Biaxial Compression Tests Research Articles

Experimental investigation on biaxial mechanical properties of laminated rock and floor heave mechanism of tunnel based on DIC

Floor heave at the bottom of a tunnel is a common defect during tunnel service. To study the deformation behavior and failure characteristics of the floor, laminated sandstone specimens with precracked U-shaped grooves were prepared, and a series of biaxial compression tests were carried out in the laboratory. Two typical floor structures of a tunnel (with and without inverts) and four lateral stress levels (σ2 = 0, 5, 10 and 15 MPa) were considered. The effects of lateral stress and inverted structure on the deformation and failure characteristics of the floor were investigated. The displacement, strain evolution and failure extent of the floor were analyzed based on the digital image correlation (DIC) technique. The testing results show that the stress concentration at the U-shaped grooves of the specimens can be effectively reduced by the inverted structure. The specimen with an invert can bear more axial deformation and axial stress than that without an invert. The V-shaped notch failure behavior occurred near the tunnel sidewalls for the specimen without an invert, and it occurred in the middle of the tunnel floor for the specimen with an invert. The failure intensity of the V-shaped notch was positively related to the lateral stresses. With the increasing of axial stress, the heave displacement at the tunnel floor gradually increased, while the settlement displacement of the sidewall showed a trend of first increasing and then decreasing. The tensile strain concentration area of the floor undergoes a process of increasing and then decreasing and finally transforms to compressive strain. The failure characteristics of the sandstone specimen observed in the laboratory tests are consistent with the phenomena in real tunnel engineering. In addition, the deformation and failure mechanism of the tunnel floor was analyzed. The key points for the treatment of tunnel floor heave are put forward based on the testing results.

Classification of Multiaxial Behaviour of Fine-Grained Concrete for the Calibration of a Microplane Plasticity Model

Fine-grained high-strength concrete has already been tested extensively regarding its uniaxial strength. However, there is a lack of research on the multiaxial performance. In this contribution, some biaxial tests are investigated in order to compare the multiaxial load-bearing behaviour of fine-grained concretes with that of high-strength concretes with normal aggregate from the literature. The comparison pertains to the general biaxial load-bearing behaviour of concrete, the applicability of already existing fracture criteria and the extrapolation for the numerical investigation. This provides an insight into the applicability of existing data for the material characterisation of this fine-grained concrete and, in particular, to compensate for the lack of investigations on fine-grained concretes in general. It is shown, that the calibration of material models for fine-grained concretes based on literature results or normal-grained concrete with similar strength capacity is possible, as long as the uniaxial strength values and the modulus of elasticity are known. For the numerical simulation, a Microplane Drucker–Prager cap plasticity model is introduced and fitted in the first step to the biaxial compression tests. The model parameters are set into relation with the macroscopic quantities, gained from the observable behaviour of the concrete under uniaxial and biaxial compressive loading. It is shown that the model is able to capture the yielding and hardening effects of fine-grained high-strength concrete in different directions.

Effects of particle angularity on the bulk-characteristics of granular assemblies under plane strain condition

We investigate the influence of particle angularity on the stress–strain behaviour, rotation and micromechanics of granular assemblies under plane strain condition. Granular particles are modelled as regular polygons and numerical biaxial compression tests have been performed using the discrete element method. The packings consist of regular polygons with increasing corner number ranging from 5 to 64. The packings are analysed in the critical state in terms of their shear strength, bulk friction angle and porosity, as well as their cumulative rotation and contact characteristics with respect to varying particle angularity. We find that packings of more angular particles are looser but have higher shear strength, while packings of more rounded particles are denser, but are less resistant to shear. We further show that packings of angular particles are more resistant to reorganization by rotation, and that the mean rotation magnitude saturates at a constant value with increasing roundness. Lastly, we find that packings of more angular particles tend to have less contacts with higher normal and tangential forces when compared with packings of more rounded particles.

