Large-scale Triaxial Compression Tests Research Articles

Large-Scale Triaxial Test on Mechanical Behavior of Coral Sand Gravel Layered Samples

Layered structures comprising coral sand and gravel have been observed in hydraulic filled foundations in the coral reefs in the South China Sea, leading to anisotropy in their physical and mechanical properties. However, the effect of a layered structure on the strength and deformation of the coral soil foundation remains unclear. In this study, a series of large-scale triaxial compression tests and step-loading tests were carried out on four types of samples, i.e., clean coral sand, clean coral gravel, sand-over-gravel layered sample, and gravel-over-sand layered sample, to investigate the impact of confining pressure and the layered structure on the strength and failure modes of these soils. The results indicate that the stress–strain relationships of all samples predominantly exhibit strain hardening under drained conditions. Under identical confining pressures, the peak strength of clean coral sand is the lowest, while that of coral gravel is the highest. The peak strengths of the two layered samples fall between these extremes, with the gravel-over-sand layered sample exhibiting higher strength. All four samples have similar peak friction angles, slightly exceeding 40°. The difference in peak strength among the four types of samples is attributed to the variations in cohesion, with the cohesion of clean coral gravel being up to four times that of clean sand, and the cohesion of layered samples falling between these two. Both clean sand and clean gravel samples exhibit a bulging phenomenon in the middle, while the layered samples primarily exhibit bulging near the coral gravel layer. In the step-loading tests, the bearing capacity of the layered samples falls between those of clean coral sand and coral gravel, with the gravel-over-sand layered samples demonstrating higher strength. Moreover, the p-s curve of the gravel-over-sand layered samples obtained from the large-scale triaxial apparatus under a confining pressure of 400 kPa resembles that from the plate load tests on the same samples.

Breakage critical state of gravels with different gradings. Part II: Constitutive modelling

A unified critical state surface for gravels considering the effect of initial grading and particle breakage was proposed in Part I based on the test results of large-scale triaxial compression tests. In this study, we proposed a state-dependent elastoplastic constitutive model to predict the shear behaviors of gravels in the framework of bounding surface plasticity and critical state theory. A simple state parameter with a reference to the unified critical state surface was applied to the dilatancy equation and the bounding stress ratio in the proposed model. The proposed constitutive model can well capture the grading state index, the relative breakage index, the deviatoric stress and the void ratio at the end of tests, and the stress–strain relationships during the shearing process. The model can still be improved by considering the influence of the confining pressure on the critical state strength.

Breakage critical state of gravels with different gradings. Part I: Experimental results

Grading and particle breakage have significant influence on mechanical properties of crushing geomaterials. A series of large-scale triaxial compression tests have been carried out to investigate the influence of grading and particle breakage on the stress-strain relationship, grading state index, critical state, and breakage index of coarse grains. The results confirm that the slope of critical state line (CSL) is a constant in the CSL stress plane and independent of initial grading, whereas, the location and slope of the CSL in the CSL deformation plane are strongly dependent on the initial grading. In addition, a hyperbolical function is used to capture the relationship between breakage index and input work for gravels with different initial gradings. A unified critical state surface considering the grading state index is established in three-dimensional CSL space for the gravels used in this study and also validated by the test data on other soils from literatures.

Experimental Study on the Triaxial Compression Properties of Coarse-Grained Filling Soil under Drying–Wetting Cycles

To explore the mechanical properties of coarse-grained filling soil in the hydrofluctuation belt of the Baihetan reservoir, a fast drying–wetting cycle method for large-scale triaxial tests was developed and a series of large-scale triaxial compression tests of coarse-grained soil were conducted under drying–wetting cycles. The results show that the drying–wetting cycles and the confining pressure are both important factors affecting the mechanical properties of coarse-grained soil. The influences of the first and second cycles on the deviatoric stress–strain curve of the coarse-grained soil are the greatest, while the influences of the third to seventh cycles tend to be stable. The peak strain is not affected by the drying–wetting cycles but only increases with increasing confining pressure. The axial strain and volumetric strain at the volume expansion point decrease with increasing number of drying–wetting cycles but increase with increasing confining pressure. The secant modulus of the peak point decreases with increasing number of drying–wetting cycles, and the initial tangent modulus decreases slightly. The influence of the drying–wetting cycles on the cohesion of the coarse-grained soil is greater than that on the internal friction angle. The typical “bulging” phenomenon occurred after the specimens were destroyed. A damage constitutive equation was developed by introducing a damage variable into the hyperbolic model to reflect the influence of the number of drying–wetting cycles. The model parameters were obtained and the proposed model was verified by fitting the experimental results.

