Biaxial Compression Tests Research Articles

Experimental investigations on the failure characteristics of the slit-contained circular opening under biaxial compression: Insights into the rockburst prevention

The slit-cut method for the rockburst prevention and control is believed effective with its easiness in operation, adjustment and compatibility. However, there is limited advance knowledge of the physics of the slit-cut method, which is vital for the engineering designs. In this study, the biaxial compression tests with a synchronous AE (acoustic emission)-DIC (digital image correlation) monitoring are creatively carried out on the slit-contained circular opening specimens with different slit configurations to demonstrate the academic thoughts and mechanisms of the slit-cut method. The experiments document the two typical failure types namely the internal crack propagation and the dynamic rockburst, which occupy different AE hit rate characteristics and different entropy properties. With the occurrence of rockburst, the AE hit rate presents a bouncing ascend-descend trend, and a higher disorder and chaos is faithfully exhibited. Depending on the slit parameters, the slit-cut method can efficiently mitigate rockburst in terms of the occurrence frequency and magnitude. The underlying mechanism lies in the enhancement of the shear mechanism and the development of the internal cracks through which the stored energy can be greatly dissipated. However, due to the unrestricted shear failure in the slit-contained opening specimens, the opening-scale inward instability can be triggered by the internal crack coalescence, thus posing a threat to the safety of the opening. Implementing the slit-cut method with a consideration of the in-situ stress conditions is evidently essential for the safe excavation.

Mechanical behavior and energy characteristics of red sandstone with different seawater immersion heights under biaxial compression

Rock structures engineered during undersea mining are typically subjected to varying water distributions due to the motion of seawater, which considerably influences their stability. Hence, it is essential to understand the influence of seawater distribution on the mechanical behavior and energy characteristics of rocks. In this study, biaxial compression tests were conducted on red sandstone at various seawater immersion heights, and acoustic emission signals during compression were monitored. The results illustrate that the mechanical properties of the red sandstone deteriorate significantly upon seawater immersion. With an increase in seawater immersion height, the peak strength and elastic modulus of the rock specimens decreased exponentially. When the seawater immersion height was varied from 0 to 1/4 H under lateral stresses of 5, 10, 15, and 20 MPa, the peak strength decreased by 18.94%, 20.29%, 17.47%, and 14.87%, respectively, and the elastic modulus decreased by 4.6%, 8.1%, 11.7%, and 10.9%, respectively. Brittleness also decreased gradually. During compression, the acoustic emission (AE) and accumulated AE counts exhibited a stationary phenomenon, first increasing slowly and then suddenly. However, the AE counts decrease with increasing seawater immersion height. Meanwhile, with increasing seawater immersion height, the proportion of tensile cracks gradually increased and that of shear cracks gradually decreased. As the seawater immersion height increased, both the peak total input strain energy and peak total elastic strain energy decreased, whereas the peak total dissipated strain energy exhibited the opposite trend.

Study on crack propagation characteristics of rocks with different lateral pressure based on joint monitoring of DIC and AE

During the process of rock failure, the characteristics of crack propagation affect the fracture characteristics and macroscopic mechanical behavior of rocks, indirectly affecting the safety and stability of rock engineering. In order to study the evolution characteristics of cracks during rock failure under different lateral pressure, based on an improved digital image correlation (DIC) and acoustic emission (AE) signal recognition method, a visual biaxial servo loading device was developed to conduct biaxial compression tests on mudstone with prefabricated cracks of the same inclination angle. The research results indicate that the stages of crack propagation include microcracks propagation, crack tip formation, stable macroscopic cracks propagation, and unstable macroscopic cracks propagation. As the lateral pressure increased, the initiation frequency of cracks decreased, the quantity of propagation decreased, and the propagation path shortened, indirectly increasing the bearing strength of rocks. The initiation stress, peak stress, and elastic modulus of pre-cracked rocks with lateral pressure ≤ 2 MPa were lower than those of pre-cracked rocks with lateral pressure > 3 MPa, with the minimum reduction amplitude of 14.1%, 21.2%, and 12.6%, respectively. As the lateral pressure decreased, the dispersion of the AE main frequency distribution increased and accelerated its downward expansion. The surface temperature curves of rocks were prone to fluctuations and rapid upward evolution characteristics corresponding to crack tip formation and crack propagation, respectively. The research results provide theoretical and engineering references for the mining of weak coal seams.

