Dynamic Uniaxial Compressive Strength Research Articles

Initial pore distribution characteristics and crack failure development of cemented tailings backfill under low impact amplitude

The stability of the cemented paste backfill is threatened by the dynamic disturbance during the excavation of the surrounding ore body. In this paper, the computerized tomography (CT) and Split Hopkinson Pressure Bar (SHPB) tests were conducted to explore the initial pore distribution characteristics of the cemented tailings backfill (CTB) and the development of the crack under low impact amplitude. SHPB tests were conducted with impact amplitudes of 34, 37, and 39 mV, respectively. Results show that the initial pores of CTB were steadily distributed with the height of CTB. The CTB contained many initial pores with similar pore size distribution characteristics, and the largest number of pores is between 0.1 and 0.3 mm. Most of the cracks in CTB after low impact amplitude develop and expand along the initial pores, and the damage of CTB mainly exists in shear cracks. A dependence has been established that the dynamic uniaxial compressive strength of the CTB increases, the total crack volume first increases and then decreases, and the number of cracks increases as the impact amplitude increases. The research results can provide a valuable reference for the dynamic performance of CTB under low impact amplitude and the design of mining backfill.

Dynamic mechanical response and fracture mechanism of granite with structural plane containing planar and circular filling simultaneously: The influence of extension length

Weakly filled structural planes are an important influence on the stability control of engineering rock mass. Weak fillings with different geometrical characteristics are randomly distributed in the natural rock mass. To investigate the dynamic mechanical response and fracture mechanism of granite containing both planar and circular weak filling, the Split Hopkinson Pressure Bar (SHPB) experimental system was used to conduct the dynamic impact test. In this paper, the dynamic mechanical properties, the fracture process, the energy evolution and the micro-morphological characteristics of the fragments are mainly analyzed. The results show that: (1) the dynamic uniaxial compressive strength (DUCS), peak strain, and deformation modulus are synergistically affected by the extension length (L) of planar filling and strain rate. The presence of planar weak filling leads to a decrease in strain rate dependence while weakening the specimens' DUCS, and improves the overall synergistic deformation capacity of the specimen. (2) Strain rate and L jointly affect the energy distribution and evolution characteristics of the specimen; L has an enhancement effect on the distribution of reflected energy and a weakening effect on the distribution of transmitted energy. The effect of L on the dissipation of energy is transformed by the strain rate. The strain rate and L are the dominant factors in the variability of stress waveform conversion. (3) At a low strain rate, L has a positive effect on the intensification of the degree of fragmentation, however, at a high strain rate, the effect is characterised by a shift between positive and negative. The degree of fragmentation has a strain rate effect. As L increases, L has a “negative effect” on the “strain rate effect” that affects the degree of fragmentation. (4) The degree of fragmentation of macroscopic fragments is closely related to the energy dissipation density, the relationship between the two appears as a logarithmic function change. The presence of weak filling does not significantly change the microscopic morphology of granite fragments. The loose and porous microstructure of gypsum is the main reason for the altered propagation characteristics of the stress waves.

Dynamic behavior and modeling of icy lunar regolith subjected to dynamic loading

The icy lunar regolith (ILR) in the permanently shadowed regions (PSRs) of the lunar poles is an important lunar water resource for humans, which can be efficiently explored in situ by using a high-speed kinetic penetrator. The dynamic mechanical properties of ILR need priority research since they are related to the ballistics and overload of the penetrator. Based on ILR occurrence form and microscopic composition analyses, four types of ILR simulant (ILRS) specimens with typical water contents are prepared in the present study according to the principle of similar mineral composition and physical state. The dynamic mechanical properties of the ILRS specimens are investigated by combining the results of split Hopkinson pressure bar (SHPB), quasi-static unconfined compression, and variable angle shear (VAS) tests. The results show that the loading strain rates alter the microcrack number and expansion rate of the specimens, which in turn significantly affects their damage mode, dynamic uniaxial compressive strength (DUCS) and toughness ratio. Finally, based on the relationship between stress and strain in the SHPB tests, the damage properties of the ILRS under dynamic loading are analyzed, and a segmental constitutive model is provided by combining it with the Drucker–Prager strength theory. The research results could form the basis for subsequent penetrator structure design and ballistic prediction.