An efficient material point method framework based on the affine matrix

The material point method (MPM) has proven to be an effective numerical approach for simulating extreme events involving large deformations. Although numerous MPM variants have been proposed to enhance the capability of MPM, its application is currently limited by issues of computational efficiency and stability. This study presents an innovative affine matrix-based MPM scheme (AM-MPM) that can substantially promote the computational efficiency of MPM while maintaining exceptional stability. The proposed scheme is featured by its use of the affine matrix as a first-order approximation of the velocity gradient in the subsequent step and the direct computation of associated strain/stress for particles. This approach reduces the information interpolation between particles and grid nodes from two times to a single time, significantly leveraging computational efficiency. The proposed MPM scheme is firstly validated by the quasi-static compaction under self-weight and then applied to various classical problems involving large deformations, including biaxial compression test, column collapse, the impact of two rings, and the twisting of a 3D elastic column. The performance of the proposed scheme in terms of accuracy and efficiency is also discussed in detail.

Effects of hydrate distribution heterogeneity on the mechanical properties of hydrate-bearing sediments with consideration of the coalescence phenomenon: Insights from DEM simulations

The heterogeneous distribution of hydrates in natural or synthetic hydrate-bearing sediments (HBSs) has definitively proven its influences on hydrate dissociation and related mechanical responses, which can alter hydrate production strategies. However, due to the limitations of experimental conditions, applicable hydrate heterogeneity characterization and corresponding evaluation of the mechanical properties of HBSs remain lacking. In this study, discrete element modelling of numerical HBSs with different hydrate heterogeneous distributions considering the coalescence phenomenon is performed. The hydrate heterogeneity degree is quantified through image recognition, and the constructed HBSs are subjected to biaxial compression tests. The results show that for two HBSs with the same hydrate saturation above 10 % but highly different hydrate heterogeneity degrees exceeding 0.1, the peak strength, secant modulus and maximum dilation angle of the strongly heterogenous HBSs are lower than those of the weakly heterogenous HBSs. In addition, the weakly heterogeneous HBSs exhibit a more uniform contact force chain structure, more hydrates participate in loading support, and more particle rearrangements, which are the meso-mechanisms for the above macro-mechanical behaviors. Combined with the consideration of the hydrate morphology and cementing ratio, the mechanical properties of HBSs fluctuate due to the randomness of hydrate coalescence but generally show a decreasing trend with increasing hydrate heterogeneity degree. These findings provide an important supplement for understanding the macro- and meso-mechanical properties of HBSs with different hydrate heterogeneity degrees, which indicates that we should precisely describe hydrate coalescence in laboratory experiments and numerical simulations and consider its representativeness in field applications.

Comparison of local and nonlocal regularization approaches in Eulerian-based finite element analyses

Finite element (FE) results might suffer from significant mesh dependency, especially when modeling shear bands in strain-softening soils. On the other hand, the shear bands in the field are generally thin. The computational costs might dramatically increase when modeling such thin shear bands, especially for large-scale problems, such as progressive landslides. To overcome these issues, FE analyses are generally performed with larger shear band thicknesses by adopting ‘softening scaling’ rules based on local and nonlocal (averaged) strains at the shear bands. The present study compares the performances of local and nonlocal approaches, with a specific focus on softening scaling and large deformation. Analyses are performed using a Eulerian-based FE program, which can model large strains in the shear band without numerical issues related to mesh distortion. Implementing strain-softening behavior in local and nonlocal methods, two idealized cases are simulated: (i) biaxial compression test, (ii) slope failure due to upslope surface loading. FE simulations show that the macroscopic response (i.e., load–displacement behavior) can be modeled using both local and nonlocal regularization techniques. Shear band thickness increases with the progress of shearing. In local analysis, mesh orientation has a considerable effect on shear band thickness. The existence of neighboring shear bands could affect nonlocal strain calculation and the simulation results.

