Load Triaxial Tests Research Articles

A dynamic strength criterion for frozen sulfate saline silty clay under cyclic loading

The strength properties of frozen saline soils under cyclic loading are more complex than those of unfrozen soils and frozen salt-free soils due to the existence of salt crystals in soil matrix. In this paper, a serious of tri-axial cyclic loading tests are conducted for frozen sulfate saline silty clay (FSSSC) with different Na2SO4 contents under different confining pressures and loading dynamic stress ratios at the temperature of −6 °C, respectively. The experimental results illustrate that the dynamic strength of FSSSC decreases with the increase of axial strain. The critical state line of FSSSC under cyclic loading is moving constantly. The dynamic critical state lines of FSSSC with different salt contents are different under same axial strains. The dynamic friction angle of FSSSC is related to salt content, loading dynamic stress ratio and confining pressure. Based on the experimental results, a dynamic strength criterion for FSSSC under cyclic loading is proposed, and the method of determining dynamic strength parameters is given. This strength criterion not only can reflect the effect of loading dynamic stress ratio and salt content on dynamic strength, but also can describe the influence of high confining pressure on dynamic strength.

Dynamic and damage properties of artificial jointed rock samples subjected to cyclic triaxial loading at various frequencies

Jointed rock samples with dip angles of 15°, 30°, 45°, and 60° (0° corresponds to the horizontal axis) are manufactured artificially, and a series of cyclic triaxial loading tests are performed at different frequencies (0.1, 1.0, 3.0, and 0.5 Hz) to assess the effects of the frequency on the dynamic characteristics, number of cycles until failure, irreversible strain, dissipated energy density, and failure mechanisms. Analysis of the test results and the phenomenon of cyclic loading reveals that frequency has a significant effect on the dynamic characteristics of the jointed rock samples. With the increasing frequency, the number of cycles until failure increases, the maximum value of the contractive volumetric strain increases, and the volume expansion significantly decreases. A higher frequency corresponds to a higher irreversible axial strain, but the irreversible volumetric strain exhibits two different trends, depending on the dip angle. Further, a higher frequency corresponds to a higher initial average axial stiffness and a higher dissipated energy density. Two new damage variables are defined in this study (the density method and residual strain method), which can describe the three stages of fatigue damage in jointed rock samples subjected to cyclic loading (initial damage, damage accumulation, and accelerated stage).

Experimental simulation study of spalling in deep rectangular tunnel with plastic fine grain marble

Following the excavation of the rectangular tunnel faces with deep harder rock, spalling often occurs on both sidewalls of the tunnel, which affects the tunnel support and stability. To investigate the spalling phenomenon of the harder rock under the deep initial three-dimensional (3D) stress conditions, true triaxial loading tests of different depths (250, 500, 750, and 1000 m) and different stress combinations (four different horizontal stresses at the same depth 1000 m) were designed on plastic fine grain marble. The overall failure process of the surrounding rock was monitored and recorded by a wireless micro-camera. The results show that, under the shallow stress condition (250 and 500 m), the V-shaped bands will appear on the both sidewall due to the overall shear failure of the specimen. Under the deep stress condition (750 and 1000 m), the obvious spalling was observed on the hole sidewalls of all marble specimens. Moreover, at the time of hole sidewall spalling, the vertical stress is the maximum principal stress. The spalling process of the hole sidewall can be divided into four stages: the microcrack initiation, the appearance of thin rock slabs, the formation of arc-shaped notches, and the expansion to V-shaped notches. If the excavation depth remains the same (1000 m) when the maximum principal stress is perpendicular to the tunnel axis, excavation along the minimum principal stress direction is beneficial to reducing the spalling damage degree. Compared to the brittle granite specimens with rectangular holes under the same 3D stress conditions, the influence of brittleness and plasticity of rock materials on spalling failure was analysed. (1) Spalling failure mode of hole sidewalls: the plastic-marble is static failure without fine particle ejection, while the brittle granite will break dynamically with fine particle ejection. (2) Spalling failure process and zones of sidewall: the cracks on the plastic marble sidewalls develop parallel to the axial direction, and run through the whole hole sidewall along the axis, while the cracks on the brittle granite hole sidewalls propagate perpendicular to the axial direction, and the sidewalls first break locally and then break through the whole hole. Failure zones on both sidewalls of the two kinds of rock specimens will transit from arc-shaped notch to V-shape notch. Those failure notches are symmetrical and finally penetrate the sidewalls. (3) Microscopic failure mode: scanning electron microscope (SEM) shows that the plastic and brittle rock materials are tensile failure during the spalling under 3D stress conditions. The plastic marble is characterized by fracture tension failure, while the brittle granite is characterized by layered tension failure. Some suggestions are given finally, under the condition that the vertical stress is the maximum principal stress, when the tunnel is excavated along the horizontal direction, the resistant ability of the tunnel sidewall in the plastic and brittle surrounding rock should be strengthened by supporting the sidewall, which can effectively reduce the degree of spalling damage.