Influence of moisture content on the mechanical properties of an ancient high-porosity building sandstone: San Cristóbal's stone (Andalusia, Spain)

San Cristóbal’s is a porous sandstone that is present in masonry structures of numerous historical buildings located in the southwest of Spain (15th–18th centuries). In order to guarantee conservation and structural integrity, most of these structures have been assessed during the last two decades after they presented severe damages in locations affected by moisture. In this study, static and dynamic elastic modulus and uniaxial compressive strength of San Cristóbal’s stone are determined in the laboratory under dry and saturated states, as well as with a moisture content of 40%, to establish the influence of moisture on them. An experimental campaign was conducted on 16 samples of ancient stone, extracted from the core of structural walls and columns during a structural intervention. The results show how the moisture content directly affects the deformability of the stone, especially between dry and 40% moisture content states. The values of the elastic modulus, both static and dynamic, are reduced with average values of 16.98% and 22.85% respectively. The loss of uniaxial compressive strength between the dry and saturated states is established with a mean value of 29.82%. Additionally, empirical relationships between the studied mechanical properties have been developed, reaching coefficients of determination over 0.90 in most cases.

Damage evaluation of geopolymer under three freezing-thawing treatments with different freezing temperatures based on ultrasonic technique

The damage monitoring of geopolymers in cold regions is important to ensure their durability. This paper presents an investigation on the damage evaluation of geopolymer under three freezing-thawing treatments with different freezing temperatures based on an ultrasonic technique. Firstly, 95 metakaolin-based geopolymer specimens were prepared and subjected to freezing-thawing (F-T) cycles under five freezing temperatures (−1.0 °C, −10.0 °C, −20.0 °C, −30.0 °C, −40.0 °C) by three F-T treatments, which are freezing in air and thawing in water (A-W), freezing in water and thawing in water (W–W) and freezing and thawing in air (A-A), respectively. Secondly, the nondestructive ultrasonic tests were used to monitor the degradation of the geopolymer during F-T cycles and uniaxial compressive strength tests were conducted to study the residual strength of geopolymer after F-T cycles. Thirdly, the effects of freezing temperature and F-T treatment on the longitudinal wave velocity and ultimate compressive strength were analyzed. Finally, a nondestructive method was proposed to predict the compressive strength of geopolymer after F-T cycles. The results showed that the longitudinal wave velocity, dynamic elastic modulus and uniaxial compressive strength of geopolymer all decrease as the freezing temperature decreases. The damage difference of geopolymer under three F-T treatment methods gradually increases as the freezing temperature decreases. Compared with W–W method and A-A method, the geopolymer under A-W treatment method showed the most serious deterioration. The results also showed that the compressive strength damage and longitudinal wave velocity damage exhibited exponential relations. The nondestructive method based on longitudinal wave velocity can efficiently predict the damage evaluation of geopolymer under different F-T treatments and freezing temperatures.

Dynamic Deformation and Failure Characteristics of Deep Underground Coal Measures Sandstone: The Influence of Accumulated Damage

Understanding accumulated damage effects is essential when undertaking deep underground rock engineering, as complex in situ environments and intense engineering disturbances realistically affect the physical and mechanical properties of rocks. Accumulated damage mainly causes the extension of micro-cracks and the sprouting of specific defects in the rocks, altering the microstructural parameters. In this investigation, loading and unloading tests were used to simulate the damage states of the deep underground coal measures sandstone. The accumulated damage factor was formed by combining the P-wave and energy damage variables. The effect of accumulated damage on the bearing capacity and deformation behavior of sandstone was particularly pronounced after experiencing impact loading. The experimental results demonstrate that the accumulated damage factor can depict the initial damage state of sandstone as well as the subsequent dynamic and progressive damage. There is a mutually governing effect between accumulated damage and strain rate. In contrast, accumulated damage significantly extends the range of strain rates, which is fed back into the dynamic uniaxial compressive strength of the sandstone. There is a negative correlation between dynamic fracture energy and accumulated damage, which strongly agrees with the sandstone’s deformation mechanism. The combination of accumulated damage and impact loads can be used to assess the long-term safety of deep underground rock engineering.