Study on Mechanical and Acoustic Emission Characteristics of Backfill-Rock Instability under Different Stress Conditions.

Unveiling the mechanical properties and damage mechanism of the complex composite structure, comprising backfill and surrounding rock, is crucial for ensuring the safe development of the downward-approach backfill mining method. This work conducts biaxial compression tests on backfill-rock under various loading conditions. The damage process is analyzed using DIC and acoustic emission (AE) techniques, while the distribution of AE events at different loading stages is explored. Additionally, the dominant failure forms of specimens are studied through multifractal analysis. The damage evolution law of backfill-rock combinations is elucidated. The results indicate that DIC and AE provide consistent descriptions of specimen damage, and the damage evolution of backfill-rock composite specimens varies notably under different loading conditions, offering valuable insights for engineering site safety protection.

Spalling characteristics of sandstone containing rectangle hole with gradient cavity height: An experimental and DEM numerical study

Spalling failure commonly exists during the progressive excavation of underground engineering, and the stepped excavation of underground mining stope is rarely simulated in previous studies. Therefore, the biaxial compression test is finished by sandstone samples containing rectangular hole (with different cavity heights), and the corresponding numerical simulation is conducted by the discrete element method. Based on the mechanical, optical and acoustic data captured during the test, the main results and conclusions are as follows: 1) There are two paths to activate the typical spalling failure, the first way is caused by the rapid energy release (manifested as sudden AE burst), and the second way is that the released energy accumulates to a threshold over a long period and finally causes large-scale damage. 2) The relatively larger cavity height delayed the initiation of central tensile failure in the roof & bottom surrounding rocks and the spalling failure in sidewalls. 3) The relatively larger confining pressure contributes to the early construction of the spalling failure and limits the extension of the central tensile crack bands. 4) The construction of spalling failure is very fast, the ejected rock plates are caused by several rounds of spalling failure, and the final V-shaped groove is built by the spalling failure and internal crushing failure.

Experimental and numerical investigation of the fracture behavior in granite specimens with two co-planar side flaws under biaxial loading

It is well known that the structural plane plays a decisive role in the mechanical properties of rock mass. To better understand the cracking mechanism and stress characteristics of co-planar intermittent double-flawed granite specimens with various dip angles (Abbreviated as flawed specimens), two co-planar flaws were prefabricated on both sides of the granite, and biaxial compression tests were conducted. The failure characteristics and local microstrain were recorded by the camera and strain acquisition instrument. The evolution law of mechanical parameters of flawed specimens was analyzed. Simultaneously, a two-dimensional particle flow code (PFC2D) was used to establish a biaxial compression model of flawed specimens. The crack initiation and propagation of the specimens were explained by the crack hot spot, the evolution law of crack, and the evolution law of the number of microcracks. It is found that (a) as flaw dip angle (α) increases, peak strength and elastic modulus initially decrease and then increase, with reductions ranging from 17.6% to 46.1% and 15.6% to 48.2%, respectively. (b) Increasing lateral stress (σ2) triggers a shift in failure mode from a tensile-shear mixed failure to a shear failure. The cohesion (c) and friction angle (φ) of flawed specimens were notably smaller compared to those of intact specimens, with reductions correlating with the α. (c) Flawed specimens exhibit pronounced contact force concentration and stress concentration at the flaw tip. The analysis of the microstrain confirms stress concentration at the flaw tip, leading to failure. However, various σ2 induces the differences in the magnitude and extent of tip stress.

3D SPH implementation to large deformation of granular mass using an advanced hypoplastic constitutive model

In this study, an advanced hypoplastic model incorporating the critical state is integrated into smoothed particle hydrodynamics (SPH) for the numerical investigation of geotechnical large-deformation problems. Moreover, a return mapping strategy is proposed to regulate the stress state to avoid calculation failure. Furthermore, a closed-form solution to predicting the failure surface implicitly incorporated in the hypoplastic model is derived. The proposed SPH method is examined with element tests such as the oedometer test and biaxial compression test to observe a good performance of the SPH model. Finally, the SPH model is applied to numerically reproducing sand column collapse to investigate the effect of the initial void ratio on the behavior of soil.