Fatigue and damage properties of non-consecutive jointed mudstone samples subjected to cyclic triaxial loading

Non-consecutive jointed mudstone samples with dip angles (0° is the horizontal axis) of 15°, 30°, and 45° were subjected to a series of cyclic triaxial loading tests at various frequencies, with the aim of evaluating the fatigue and damage properties of jointed mudstone samples, including the ultimate cycle number, maximum axial strain, damage evolution, and failure behavior. The results from the tests indicate the following: (i) the higher the frequency, the greater the maximum axial strain difference and the higher the value of Eavl (average Young’s modulus during loading) and Eavu (average Young’s modulus during unloading), and (ii) compared with triaxial compressive loading, additional cracks are found in the samples subjected to cyclic loading, and the propagation of cracks initiates from the joint tip toward the two ends of the sample. Meanwhile, under the condition of a lower frequency, additional cracks occur after failure. Based on the experimental data, a new indicator (RS), considering the effect of the amplitude, peak deviatoric stress, and valley deviatoric stress, was proposed to indicate the stress level. At the same RS and dip angle, there is a positive correlation between frequency and lgNf (Nf is the number of cycles before the failure of the rock samples, and lgNf is the logarithmic value of Nf). Furthermore, a newly proposed damage variable based on shear strain is used to describe the damage evolution of jointed mudstone subjected to cyclic triaxial loading.

Combined Effect of Compaction Methods and Loading Conditions on the Deformation Behaviour of Unbound Granular Material

Unbound granular material is an important construction material that is used for flexible pavement. As one of the most important indices used for describing the properties of unbound granular material, the deformation characteristic is significantly influenced by the compaction method used. However, the effects of the compaction method on the deformation behaviour are neglected in the existing studies. Hence, we investigate the deformation characteristics of unbound granular material produced by the vertical vibration compaction method (VVCM) and the modified Proctor compaction method (MPCM) via repeated triaxial load tests and reveal the effect of loading conditions and aggregate gradations. The results show that (a) the deformation of unbound granular material decreases as the stress level and confining pressure are increased; (b) the deformation resistance of unbound granular material produced by the VVCM is superior to that produced by the MPCM; (c) the difference of deformation resistance arising between using the VVCM and MPCM decreases as confining pressure increases and is not significant with changes in the stress level; (d) the extent of various factors on deformation characteristics from greatest to least is as follows: stress level > aggregate gradation > compaction method > confining pressure; and (e) increasing the content of coarse aggregates is conducive to enhancing the deformation resistance of unbound granular material, particularly for the VVCM. Finally, a simple approach for modelling and predicting deformation is established.

Experimental Study of Rock Creep under True Triaxial Loading

The aim of this sutdy is to research the time dependence of deformations under complex stress conditions arising in the ground formation during the exploitation of oil and gas fields. The experiments were carried out on the IP Mech triaxial independent loading test system on the rocks of the Prirazlomnoye oil field using loading programs simulating the stress state in the near-borehole region with a decrease in pressure in the well. Experimental dependences of deformations on time during step loading are presented. The basic requirements for constructing a model of a stress-strain state taking into account the influence of time effects are discussed.