Research on the Conversion Relationship between Dynamic Point Load Strength and Dynamic Compressive Strength Based on Energy System

Point load test is a simple and fast method to obtain the compressive strength of rock. According to the maximum load at failure and the distance between two cone ends, the compressive strength of specimens can be calculated by the empirical formula. Because the test process does not need to process the sample into laboratory size type, so it is widely used. At the same time, with the rapid development of rock dynamics, the dynamic strength index of rock becomes more and more important under dynamic load. But dynamic point load is seldom mentioned by scholars. In order to explore the correlation between dynamic point load test and dynamic compressive strength, a large number of dynamic point load and uniaxial dynamic compressive strength tests were carried out using a split Hopkinson pressure bar for granite. Based on the energy theory, the crushing energy consumption during dynamic point load and uniaxial dynamic compressive strength tests were statistically analyzed, and the transformational relation between uniaxial dynamic compressive strength and crushing energy consumption during the dynamic point load test was determined. At the same time, the influence of the cone angle on crushing energy consumption during the dynamic point load test was investigated by changing the conical angle. The results show that with the increase in conical angle, the energy consumption of rock crushing decreases gradually during the dynamic point load test. And the decreasing trend tends to be gentle. The conclusion can provide a reference for obtaining the numerical value of the dynamic compressive strength of the rock in the future.

Experimental Investigation on Multiscale Fracturing in Thermally Treated Sandstone under SHPB Impact Loading

To study the effect of temperature on the dynamic mechanical behaviors and fracture characteristics of thermally treated sandstone, compressive dynamic loading experiments were performed on a series of samples using a split Hopkinson pressure bar (SHPB). In the tests, the dynamic uniaxial compressive strength of the thermally treated sandstone was inversely proportional to the temperature treatment in the range of 200°C–1000°C, while the dynamic elastic modulus first increased and then gradually decreased after different temperature treatments. The results show that two classical mechanical types (i.e., Class I and Class II) are observed from the dynamic stress-strain responses of SHPB tests for thermally treated sandstone. By means of scanning electron microscopy (SEM), the microdifference of postloading microfracture characteristics in Class I and Class II behavior was identified. In Class I behavior, intercrystalline cracks (IE) are the chief form of cracks on the fracture surface of a specimen fractured by SHPB loading even though there some intracrystalline cracks may also be present. In contrast, Class II behavior results from the chief cracking type being intracrystalline cracks.

Degradation of Mechanical Behavior of Sandstone under Freeze-Thaw Conditions with Different Low Temperatures

This study investigated the effects of freezing temperature under freeze-thaw cycling conditions on the mechanical behavior of sandstone. First, the sandstone specimens were subjected to 10-time freeze-thaw cycling treatments at different freezing temperatures (−20, −40, −50, and −60 °C). Subsequently, a series of density, ultrasonic wave, and static and dynamic mechanical behavior tests were carried out. Finally, the effects of freezing temperature on the density, P-wave velocity, stress–strain curves, static and dynamic uniaxial compressive strength, static elastic modulus, and dynamic energy absorption of sandstone were discussed. The results show that the density slightly decreases as temperature decreases, approximately by 1.0% at −60 °C compared with that at 20 °C. The P-wave velocity, static and dynamic uniaxial compressive strength, static elastic modulus, and dynamic energy absorption obviously decrease. As freezing temperature decreases from 20 to −60 °C, the static uniaxial compressive strength, static elastic modulus, dynamic strength, and dynamic energy absorption of sandstone decrease by 16.8%, 21.2%, 30.8%, and 30.7%, respectively. The dynamic mechanical behavior is more sensitive to the freezing temperature during freeze-thawing cycling compared with the static mechanical behavior. In addition, a higher strain rate can induce a higher dynamic strength and energy absorption.