The Fracture Evolution Mechanism of Tunnels with Different Cross-Sections under Biaxial Loading

Biaxial compression tests based on an elliptical tunnel were conducted to study the failure characteristics and the meso-crack evolution mechanism of tunnels with different cross-sections constructed in sandstone. The progressive crack propagation process around the elliptical tunnel was investigated using a real-time digital image correlation (DIC) system. Numerical simulations were performed on egg-shaped, U-shaped, and straight-walled arched tunnels based on the mesoscopic parameters of the elliptical tunnel and following the principle of an equal cross-sectional area. The meso-crack evolution and stress conditions of the four types of tunnels were compared. The results show that (1) fractures around an elliptical tunnel were mainly distributed at the end of the long axis and mainly induce slabbing failure, and the failure mode is similar to a V-shaped notch; (2) strain localization is an important characteristic of rock fracturing, which forebodes the initiation, propagation, and coalescence paths of macro-cracks; and (3) the peak loads of tunnels with egg-shaped, U-shaped, and straight-walled arched cross-sections are 98.76%, 97.56%, and 90.57% that of an elliptical cross-section. The elliptical cross-section shows the optimal bearing capacity.

Study on the biaxial mechanical behavior of steel fiber reinforced recycled aggregate concrete

Steel fiber reinforced recycled aggregate concrete (SFRAC) was proposed considering the enhancement of mechanical properties by incorporating steel fibers and the reduction of environmental impacts and cost due to the addition of recycled aggregates. To better understand the mechanical properties of SFRAC under biaxial compression and its failure criteria, 150 SFRAC specimens were prepared with different replacement ratios of recycled coarse aggregate (50%, 75%, 100%) and volume fractions of steel fibers (0%, 0.6%, 1.2%, 1.8%). In addition, various stress ratios were considered in the biaxial compression test (σ1:σ2=0:1.0, 0.2:1.0, 0.5:1.0, 0.7:1.0, 1.0:1.0). The results indicated that the failure of SFRAC was observed as columnar and splitting modes depending on the biaxial stress ratio. The impacts of the considered variables on the biaxial peak strength were studied. The increasing volume fraction of steel fibers tended to improve the biaxial peak strength of SFRAC within a threshold beyond which the impact was negative. Similarly, the biaxial strength of most SFRAC samples showed a pessimum effect when the biaxial loading stress ratio was higher than 0.7, though they were still higher than the uniaxial compressive strength. A failure criterion for SFRAC under biaxial compression loading is proposed, in which four parameters are included. The calculations are in good agreement with the experimental results.

Critical Failure Characteristics of a Straight-Walled Arched Tunnel Constructed in Sandstone under Biaxial Loading

To characterize the failure of rock mass surrounding underground tunnels, biaxial compression tests were conducted on a real sandstone model with a straight-walled arched hole. The acoustic emission (AE) system and digital image correlation (DIC) optical inspection equipment were used to investigate the crack evolution process and failure precursors of the tunnel. A two-dimensional particle flow code (PFC2D) was used to conduct numerical simulations on the sample, so as to investigate the mesoscopic failure mechanism of rock mass. The results show that the failure of the single tunnel constructed in sandstone occurs mainly in the walls on both sides (between the spandrels and arch feet), showing slabbing failure characteristics and a certain abruptness. The crack initiation in sandstone in early stage is not obvious, and the crack propagation in rock mass is rapid when acoustic emissions are enhanced. The small increments in the AE count and amplitude and the continuous reduction in the b-value can be used as precursors for the failure of rock mass. When the height–span ratio is 0.8 and 1.0, the stress distribution around the chamber is more uniform, and when the height–span ratio is greater than 1.0, the stress is mainly concentrated in the vault and arch bottom. In the PFC simulations, tensile fractures firstly initiate in the middle of walls and at the arch feet, arcuate fracture concentration zones are then formed, in which shear fractures appear and a few particles spall from the surfaces. When approaching the ultimate bearing capacity, rock masses on both sides of the tunnel are fractured over large areas, and the slender coalesced fractured zone develops to the deep part of rock mass, causing failure of the sample.