Experimental micro–macromechanics: particle shape effect on the biaxial shear response of particulate systems

The effect of particle shape on micro and macroscopic behaviour is investigated experimentally through a series of biaxial compression tests on round and angular rods. Dual-size assemblies of thousands of circular (round) or hexagonal (angular) particles were sheared under three confining pressures. The displacement, rotation and evolution of coordination number of all the particles during the shearing process were identified through image analysis. The macroscopic results show that the angular assemblies exhibit higher strengths and dilations. Angular particles tend to be more dilative and have higher critical state strengths. The microscale features show that the magnitude of particles' rotation is higher in circular assemblies. Particle rotations are normally distributed within both samples. However, rotating clusters are observed in angular samples. Angular particles are more resistant to rotations due to interlocking and clustering, causing higher strength at the macroscale. The absolute mean cumulative rotation for the circular assemblies at the end of shearing was approximately twice that of the angular assemblies. In circular assemblies, small particles rotate almost twice as much as big particles, which initiates a ball-bearing effect that further contributes to strength reduction. Particle rotation further governs the average coordination number, with hexagonal aggregates having higher coordination numbers.

Influence of the loading rate on the evolution characteristics of AE and MS signals during granite failure

To explore precursor rock failure information near a tunnel excavation boundary under different excavation rates, biaxial compression tests of granite failure under different loading rates were conducted, and the acoustic emission (AE) and microseismic (MS) signals produced by granite failure were monitored and analyzed. The results indicate that loading rate has an important effect on the evolution of AE and MS signal characteristic parameters. Hence, a multi-parameter rock failure warning method based on AE and MS according to different loading rates was proposed, which provides the scientific basis for safety warning of deep engineering. A damage evolution equation based on AE and MS is established to quantitatively describe the damage process of granite under different loading rates. Moreover, it is confirmed that there are vast variations in the sensitivity of AE and MS signals to the formation of cracks of different sizes. Therefore, combining AE and MS signals to monitor the granite failure process will help to reveal the mechanism of granite failure and improve the accuracy of granite failure prediction.

Numerical study on the nonlinear thermomechanical behaviour of refractory masonry with dry joints

Refractory masonry with dry joints is widely employed as a protective lining in industrial applications requiring high-temperature treatments. The thermal and mechanical behaviour of alumina spinel refractory masonry is investigated for a wide range of mechanical loading conditions at ambient and high temperature up to 1500 °C within the framework of the ATHOR project. This paper discusses the different numerical analysis approaches for the simulation of the experimental results. Micro and macro modelling approaches show good agreement with the large scale uniaxial and biaxial compression tests for loading and unloading at the ambient temperature. Simulations carried out for large scale uniaxial and biaxial creep tests as well as biaxial relaxation tests at 1500 °C show good agreement. The numerical results indicate the ability of these modelling approaches to represent the complex thermomechanical behaviour of the refractory masonry. Both methods demonstrate an orthotropic and highly nonlinear behaviour of the refractory masonry as observed in the experimental campaign. The numerical outcome, validated with experimental results demonstrate compatibility between micro and macro modelling approach that can be employed to evaluate local and global behaviour of large industrial installations.

Dynamic responses of radiation-induced heavyweight concrete subjected to biaxial compression

As a good shielding material, heavyweight concrete is largely utilized for building the Concrete Biological Shielding (CBS) wall of the commercial nuclear power plant, and is often subjected to the biaxial loadings with a strain rate possibly varying from 10−5s−1 to 10−2s−1. This paper assesses the combined effects of the lateral stress and the strain rate on the compressive strength of radiation-induced heavyweight concrete on the example of serpentine concrete. For this goal, a 3-dimensional rectangular numerical concrete specimen containing special aggregates and ITZs was firstly established. Exposed to neutrons of different fluences, the initial damages caused by the fast neutron were assessed by a thermal-mechanical approach. Then, a series of static and dynamic biaxial compressive tests on the rectangular specimens having initial damages of different degrees were carried out by finite element analysis via ABAQUS. The failure modes, the crack patterns, and the ultimate compressive strengths of the pre-irradiated serpentine concrete were obtained numerically. Finally, based on the numerical results and the classical "Kupfer-Gerstle" ("K-G") criterion, this paper derives the biaxial strength criterion of the radiation-induced heavyweight concrete undergoing both lateral stress and strain rate. Through the formula, it was found that the fast neutron fluence of high level reduces significantly the biaxial compressive strength of the heavyweight concrete. Besides, if the heavyweight concrete undergoes additionally lateral stress and the strain rate, the biaxial compressive strength could be enhanced. The enhancement effect was maximized at the lateral stress ratio of 0.5 and the strain rate of 10−2s−1. The current numerical approach was developed with a potential application to the lifetime extension of the heavyweight concrete infrastructures used in commercial nuclear reactors.