Application of Taguchi Method for Optimization of Various Factors in Improving the Shear Modulus of Enzyme Treated Soil

In this study, the effect of dosage of enzyme (50 to 400 ml/m3), curing period (7 to 28 days) and cyclic triaxial loading test parameters such as confining pressure (50 to 200 kPa), loading frequency (0.25 to 1 Hz) and loading rate (0.3 to 0.9%) on the shear modulus of enzyme stabilized soil is investigated. Taguchi method has been applied to the cyclic triaxial tests, and L16 orthogonal array with five factors and four levels was selected. Analysis of variance was carried out to study the significant factors affecting the response. Among the selected factors considered for the study, curing period is found to be the most effective parameter on the shear modulus of the enzyme treated soil. With the increase in dosage of the stabilizer, there is a significant improvement in the shear modulus of the treated soil. The increase in shear strain amplitude and loading frequency decreases the shear modulus. The higher value of shear modulus was obtained for the specimen with dosage of enzyme 400 ml/m3 after a curing period of 28 days under a confining pressure of 200 kPa, loading frequency of 0.25 Hz and cyclic shear strain amplitude of 0.3%.

Dynamic mechanical behavior, damage mode and mechanism of multi-scale high energy insensitive particulate reinforced composites of a new type of anti-missile sandwich wall structure

Multi-scale high energy insensitive particle reinforced composites (MSHEIPRC) are the most important part of the anti-missile sandwich wall structure. Its dynamic mechanical behavior and damage mode will directly affect the success of anti-missile capability. In this paper, based on a modified Split Hopkinson Pressure Bar (SHPB) experimental technique and the design of triaxial impact loading test device, combining with theoretical study and meso analysis, the dynamic mechanical response under high strain rates and damage characteristics of MSHEIPRC under high overload were carried out. The results show that the dynamic stress-strain curves of MSHEIPRC are highly non-linear and have obvious strain rate effect and density effect. Compared with the experimental results, the modified constitutive model can accurately describe its dynamic mechanical behavior at high strain rates. Based on the energy model and related parameters, the bond strength between the surface of crystal particles and the binder is calculated to be about 0.6 MPa. Therefore, the shear debonding damage mode between the surface of crystal particles and the binder is first found at the stage of small impact loading. The critical fracture strength of cracks with different sizes is calculated by Griffith method. With the increase of impact loading pressure, the fracture phenomenon gradually appeared on the surface of crystal particles, which was basically consistent with the theory model. Continuing to increase the impact loading pressure, sufficient shock wave energy forces crystal particles to produce transgranular fracture, which leads to crystal particles breaking.

A constitutive model for rock based on energy dissipation and transformation principles

The study on constitutive relations is of great significance to the understanding of the complex mechanical properties and failure criteria of rock. The establishment of a scientific rock constitutive model can provide theoretical support for engineering safety and geological disaster prevention. From the view point of energy, a new rock constitutive model under triaxial compression was developed by connecting the law of energy transformation with the mechanical properties of rock in the paper. First, the concepts of elastic energy conversion rate and elastic energy conversion function were proposed to describe principles of energy dissipation and energy transformation in the whole stress-strain process of rock. And, a mathematical description was made of the work of external force transforming into elastic energy. Second, assumptions were proposed to describe the evolution laws of some important mechanical parameters. Finally, the quasi-static constitutive model for rock under triaxial compression based on conservation of energy was established, combining theories of damage mechanics and thermodynamic, and the fitted results of this constitutive model were compared to experimental data of sandstone under triaxial cyclic loading tests. The rationality of the constitutive model for rock was proved by describing the whole stress-strain process of triaxial compression under different confining pressures. And, the model can describe the initial compaction stage and residual stress of stress-strain curve, which can reflect the nonlinear characteristics of rock.

Experimental investigation of the mechanical properties of fine-grained sandstone in the triaxial cyclic loading test