Enhancing gas drainage efficiency in soft coal seams: Mechanically over-excavated cavities along borehole in floor tunnels

In addressing the challenge of gas drainage in soft coal seams, this study introduces an innovative technique involving mechanically over-excavated cavities along boreholes in floor tunnels. Through a combination of laboratory experiments, numerical simulations, and field tests, we investigate the efficacy of this method in weakening the coal structure and enhancing gas extraction. The influence of cavity parameters (inclined angle, cavity length and diameter, cavity number) on the mechanical properties and AE behavior of coal specimens were investigated by biaxial compression tests. Besides, the stress distribution and permeability evolution around the cavity were studied using FLAC3D combined with COMSOL software. Finally, the field tests were conducted in LiCun Coal Mine. The results indicate that a significant reduction in the compressive strength of coal specimens with increasing cavity dimensions. Notably, the peak stress in coal specimens decreased significantly as the cavity length, diameter, and number increased, by up to 42.09%, 39.38%, and 57.83%, respectively. In contrast, the peak stress increased with the cavity inclination angle, and it was about 1.14 times higher for specimens with a 90° angle than for those with a 15° angle. It is also found that the cavity inclination reduces the damage and acoustic emission activity of the specimen, while the cavity volume increases them and weakens the rock-bearing capacity. The failure modes of all specimens are shear failure combined with surface spalling. Numerical simulations employing FLAC3D and COMSOL software corroborated these results, demonstrating enhanced pressure relief and increased permeability around the cavities. Field tests conducted in the LiCun Coal Mine provided practical evidence of the method's efficacy, with a more than 3-fold increase in gas drainage concentration and volume observed compared to traditional drilling methods. By adopting mechanically over-excavated cavities, not only is the efficiency of gas drainage significantly improved, but also the safety and productivity of mining operations are enhanced. This study offers a promising and impactful solution for addressing gas drainage challenges in soft coal seams.

Mechanical and fracturing characteristics of defected rock-like materials under biaxial compression

Various types of discontinuities have great influences on rock stability in underground excavation, as engineering disasters can be triggered by crack growth from flaws. Motivated by this concern, quasi-static biaxial compression tests are conducted on rock-like cement mortar materials. These tests examine the effect of shape, orientation, and interaction of pre-existing defects, representing natural or preconditioning fractures. Mechanical and fracturing characteristics are analyzed using stress-strain response, acoustic emission (AE), digital image correlation (DIC), and synchrotron X-ray computed tomography (CT) techniques. It could be found that the confinement increases the sample peak stress and restricts the development of cracks in intact samples. In hole samples, cracks initiate from the boundary of the hole, and the application of the confining pressure restricts the tensile cracks originating at the roof and floor of the hole to some extent. The hole-flaw interaction changes stress distribution and cracking behaviours. Cracks initiating from intersected flaw tips with a high inclination angle coalesce with the hole, while parallel independent flaws block the propagation of the cracks initiating from the hole boundary. Besides, it is proved that the integrated analysis framework, coupled with acoustic emission information interpretation, is feasible to accurately investigate fracturing characteristics in samples with complex pre-existing defects. The findings would benefit underground civil and resource-related projects.

Effect of pre-existing infilled fracture on characteristics of failure zones around circular opening

Understanding and predicting failure zones in fractured rocks around an underground opening is critical in designing excavation and support schemes. It is known that two symmetrical V-shaped failure zones of a circular opening occur in a homogeneous rock mass, but effect of infilled fractures on geometry of failure zone still remains poorly understood. In this study, biaxial compression tests were experimentally and numerically conducted on rock samples including an infilled fracture, and the effect of infilled fractures on the geometrical parameters (width, depth, and area) of failure zones around a circular hole were studied. Both the experimental and numerical results are in good agreement, and the results indicate that inclination angles and positions of infilled fractures can significantly affect the geometrical parameters of failure zones. Three characteristic stresses including crack initiation stress, crack damage stress and peak stress are defined and determined from the stress–strain and AE parameter-strain curves, and the ratios of crack initiation stress and crack damage stress over peak stress are comparable with the previous empirical formulars. In addition, stress distributions in the rock samples are compared to explain how infilled fractures affect geometrical parameters of failure zones.