Modeling pipe-soil interaction under vertical downward relative offset using B-spline material point method

To analyze the pipeline response under permanent ground deformation, the evolution of resistance acting on the pipe during the vertical downward offset is an essential ingredient. However, the efficient simulation of pipe penetration into soil is challenging for the conventional finite element (FE) method due to the large deformation of the surrounding soils. In this study, the B-spline material point method (MPM) is employed to investigate the pipe-soil interaction during the downward movement of rigid pipes buried in medium and dense sand. To describe the density- and stress-dependent behaviors of sand, the J2-deformation type model with state-dependent dilatancy is adopted. The effectiveness of the model is demonstrated by element tests and biaxial compression tests. Afterwards, the pipe penetration process is simulated, and the numerical outcomes are compared with the physical model tests. The effects of pipe size and burial depth are investigated with an emphasis on the mobilization of the soil resistance and the failure mechanisms. The simulation results indicate that the bearing capacity formulas given in the guidelines can provide essentially reasonable estimates for the ultimate force acting on buried pipes, and the recommended value of yield displacement may be underestimated to a certain extent.

An efficient phase-field model of shear fractures using deviatoric stress split

We propose a phase-field model of shear fractures using the deviatoric stress decomposition. This choice allows us to use general three-dimensional Mohr–Coulomb’s failure function for formulating the relations and evaluating peak and residual stresses. We apply the model to a few benchmark problems of shear fracture and strain localization and report remarkable performance. Our model is able to capture conjugate failure modes under biaxial compression test and for the slope stability problem, a challenging task for most models of geomechanics.

Effects of un-parallel notches on the granite fracture properties and acoustic emission phenomena under biaxial test: PFC computation and experimental verification

To assess connections between these notches throughout fracture formation under stress, the mechanical performance of granite specimens with two non-parallel notches underwent a biaxial compression test. The first notch had a 45° angle, whereas the second had a variable angle (0°, 45°, 90°, 130°, and 180°). Three ligament angles (0°, 45°, and 90°) were measured between the inner tips of two notches and the horizontal axis. The ligament length, distance between the inner tips of the two notches, and both notch lengths were each 2 cm. These samples were examined at 0, 1, and 3 MPa confining pressures. By using the particle flow code (PFC), simulated results of the evaluations were run after the trials. It was discovered that the specimens’ failure strengths were influenced by the notch’s geometric characteristics, which in turn impacted the failure mechanism as well as fracture geometry. The number of tensile fractures had an impact on the specimens’ failure strengths. Whilst also increasing the ligament angle, more induced tensile cracks were produced. In the early stages of loading, just a few AE events were found, but after that, they multiplied quickly before approaching the peak stress. By raising the confining pressure, the AE hits were increased and delay failure was occured. The failure strength and AE hits are significantly impacted by the second notch’s angle. In constant ligament angel, the compressive strength of models containing overlapped joint were more than that with non-overlapped joints. In models containing non-overlapped notches, the model with ligament angles of 0° and 45° had maximum and minimum compressive strength, respectively. In models containing overlapped notches, the model with ligament angles of 90° and 45° had maximum compressive strength and minimum compressive strength, respectively. In all ligament angles, minimum compressive strength was occurred when the upper joint angle was 45°. Both the computational and experimental methodologies showed that the specimens’ failure pattern and compressive strength were very comparable.