Fine-grained sandstone is widely distributed in the Xiangjiaba Hydropower Station. To investigate the mechanical properties of the sandstone, a series of triaxial cyclic loading tests were conducted under the confining pressures of 5 MPa, 15 MPa, and 20 MPa. According to the mechanical characteristics of the sandstone, a typical stress–strain curve can be divided into five stages: initial compression stage, linear elastic stage, strain hardening stage, strain softening stage and residual strength stage. Initial yield strength, peak strength, and residual strength linearly increase with the increasing of confining pressure, respectively. The initial yield stresses $$ {\sigma_{\text{y}}} $$ are approximately 69.0–74.0% of the peak strengths; while the residual strengths $$ {\sigma_{\text{s}}} $$ are 39.5–43.5% of the peak strengths under various confining pressures. Based on the experimental results, the strength and deformation parameters are obtained and analyzed. The strength parameters are calculated according to the Mohr–Coulomb criterion. The cohesion increases at first and then decreases with the increasing of internal variable $$ \kappa $$ . The friction angle first gradually decreases, then it increases to the maximum and finally decreases to a stable value with the increasing of internal variable $$ \kappa $$ . The maximal variations of friction angle and cohesion are approximately 29.1% and 62.5%, respectively. Variations in elastic modulus decrease, respectively, 10.3%, 6.1%, and 6.8% pre-peak strength under different confining pressures, and the variations in Poisson’s ratio are, respectively, 71.1%, 36.2%, and 63.5%. The nonlinear mechanical behaviors of fine-grained sandstone are the result of the accumulation of irreversible deformation and damage evolution. Moreover, the hysteresis loop is obvious with the accumulation of irreversible deformation and damage evolution. Damage starts to evolve when the stress level is more than the damage threshold. The experimental investigation can enhance the knowledge of the nonlinear behaviors of rock material.

Study on damage evolution of intact and jointed marble subjected to cyclic true triaxial loading

Rock masses in underground engineering, such as mining and deep tunnelling engineering, are often repeatedly loaded and unloaded. Here, a series of true triaxial multi-level single cyclic loading tests (σ1 > σ2 > σ3) were carried out on intact and jointed marble (50 × 50 × 100 mm3) to investigate the influence of the stress state and the absence or presence of a joint on the progressive damage process. The test results show that rock damage is directional and anisotropic. The irreversible strains in the σ1 and σ3 directions change linearly with cumulative damage, whereas the irreversible strains in the σ2 direction decrease nonlinearly. The dissipated energy is appropriately linearly related to the equivalent irreversible strains. The damage variables based on the irreversible strains and dissipated energy are closely correlated, as determined by their similar fitted parameters. The stress state (σ2 and σ3), the absence or presence of a joint and the brittleness play a vital role in the rock damage evolution. More specifically, the parameter a has a logarithmic relationship with σ3 and a linear relationship with σ2. The relationship between the parameter a and the joint dip angle β can be fitted by a Gaussian distribution. In addition, the fitted parameter a decreases with increasing brittleness.

Effects of Compaction Moisture Content on Permanent Deformation of Soils Subjected to Repeated Triaxial Load Tests

Water content is one of the key parameters that can cause variations in the elastic and plastic deformation of pavement layers. More specifically, both the compaction moisture and the post-compaction moisture should be carefully controlled. The objective of this study is to analyze the effects of compaction moisture on the accumulation of permanent deformation when unbound materials are subjected to repeated triaxial loads. The moisture contents were defined according to the compaction curve of each soil, being one above and one below the optimum moisture content which has been typically considered for the design of asphalt pavements in Brazil. The four soils evaluated in the study are commonly used in flexible pavements and have different classifications according to the Brazilian Miniature, Compacted, Tropical (MCT) methodology tailored to classify tropical soils. Specimens were subjected to permanent deformation tests and the results obtained show that the moisture content can significantly affect the mechanical behavior of soils. That effect is significant on both sides of the compaction curve, but the permanent deformation increases as the compaction moisture content increases on the wet side. This work is part of a broader and comprehensive research project in progress in Brazil to continuously update the current version of the Brazilian mechanistic-empirical pavement design guide. The proper understanding and characterization of the resistance to permanent deformation of unbound materials is a key part of that design guide.

Study of the mechanical behaviour of reclaimed asphalt aggregates without binder addition

The objective of this paper is to investigate a source of reclaimed asphalt aggregates without binder addition. To this end, a series of triaxial tests was performed. First, monotonic triaxial tests allowed to determine failure resistance of the material at 20°C, 50°C and at 20°C with a prior preheating at 50°C. Then, the resilient and permanent behaviours at 20°C were studied thanks to Repeated Load Triaxial (RLT) tests with different levels of loading. The results show that increasing the temperature test implies a drop of failure resistance but a higher cohesion. RLT tests reveal a stress dependent resilient behaviour. Finally, the experimental results were compared with calibrations of the non-linear elastic Boyce model.