Crack Evolution and Failure Mechanisms of Rock Specimens with Oblique Cylindrical Holes in Biaxial Compression Tests

Crack Evolution and Failure Mechanisms of Rock Specimens with Oblique Cylindrical Holes in Biaxial Compression Tests

Spalling failure and fracturing of brittle sandstone containing rectangular cavity based on biaxial compression test and discrete element method

Excavation of tunnel in hard brittle rock commonly induces disasters like spalling failure and rock burst, and numerous relevant theoretical, experimental and numerical studies are finished to detect the specific destruction mechanism. However, only a small number of researches focused on the excavation of underground mining stope, and the largest feature is the rectangular cavity and the gradient height of the cavity. Therefore, to mimic the failure procedure of the surrounding rock of the mining stope, six groups of rock samples containing various cavity heights are applied in the biaxial compression test (two types of confining pressures), and the PFC method is utilized to explain the failure mechanism in microscopic scale. The main results and conclusions are as follows: (1) The confining pressure directly affects the time relationship between the visible spalling failure and the typical AE burst incident. (2) As the cavity height increases, the spalling failure cannot cover the whole sidewall at once and transforms into localized destruction, which advances the corresponding initial appearance time. (3) The striped tensile failures distribute in the roof and bottom surrounding rock of rectangular cavity; tensile failures cover the whole spalling regions in sidewalls, while the shear failures are mainly concentrated in the lower part of the spalling areas. (4) When the cavity height is relatively large, the spalling failure in both sidewalls exhibits asymmetric distribution during the majority period of the test, and the final failure surface only shows an approximate symmetrical distribution.

Influence of principal stress orientation on stress distribution and plastic zone evolution of rock surrounding tunnels

During the excavation of tunnels, the principal stress orientation changes, with a significant impact on the stress distribution, mechanical properties, and plastic zone evolution of rocks surrounding tunnels, causing severe deformation control, and monitoring problems in the stability of tunnels. Currently, biaxial compression tests were conducted to explore the influence of principal stress orientation on mechanical properties of rocks surrounding tunnels. The analytical solution of stress and the model of plastic zone of rocks considering the principal stress orientation and the distance from the excavation boundary were established to reveal the failure mechanism of surrounding rock under different principal stress orientations. With an increase in the angle between the principal stress orientation and the long axis of tunnel, the maximum tangential stress around the tunnel gradually changed to the minimum horizontal principal stress direction, and its value gradually increased, leading to quicker failure of surrounding rocks, and reducing the strength enhancement effect of the same section size of the tunnel. However, the increase in the angle reduced the damage range and the range of the plastic zone around the tunnel and caused the plastic zone to gradually approach the bottom and roof. The maximum depth of the plastic zone remained parallel to or nearly parallel to the minimum horizontal principal stress direction. When the principal stress orientation was kept constant, the maximum depth of the plastic zone shifted to the minimum horizontal principal stress direction with an increase in the vertical principal stress.

Mechanical properties and risk characterization of surrounding rocks containing various blocks of arch-shaped holes under biaxial compression

In most rock engineering, the rock mass was cut into different blocks by natural structural planes, and the stability of underground openings was affected by the type, number, position and mechanical properties of rock blocks. In this study, the biaxial compression tests were conducted using rock-like material specimens with prefabricated arch-shaped holes and different types of blocks, and the failure and crack propagation properties of surrounding rocks were studied. Firstly, the location and danger degree of blocks in rock mass were classified, and the rock block danger analysis (BDA) method was proposed. Secondly, the support effect on the stability of surrounding rocks with different blocks was studied. Then, the deformation, strength, crack area ratio, and debris distribution characteristics of specimens were analyzed. Finally, the reasonable support schemes were proposed for surrounding rocks with different types and sizes of blocks. The test results showed that the σ-ε curve of holed rocks was similar to that of intact rocks, and the micro-cracks evolved into macro-cracks at crack destabilization expansion stage, followed by large deformation of whole specimens and spalling. Under support condition, as the block size increased, the crack area ratio decreased and the stability of holed specimens improved. Furthermore, the biaxial compressive strength of the surrounding rock and the stability of the holed specimens decreased as the block danger coefficients increased.