Fracture evolution and fracture mechanism of tunnel surrounding rock: A case study based on laboratory tests and theoretical analysis

AbstractTo explore the fracture characteristics and fracture mechanism of the surrounding rock of straight‐walled arched tunnels, this study first carried out biaxial compression tests on rock samples and analyzed the disaster evolution characteristics under loading. Then, a stress distribution function was established using a complex function, and the distribution characteristics of the surrounding rock stress field were determined. Finally, combining the stress field with results obtained by computational tomography and acoustic emission measurements, the dynamic evolution of the main fracture mechanism of the tunnel surrounding rock was comprehensively analyzed, and the fracture mechanism of the surrounding rock of the straight‐walled arched tunnel was elucidated. The research results strengthen the understanding of rock fracture evolution during tunnel construction and operation and provide a basis for tunnel construction and surrounding rock stability analysis.

Discrete Element Modeling of the Effect of Hydrate Distribution Heterogeneity on the Mechanical Behavior of Cemented Hydrate-Bearing Sediments

Hydrate distribution heterogeneity is often observed in natural and artificial hydrate-bearing sediments (HBSs). To capture hydrate distribution heterogeneity, a pore-scale method is proposed to model cemented HBSs using the discrete element method (DEM). This method takes into account the quantitative effect of hydrate saturation in a sediment pore on the contact bond parameters surrounding the pore. A series of DEM specimens with different macroscopically and microscopically heterogeneous hydrate distributions are prepared. The mechanical behavior of heterogeneous HBSs is investigated by performing biaxial compression tests with flexible boundaries. The simulation results show that both macroscopic and microscopic hydrate distribution heterogeneity can influence the mechanical properties of HBSs. The shear strength is promoted in both macroscopically and microscopically heterogeneous HBSs. Longitudinally heterogeneous HBSs have a higher secant modulus, while transversely heterogeneous HBSs have a lower secant modulus than homogeneous HBSs. The secant modulus of microscopically heterogeneous HBSs first increases and then decreases with increasing pore hydrate saturation. It is found that the deformation behavior and bond breakage evolution of HBSs depend on hydrate distribution heterogeneity. These findings can provide insights into understanding the mechanical behavior of natural HBSs with heterogeneous hydrate distributions.

Implications for identification of principal stress directions from acoustic emission characteristics of granite under biaxial compression experiments

The rock fracture characteristics and principal stress directions are crucial for prevention of geological disasters. In this study, we carried out biaxial compression tests on cubic granite samples of 100 mm in side length with different intermediate principal stress gradients in combination with acoustic emission (AE) technique. Results show that the fracture characteristics of granite samples change from ‘sudden and aggregated’ to ‘continuous and dispersed’ with the increase of the intermediate principal stress. The effect of increasing intermediate principal stress on AE amplitude is not significant, but it increases the proportions of high-frequency AE signals and shear cracks, which in turn increases the possibility of unstable rock failure. The difference of stress in different directions causes the anisotropy of rock fracture and thus leads to the obvious anisotropic characteristics of wave velocity variations. The anisotropy of wave velocity variations with stress difference is probable to identify the principal stress directions. The AE characteristics and the anisotropy of wave velocity variations of granite under two-dimensional stress are not only beneficial complements for rock fracture characteristic and principal stress direction identification, but also can provide a new analysis method for stability monitoring in practical rock engineering.