Innovative stabilization techniques for weak crushed rocks used in road unbound layers: A laboratory investigation

Abstract The “Ferry-free coastal highway route E39” project includes building several long tunnels along the southwestern Norwegian coast. The tunnelling operations will generate a large quantity of blasted rocks; these could be used in the road unbound layers close to the place of production to provide a sustainable cost-benefit application. The existing design guidelines define strength requirements for road unbound layers in terms of Los Angeles and micro-Deval tests. Even if the major part of the rocks has igneous origin and could potentially fulfil the standard tests, the damage induced by the confined heavy blasting makes the materials fail the check procedures. The research investigates how to enable the use of the “weak” rocks by investigating three possible techniques. The first approach is the mixture between the different types of rocks available in situ. The second approach is additive application; two different non-traditional additive types are examined: one is polymer-based, the other one is lignin-based. The third approach is the attempt to modify the rocks mineralogical structure by overheating. The research test campaign uses both the aforementioned standard tests and repeated triaxial load tests. Rocks mixture and additives application are viable and sustainable methods to improve the mechanical properties of the “weak” crushed metamorphic rocks. Overheating does not turn out to be an effective and convenient procedure.

Global topology of failure surfaces of metallic foams in principal-stress space and principal-strain space studied by numerical simulations

Failure surfaces of foam materials in the principal-stress/strain space are quite different from those of dense materials. They cannot be deduced directly by several simple tests such as uniaxial loading and pure shear loading. However, it is difficult to carry out arbitrary proportional triaxial loading tests on materials, resulting in the global topology of failure surfaces of metallic foams remaining unknown. To overcome the difficulty of triaxial loading tests, we carried out numerical simulations by applying triaxial loadings on 3D Voronoi models, which have been used successfully to study the global topology of yield surfaces of metallic foams. In the numerical simulations, 3D Voronoi structures were used to simulate the meso–structures of metallic foams, and only the parameters of the matrix material (aluminum) are needed. Considering the influence of multi-axial effects on the failure of metallic foam, a new failure criterion for metallic foams based on the mass of failed elements was proposed, and sufficient failure points in the principal-stress/strain space were obtained to characterize the failure surfaces well. Results indicated that the failure surface in the in the principal-stress space looks more complicated than the failure surface in the principal-strain space, so it is better to depict the failure surface in the principal-strain space, which approaches an ellipsoid. Furthermore, self-similarity exists in failure surfaces of metallic foams with different relative densities. Because a metallic foam may not fail when the compressive loadings are dominant, the failure surface appears to be a “missing area” in the permissible principal-stress/strain space. The boundaries of missing areas were determined analytically and validated numerically.

A simple permanent deformation model of rockfill materials

Existing experimental results have shown that using a semi-log linear relationship between the permanent volumetric strain and cyclic number underestimates the volumetric deformation of rockfill materials with a large cyclic number, and that the error increases with the confining pressure. The existing permanent deformation models are not suitable for the seismic safety analysis of high dams during strong earthquakes. In this study, a series of large-scale triaxial cyclic loading tests of rockfill materials were performed, and a new permanent deformation model of rockfill materials was developed and validated with three kinds of rockfill materials. The results show that the proposed model can properly reflect the general features of the permanent deformation of rockfill materials. The main features of the model are as follows: (1) relations between the cyclic number and permanent volumetric/shear strain are described by hyperbolic functions, which can avoid underestimating the volumetric deformation occurring during strong earthquakes; (2) the model can capture the effect of the mean effective stress on the permanent volumetric strain, with greater confining pressure correlating to greater permanent volumetric deformation, and the permanent volumetric strain under low confining pressure near the dam crest can be well represented; and (3) the model can reflect the effect of the consolidation stress ratio on the permanent shear strain.