Heterogeneities of grain boundary contact for simulation of laboratory-scale mechanical behavior of granitic rocks

From a practical point of view, grain structure heterogeneities are key parameters that control the rock response and still remains a challenge to incorporate in a quantitative manner. One of the less discussed topics in the context of the grain-based model (GBM) in the particle flow code (PFC) is the contact heterogeneities and the appropriate contact model to mimic the grain boundary behavior. Generally, the smooth joint (SJ) model and linear parallel bond (LPB) model are used to simulate the grain boundary behavior. However, the literature does not document the suitability of different models for specific problems. Another challenge in implementing GBM in PFC is that only a single bonding parameter is used at the grain boundaries. The aim of this study is to investigate the responses of a laboratory-scale specimen with SJ and LPB models, considering grain boundary heterogeneous and homogeneous contact parameters. Uniaxial and biaxial compression tests are performed to calibrate the response of Creighton granite. The stress–strain curves, volumetric dilation, inter-crack (crack in the grain boundary), and intra-crack (crack within the grain) development, and failure patterns associated with different contact models are examined. It was found that both the SJ and LPB models can reproduce the pre-peak behavior observed for a granitic rock type. However, the LPB model is unable to reproduce the post-peak behavior. Due to the large interlocking effect originating from the balls in contact and the ball size in the LPB model, local dilation is induced at the grain boundaries. This overestimates the volumetric dilation and residual shear strength. The LPB model tends to result in discontinuous inter-cracks and stress localization in the rock specimen, resulting in fine fragments at the rock surface during failure.

Investigation of particle-scale mechanical behavior of hydrate-bearing sands using DEM: Focus on hydrate habits

Natural gas hydrates are commonly found in onshore permafrost and seabed regions. Hydrates often exhibit different pore habits in different regions, which may affect the mechanical behavior of hydrate-bearing sands (HBSs). In this study, pore-filling, load-bearing and cementing HBSs were prepared using discrete element method (DEM). The numerical biaxial compression tests show that the digital cementing HBS specimen well capture the stress-strain response of the gas-saturated HBS specimen in experiments under σc = 3 MPa, which justify the DEM model in this study. The pore-filling hydrates contribute little to the stress-strain response of sediments, while the load-bearing and cementing hydrates significantly enhance the failure strength and stiffness, and also induce strain-softening behavior and higher volume dilation. Hydrate cementation hinders the lateral fabric development in HBS, and renders orderly particle displacement and abundant strong force chains. The yielding of HBS is dominated by the buckling of strong force chains. Particle sliding, dislocation and rotation are responsible for the formation and development of shear bands, which governs the volume dilation and non-uniform deformation of HBS. Comprehending the geomechanical behavior of HBS under different hydrate habits facilitates the optimization of engineering solutions for hydrate exploitation.

Failure behaviors of square-tunnel-structured rock specimens under various inclined boundary stress conditions

In most engineering practices, the directions of geo-stress are not always perpendicular or parallel to the center line of underground tunnels’ cross section, and the surrounding rocks are subjected to inclined boundary stresses, which is often ignored in most previous studies. The investigations on the failure characteristics of surrounding rocks around tunnels under inclined boundary stresses have great practical significance. In this study, a series of uniaxial or biaxial compression tests were conducted using cubic red sandstone specimens with different prefabricated square holes. The horizontal centerline inclinations of square holes were designed as 0°, 15°, 30°, and 45°. The test results show that the horizontal centerline inclinations of holes have a great impact on the strength of rock specimens which tend to increase firstly and followed by decreasing. The confining pressure stress σ2 amplify the horizontal centerline inclination effect of holes on the strength of rock specimens. In addition, the horizontal centerline inclination of holes also affected the failure mode and crack propagation of surrounding rock significantly, the stress concentration zone at the right-angle boundary of the hole can induce the propagation of cracks. The findings of this study may provide theoretical guidance and reference significance for the stability evaluation, support parameter and disaster control of underground tunnels.