle failure. The degree of failure is low, and the confining pressure's effect on the coal sample is obvious.

Mechanical properties and microscopic characterization of cemented paste backfill with electrolytic manganese residue matrix binder

With the exploitation of mineral resources, more and more mined-out areas need to be backfilled. In order to meet the needs of backfill, a new cemented paste backfill (CPB) is developed. The new material uses electrolytic manganese residue (EMR), slag and red mud as binder, tailings as aggregates, and sodium hydroxide as alkali activator. Taking the EMR content, the mass concentration and bone glue ratio as the influencing factors, the unconfined compressive strength (UCS) of 3 d, 7 d and 28 d as the target, the single factor design was carried out, and the mixture proportion recommended was obtained. According to the 28 d UCS of CPB under mixture proportion recommended, a piecewise damage constitutive model with the end of initial compaction stage as the critical point was established. The three microscopic test methods of scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the CPB at different ages under the mixture proportion recommended. The internal morphology, composition, phase composition and chemical bond combination of the CPB were analyzed. The results show: when the EMR content is 20%, the mass concentration is 81%, and the bone glue ratio is 6, the mixture proportion recommended for the developed CPB was recommended; The established constitutive model can fit the UCS curve well; At the initial stage of reaction, C–S–H and C-A-S-H gels were formed inside the CPB; With the increase of age, the amount of gel formation increased, the diffraction peak of the reactants weakened, the diffraction peak of the product enhanced, the pore played a filling role, so that the internal structure was dense, porosity decreased; The corresponding characteristic bands originate from the asymmetric stretching and bending vibrations of H–O–H, Si–O, Si–O–Si, C–O and O–H chemical bonds. This new type of CPB can well meet the requirements of backfill, and can reasonably use solid waste.

Mechanical properties investigation and damage constitutive models of red sandstone subjected to freeze-thaw cycles

The mechanical properties of rock can be degraded by long-term freeze-thaw cycles (FTCs). However, the damage constitutive model of rock subjected to FTCs for simulating the whole deformation process is still lacking. In this paper, uniaxial compression tests for red sandstones were carried out under different numbers of FTCs, and the relationships between the mechanical parameters and the number of FTCs were established. The method for determining the fully compacted point was firstly proposed, and a random exponential function of strain difference during compaction stage and linear elastic stage was constructed. Then, a novel damage constitutive model simulating the whole deformation process of rock subjected to FTCs was proposed. The proposed model and three existing models were compared with experimental data, and their model performances were evaluated. The proposed model with the consideration of the initial compaction stage was found to be more consistent with the experimental data, while the two existing models can be improved to be more versatile. The finding enriches the research of rock in constitutive modelling in cold regions.