Progressive Failure Characteristics and Failure Symptoms of Straight-Walled Arched Sandstone Tunnels

In order to investigate the development process of crack formation in shallow-buried sandstone tunnel, biaxial compression tests were conducted on a similar model of the real straight-walled arched sandstone tunnel. The results indicate that the initial crack appeared at the arch line on both sides of the tunnel and propagated downwards, eventually leading to spalling of the rock mass on the surface of the tunnel, forming a V-shaped groove. Additionally, slab cracks were observed in the straight wall on the right side of the tunnel, which were approximately parallel to the vertical load. The failure characteristics of the tunnel were closely related to the fractal dimension of the crack geometry distribution. During the tunnel compaction and elastic deformation stage, the fractal dimension of the cracks in the tunnel surface increased linearly, while during the crack propagation stage, the fractal dimension increased gradually, with a sudden increase occurring just before the rock mass reached its peak load. The acoustic emission results revealed that AE ringing counts and amplitude were inactive during the first 4239 seconds of the test. And they only increased during the crack propagation stage. The continuous decrease of the b-value and the sudden increase of the fractal dimension of cracks can serve as a reliable precursor of tunnel failure.

Nonlocal strain regularisation for critical state models with volumetric hardening

The finite element (FE) method may suffer from numerical instability and mesh dependency when modelling the formation of shear bands using strain-softening constitutive models. Nonlocal methods have been shown to be capable of avoiding these numerical issues effectively. This paper proposes a novel generic algorithm for incorporating existing nonlocal methods into the FE formulation of elasto-plastic constitutive models with volumetric hardening laws. Using the example of the modified Cam Clay (MCC) model this paper shows that instabilities are inevitable if the nonlocal plastic volumetric strains are directly used to update the hardening parameter. The paper further proposes a robust regularisation algorithm to overcome this issue. This algorithm has been implemented into a bespoke FE code and its capability in significantly reducing mesh dependency while maintaining numerical stability is verified through a series of numerical analyses of biaxial compression tests for highly overconsolidated clays. The performance of various existing nonlocal methods is also critically assessed.

Crack Initiation and Propagation of Embedded Three-Dimensional Parallel Cracks in Transparent Rock-Like Material

Abstract In order to understand the initiation and propagation of three-dimensional (3D) embedded parallel cracks in brittle rock under uniaxial and biaxial compression, a preparation method of a transparent rock-like material was proposed in order to observe the propagation process of cracks in brittle rock. Uniaxial and biaxial compression tests were carried out on the transparent rock-like samples containing two embedded 3D parallel cracks. Four types of crack propagation modes were observed: wing crack, petaloid crack, flat-wing crack, and ring-like crack. It is also found that the direction of lateral pressure relative to the strike of the embedded crack has a significant effect on the propagation direction of new generated cracks: a lateral pressure parallel to the strike of the embedded cracks promotes crack propagation in the direction of the maximum principal stress; on the other hand, a lateral pressure normal to the strike of the embedded cracks promotes cracks propagation around the embedded crack at an angle of 30°–60°, forming ring-like cracks, which is similar to the zonal disintegration phenomenon monitored around deep underground excavations.

Experimental Study on the Mechanical Behavior of Sandy Soil Reinforced by Disposable Face Mask Chips under Different Stress Paths

Since 2020, with the global spread of major respiratory infectious diseases, such as COVID–19, the demand and consumption of personal protective equipment, such as masks, have increased dramatically worldwide. The environmental pollution caused by numerous waste disposable face masks has gradually attracted people’s attention. In this study, the mechanical properties of mask–chip–reinforced soil are evaluated from a new perspective, through the uniaxial, biaxial, conventional triaxial, and true triaxial compression tests on reshaped sandy soil samples mixed with different contents of mask chips. The experimental results show that the mechanical properties of the sandy soil can be improved by the mask chips. With the proper content of mask chips, the failure strength is substantially improved, and the failure of soil is delayed. Meanwhile, the strength and stiffness are significantly affected by the stress path and the content of mask chips, even if the soil samples with the same mask–chip content can also show different mechanical properties under different stress paths. Additionally, the mechanical properties of soil are not necessarily improved constantly with the increasing content of mask chips. The failure strength of sandy soil samples under conventional and true triaxial stress paths decreases when the mass content of mask chips exceeds 0.3% and 0.5%, respectively. This study confirms the potential of mask chips applied to subgrade, slope, and other engineering construction fields in a sustainable way.