Modelling of dynamic rock fracture process using the finite-discrete element method with a novel and efficient contact activation scheme

Within the framework of combined finite-discrete element method (FDEM), we propose a novel and efficient semi-adaptive contact activation approach (semi-ACAA). The semi-ACAA adaptively activates contact calculations for continuum solid elements around the cohesive element which has just been subjected to shear softening while its softening function has just satisfied a prescribed threshold. The semi-ACAA is implemented in a three-dimensional (3D) FDEM code parallelized based on general-purpose graphic processing units (GPGPU) to model dynamic fracture processes of marbles in dynamic Brazilian tensile strength (BTS) and uniaxial compressive strength (UCS) tests. The semi-ACAA not only overcomes spurious fracturing mode associated with FDEM simulations using ACAA but also is more physically sound and computationally efficient compared with brute-force contact activation approach (BCAA), which has been prevalent in FDEMs with intrinsic cohesive zone model (ICZM)-based cohesive elements. Furthermore, it is proven that the spurious fracturing mode is caused by unphysical movements of mesh due to missing contacts during the shear softening of cohesive elements instead of the sudden activation of contact force calculations as indicated in some literature. Finally, a series of simulations of the dynamic BTS and UCS tests conducted by split Hopkinson pressure bar (SHPB) facility reveals that, if the threshold is set to around unity, not only the spurious fracturing mode can be overcome but also the modelling precision of stress waves in intact rocks can be improved. The speed-up times of semi-ACAA compared with BCAA are between 1.23 and 13.5 depending on the intensity of shear cracking in the simulations including mixed-mode cracking. Since the proposed semi-ACAA is simple and can be easily implemented into any FDEM codes with ICZM-based cohesive elements, it is expected to accelerate future developments and applications of FDEM with ICZM-based cohesive elements in rock dynamics such as drilling and blasting.

Effect of bacterial nanocellulose on the fresh and hardened states of oil well cement

The evaluation of oil well cement additives is determined by the influence on the rheological and mechanical properties of the slurry. The use of nanoadditives for cement, such as bacterial nanocellulose (BNC), has been increasing in recent years due to their high tensile strength, high Young's modulus, and thermal resistance. However, the influence of BNC addition on the mechanical properties of the cement is not widely studied. The purpose of this work is to understand the effects caused by the addition of BNC in class G cement during the fresh and hardened states. Free fluid and high-pressure consistometry tests have been carried out in the fresh state. In the hardened state, dynamic thermomechanical analyses (DMA) and uniaxial compressive strength (UCS) tests have been performed. Additionally, thermogravimetric analysis (TGA) was carried out to determine the calcium hydroxide content and the hydration degree (DOH). Results indicate that BNC modifies the slurry behavior by reducing the free fluid content and by incrementing consistency. Moreover, the calcium hydroxide content and DOH increase with the addition of BNC. The mechanical properties of BNC-cement samples are increased in terms of storage modulus and mechanical strength. These properties can improve the performance of cement used in cementing operations.

Frost Resistance of Thermoinsulating Recycled-Aggregate Concretes Containing Glazed Hollow Beads

Glazed hollow bead (GHB) is an innovative material that gives concrete very good thermal-insulation properties and improves concrete frost resistance, when used as a partial replacement of ordinary fine aggregate. The improvement in frost resistance under freeze-thaw cycles and the deterioration mechanisms of recycled aggregate thermoinsulating concretes (RATICs) is the objective of this study, after adding GHBs to the mix. Fifteen cubes and ninety prisms made of five RATIC mixes (GHB content = 0, 40, 65, 90 and 130 kg/m3) were tested in compression, to identify the optimum content of GHB. The degradation mechanisms of RATIC were identified based on mass loss, dynamic elastic modulus and uniaxial compressive strength. In addition, RATIC mesostructure was investigated by means of Computed Tomography Scanning, and the deterioration mechanisms by analyzing pore amount, average and maximum pore area and porosity. The experimental results demonstrate that adding glazed hollow beads to recycled aggregate concrete (RAC) moderately changes the mesostructure of the concrete, to the advantage of frost resistance, and that the optimum content of GHB is around 90 kg/m3.