Impact of seismicity on Nice slope stability—Ligurian Basin, SE France: a geotechnical revisit

The shallow Nice submarine slope is notorious for the 1979 tsunamigenic landslide that caused eight casualties and severe infrastructural damage. Many previous studies have tackled the question whether earthquake shaking would lead to slope failure and a repetition of the deadly scenario in the region. The answers are controversial. In this study, we assess for the first time the factor of safety using peak ground accelerations (PGAs) from synthetic accelerograms from a simulated offshore Mw 6.3 earthquake at a distance of 25 km from the slope. Based on cone penetration tests (CPTu) and multichannel seismic reflection data, a coarser grained sediment layer was identified. In an innovative geotechnical approach based on uniform cyclic and arbitrary triaxial loading tests, we show that the sandy silt on the Nice submarine slope will fail under certain ground motion conditions. The uniform cyclic triaxial tests indicate that liquefaction failure is likely to occur in Nice slope sediments in the case of a Mw 6.3 earthquake 25 km away. A potential future submarine landslide could have a slide volume (7.7 × 106 m3) similar to the 1979 event. Arbitrary loading tests reveal post-loading pore water pressure rise, which might explain post-earthquake slope failures observed in the field. This study shows that some of the earlier studies offshore Nice may have overestimated the slope stability because they underestimated potential PGAs on the shallow marine slope deposits.

Resilient moduli responses of polymeric-treated pavement foundation materials under repeated loading

Polyacrylamide (PAM) has been shown to enhance strength, stiffness and durability characteristics of granular materials, in addition to providing lower carbon footprints when compared to traditional stabilisers. In this study, the responses of granular subbase materials for flexible pavements, under simulated traffic loading, have been investigated using repeat load triaxial (RLT) testing. Three types of granular materials treated with PAM have been tested to assess changes in engineering performance and responses to simulated traffic. Untreated materials were also tested for comparison. The engineering performance of these materials (treated and untreated) in terms of strength and stiffness were assessed by conducting unconfined compressive strength (UCS) and RLT tests. Results indicate that samples treated with PAM yield higher UCS and resilient modulus values than the untreated samples, with the magnitude of the resilient modulus being dependent on compaction effort. Data related to response under simulated loads has also been collected and used to calibrate an existing model to predict resilient modulus response for each soil type from its physical and engineering properties. The calibrated models were found to satisfactorily simulate the resilient modulus response of the RLT experimental test data.

Mechanical Behavior of Polyurethane Polymer Materials under Triaxial Cyclic Loading: A Particle Flow Code Approach

Polyurethane polymer grouting materials were studied with conventional triaxial tests via the particle flow code in two dimensions (PFC2D) method, and the simulation results agreed with the experimental data. The particle flow code method can simulate the mechanical properties of the polymer. The triaxial cyclic loading tests of the polymer material under different confining pressures were carried out via PFC2D to analyze its mechanical performance. The PFC2D simulation results show that the value of the elastic modulus of the polymer decreases slowly at first and fluctuated within a narrow range near the value of the peak strength; the cumulative plastic strain increases slowly at first and then increases rapidly; the peak strength and elastic modulus of polymer increase with the confining pressure; the PFC2D method can be used to quantitatively evaluate the damage behavior of the polymer material and estimate the fatigue life of the materials under fatigue load based on the number and the location of micro-cracks. Thus, the PFC2D method is an effective tool to study polymers.

Fracture Characteristics of Overlying Bedrock and Clay Aquiclude Subjected to Shallow Coal Seam Mining

Some of the coal deposits in the northwest region of China are at relatively shallow depths, covered by a thin layer of bedrock and a thick layer of wind-blown sand. We studied the mechanics and permeability of a clay aquiclude by X-ray diffraction and triaxial loading tests and the fracturing of the overlying bedrock and aquiclude in physical simulation tests. The results indicate that if the bedrock thickness is 90 m or more, the height, distribution pattern, and damage to the water-conducting fractured and caving zones (“belts”) are normal. If the bedrock thickness is 30 m or less, the “three belts make one” phenomenon occurs, and caving extends into the soil layer; the overlying rock is completely perforated by vertical fractures and the collapse spreads to the surface. When the bedrock thickness is between 30 and 90 m, the height of the water-conducting fractured zone is inhibited by the weathered rock and cohesive soil layers, and the height of the “two belts” is significantly decreased. This can be used to prevent and control coal mine water hazard.