Dynamic fragmentation of microwave irradiated rock

Abstract The microwave-assisted rock fragmentation has been proven to be a promising approach in reducing cutting tools wear and improving efficiency in rock crushing and excavation. Thus, understanding the influence of damage induced by microwave irradiation on rock fragmentation is necessary. In this context, cylindrical Fangshan granite (FG) specimens were exposed to microwave irradiation at a power of 6 kW for different durations up to 4.5 min. The damages of the specimens induced by irradiation were quantified by using both X-ray micro-CT scanning and ultrasonic wave measurement. The CT value and P-wave velocity decreased with increase of irradiation duration. The irradiated specimens were then tested using a split Hopkinson pressure bar (SHPB) system to simulate rock fragmentation. A momentum-trap technique was utilized to ensure single-pulse loading on the specimen in SHPB tests, enabling valid fragment size distribution (FSD) analysis. The dependence of dynamic uniaxial compressive strength (UCS) on the irradiation duration and loading rate was revealed. The dynamic UCS increased with increase of loading rate while decreased with increase of irradiation duration. Using the sieve analysis, three fragmentation types were proposed based on FSD, which were dictated by both loading rate and irradiation duration. In addition, an average fragment size was proposed to quantify FSD. The results showed that the average fragment size decreased with increase of loading rate. A loading rate range was identified, where a dramatic reduction of the average fragment size occurred. The dependence of fragmentation on the irradiation duration and loading rate was also discussed.

Comparative Study on Dynamic Mechanical Properties and Energy Dissipation of Rocks under Impact Loads

Geological conditions are often complex during rock engineering construction (such as blasting and drilling) and usually involve rocks of several or multiple lithologies. However, comparative studies on the dynamic mechanical response of rocks of different lithologies are relatively few. To study differences in dynamic mechanical properties and energy dissipation of rocks of different lithologies, three kinds of rock that are common in rock engineering and have large differences in wave impedance were selected: red sandstone, gray sandstone, and granite. Samples of these three rocks were subjected to impact compression tests at different impact speeds, and the rock fragments after impact compression were screened to quantify the degree of fragmentation. The characteristics of stress wave propagation, dynamic stress-strain relationships, degree of fragmentation, and energy dissipation laws of the three rock types were compared and studied. The results show the following: (1) affected by the wave impedance matching relationship, the reflected waves, strain rates, and reflected energy of the three kinds of rock showed significant differences under the same incident stress wave. (2) The dynamic mechanical characteristics and energy dissipation laws of the rocks all had obvious strain rate effects, but the dynamic uniaxial compressive strength and energy dissipation density of the three rock types had different sensitivities to the strain rate. (3) The change trend of the energy utilization efficiency of the gray sandstone with incident energy was different from that of the red sandstone and granite; there was no obvious extreme point in the incident energy range. The three kinds of rock required different incident energies to reach maximum energy utilization efficiency. (4) The equivalent average fragment size of the three rock types decreased as a function of power with increasing impact velocity and energy dissipation density. (5) Under the same incident wave, although the reflection energy, transmission energy, and degree of fragmentation of the three kinds of rock showed significant differences, the differences in dissipation energy were small.

Mechanical behaviour of rock subjected to uniaxial compression at intermediate strain rate

ABSTRACT This paper presents a study of the mechanical behaviour of rock under dynamic compression loading at intermediate strain rate (ISR). A series of laboratory tests were carried out to obtain dynamic uniaxial compressive strength by using a non-explosive powder reaction loading apparatus that was able to generate loading in the ISR range. It was found that the strength of granite and sandstone specimens increases at the ISR range. The results were discussed with a comparison with other findings from multiple references. In general, strength increases linearly at the lower rates and rises significantly at higher rates. Within the ISR range, the tendency is not consistent. Further understanding of rock behaviour within the ISR range needs to be extensively explored. This paper might be useful to understand phenomena involving a dynamic process such as mechanical excavation in which rock experiences a loading lower than high strain rate, i.e., explosive blasting, and higher than static loading.

Machine Learning Approach to Model Rock Strength: Prediction and Variable Selection with Aid of Log Data

Comprehensive knowledge and analysis of in situ rock strength and geo-mechanical characteristics of rocks are crucial in hydrocarbon and mineral exploration stage to maximize wellbore performance, maintain wellbore stability, and optimize hydraulic fracturing process. Due to the high cost of laboratory-based measurements of rock mechanics properties, the log-based prediction is a viable option. Nowadays, the machine learning tools are being used for estimation of the in situ rock properties using wireline log data. This paper proposes a machine learning approach for rock strength (uniaxial compressive strength) prediction. The main objectives are to investigate the performance of data-driven predictive model in determining this vital parameter and to select features of predictor log variables in the model. The backpropagation multilayer perception (MLP) artificial neural network (ANN) with Levenberg–Marquardt training algorithm as well as the least squares support vector machine (LS-SVM) with coupled simulated annealing (CSA) optimization technique is employed to develop the dynamic data-driven models. Capturing nonlinear, high dimensional, and complex nature of real field log data, the rock strength models’ performances are evaluated using statistical criteria to ensure concerning the model reliability and accuracy. The model predictions are compared and validated against the measured values as well as the results obtained from existing log-based correlations. Both the MLP-ANN and the CSA-based LS-SVM connectionist strategies are able to predict the rock strength so that there is a very good match between the model results and corresponding measured values. The input log parameters are ranked based on their contributions in prediction performance. The acoustic travel time and gamma ray are found to have the highest relative significance in estimating rock strength. New correlations are also developed to obtain the in situ rock strength of the siliciclastic sedimentary rocks using the most important log parameters such as dynamic sonic slowness, formation electron density, and shalyness effect. The developed correlations can be used to obtain quick estimation of dynamic uniaxial compressive strength profile using wireline logging data, instead of static data from the surface measurements or laboratory data. It is expected that the proposed models and tools will enable oil and gas engineers to better predict rock strength and thus enhance wellbore performance.

Experimental characterisation of mechanical behaviour of concrete-like materials under multiaxial confinement and high strain rate

Concrete structures and rock engineering projects are generally in uniaxial (principal stresses σ1 > σ2 = σ3 = 0), biaxial (σ1 ≥ σ2 > σ3 = 0) and triaxial (σ1 ≥ σ2 ≥ σ3 ≠ 0) stress states, and also frequently subjected to dynamic loads such as earthquake, high-velocity impact, drilling & blasting and explosive attacks. A series of experiments is conducted on cement and concrete specimens using a triaxial Hopkinson bar (Tri-HB) system, investigating mechanical properties and fracturing behaviour of concrete-like materials under the coupled static-dynamic loading conditions. Experimental results show that the values of dynamic uniaxial compressive strength (σUCd) of cement and concrete are much higher than those of static uniaxial compressive strength (σUCs), but decrease with increasing axial pre-stress from 0 to 30 MPa. The dynamic compressive strength under biaxial (σBCd) and triaxial (σTCd) static pre-stresses are around 2 and 3 times higher than σUCs, respectively. The values of σBCd are substantially sensitive to the pre-stresses (σ1 and σ2) and increase with the increase of σ2. Whereas the increment of σTCd is not obvious at triaxial compression conditions. Tested specimens are broken into small pieces under uniaxial pre-stress conditions, but exhibit slant-shear-failure mode under biaxial conditions. The peak strains under triaxial pre-stress states are much higher than those under biaxial and uniaxial conditions. The cement under dynamic triaxial compression shows significant plasticity deformation and microscopic damage due to the constraint of three-dimensional confining pre-stresses. The X-ray computed tomography (CT) images show internal microcracks under triaxial conditions.

Dynamic compressive properties of dry and saturated coral rocks at high strain rates

Coral rock is characterized by a high porosity of 48.51 ± 5.43% and a low compressive strength of 9.91 MPa due to its biological origin. To experimentally investigate the uniaxial compressive behaviors of both air-dried and fully saturated coral rocks, the quasi-static and dynamic compression tests were performed employing an electro-hydraulic servo-controlled device and a split Hopkinson pressure bar (SHPB) apparatus. The combined effects of strain rate and water saturation on the mechanical properties of coral rock were assessed in terms of the uniaxial compressive strength, Young's modulus, brittleness, energy dissipation as well as the failure pattern. The dynamic uniaxial compressive strength and Young's modulus of saturated coral rocks is consistently lower than those under dry condition, while the losses in them reduces with an increase in stain rate. The coral rock behaves in a more brittle manner in dry state than in saturated state, which is validated by the digital image correlation (DIC) in conjunction with high-speed camera images. Likewise, the change in brittleness or specific energy absorption (SEA) with strain rate under dry and saturated conditions exhibits a similar trend to the dynamic compressive strength. Furthermore, the dynamic failure of dry coral rock appears to be more dependent on the inherent flaws and defects than the saturated one, as the failure pattern doesn't follow a fixed shearing or splitting fracture mode. Overall, the difference of dynamic behaviors between dry and saturated coral rocks are mainly controlled by the coupling weakening and strengthening effects induced by free water. The strengthening effect of water saturation is manifested by making the strength and deformability of coral rock more sensitive to the loading strain rate.

Dynamic Mechanical Properties of Dry and Water-Saturated Siltstones Under Sub-Zero Temperatures

The effects of low temperature and strain rate on the dynamic mechanical properties of dry and saturated siltstones under sub-zero temperatures were investigated and presented in this paper. The siltstone specimens were first frozen to different sub-zero temperatures (from − 10 to − 50 °C), and then were tested at those temperatures using a split Hopkinson pressure bar (SHPB) system. The results indicated that compared to the dry specimens, the saturated specimens exhibit a much shorter compaction phase in the dynamic stress–strain curve due to the presence of the pore water or ice at sub-zero temperatures. The dynamic elastic modulus (Ed) and dynamic uniaxial compressive strength (UCSd) monotonically increase with the increase in the strain rate for both the dry and saturated specimens, and the dry specimens are more sensitive to the strain rate effect with respect to the UCSd. Furthermore, for both the dry and saturated specimens, the Ed and UCSd first increase with the decrease in the temperature from 18 to − 30 °C, and then decrease with a further drop in the temperature from − 30 to − 50 °C. Manifold reasons are responsible for this phenomenon, including the shrinkage of mineral grains, enhancement of the ice strength and interaction of the water/ice mixture with rock as the temperature drops. Using the NMR technique, the mechanisms of the mixed water/ice weakening and strengthening effects on the dynamic mechanical properties of siltstones at sub-zero temperatures were discussed.

A particle mechanics approach for the dynamic strength model of the jointed rock mass considering the joint orientation

SummaryIn nature, there exist several forms of anisotropy in rock masses due to the presence of bedding planes, joints, and weak layers. It is well understood that the anisotropic properties of jointed rock masses significantly affect the stability of surface and underground excavations. However, these critical anisotropic characteristics are often ignored in existing uniaxial dynamic failure criteria. This study investigates the effect of a pre‐existing persistent joint on the rate‐dependent mechanical behaviours of a rock mass using a particle mechanics approach, namely, bonded particle model (BPM), to realistically replicate the mechanical response of the rock mass. Firstly, in order to capture the rate‐dependent response of the jointed rock mass, the BPM model is validated using published experimental data. Then, a dynamic strength model is proposed based on the Jaeger criterion and simulation results. To further investigate the dynamic behaviours, the dynamic uniaxial compressive strength (UCS) for anisotropic rock masses with various joint orientations is investigated by subjecting the BPM models to uniaxial compression numerical tests with various strain rate. The proposed dynamic strength model is validated based on numerical simulation results. Finally, the fragmentation characteristics of the jointed rock masses are analysed, which demonstrate that the failure mode affects the dynamic UCS. This is further confirmed by the analysis of the orientations of microscopic cracks generated by the compression loading.