Evaluation of required stiffness and strength of cellular geosynthetics

Cellular geosynthetics have been widely used in engineering practices for their enhanced confining effects. However, limited research has been reported on methods of evaluating the required stiffness and strength of cellular geosynthetics based on the strength of geosynthetic-encased soil in engineering practices. In this study, an analytical approach was formulated to estimate the required stiffness and strength of cellular geosynthetics based on the equivalent strength of geosynthetic–soil composites considering soil nonlinearity, soil plasticity, soil dilatancy, and soil–geosynthetic interaction. Large-scale triaxial compression tests on geosynthetic-encased sand were performed to validate the effectiveness of the method. Parametric studies were conducted using the validated method to investigate the effects of nonlinear elastic parameters, the peak strength of the infill soil, and the aperture size of cellular geosynthetics on the required stiffness and strength.

Testing and constitutive modelling of the mechanical behaviours of gravelly soil material

In this study, large-scale triaxial compression tests are conducted on gravelly soil material. The experimental investigation shows that gravelly soil exhibits complicated volume-dilatation/contraction behaviours that are dependent on stress level. To better describe the mechanical behaviours, an elastoplastic constitutive model is proposed within the frameworks of generalised plasticity and the critical state soil mechanics. An evolution equation of the void ratio is incorporated into the model to describe the effect of a relatively dense or loose state on the volume-change behaviours. The model has 10 material constants that could be determined using a few large-scale conventional triaxial tests. The model is then used to simulate the mechanical behaviours of gravelly soil. Comparisons between the numerical simulations and the test data show that the model is capable of capturing the mechanical behaviours of gravelly soil under various confining pressures. This research can be beneficial for understanding the mechanical behaviours of gravelly soils.

Critical State and Grading Evolution of Rockfill Material under Different Triaxial Compression Tests

One of the most important features of soil behavior is the dependence of stress path. A series of large-scale triaxial compression tests was conducted to investigate the effect of stress path on critical state and particle breakage–induced grading evolution of rockfill material. All tests were first consolidated under constant principle stress ratio condition, and then sheared under three different stress paths, including constant lateral stress σ3, constant mean effective stress p′, and constant vertical stress σ1. Test results indicate that the mean effective stress p′, deviatoric stress q, and void ratio e along different stress paths reached the consistent critical state line (CSL) in the p′-q-e space. There was a power function between p′ and q, and a linear relation between e and (p′/pa)ξ with ξ=0.5 at the critical state. The particle breakage under different stress paths was found to be consistent at the same critical state, and a unique relation among e, p′, and the particle grading parameters at the critical state existed irrespective of stress paths. The test results at the critical state formed a straight line in the e-(p′/pa)0.5-IG space.

Investigation of Macroscopic and Microscopic Behavior of Gravels Using Triaxial Compression Test with CT Scan

Abstract A large number of studies have been conducted to investigate macroscopic responses of gravelly soils using large-scale triaxial compression tests; however, particle-level responses of gravelly soils under triaxial compression remain poorly explored. Here, we reported a series of large-scale triaxial compression tests on uniformly graded gravels subjected to a low to medium range of confining pressures (200–800 kPa). We further investigated the particle-level responses of the gravels using the combined triaxial compression test with computed tomography (CT) scan. A computer image analysis program was developed to analyze the CT images, providing visualized and quantitative particle displacement and rotation. Based on the experimental results, the stress-strain relationships at different confining pressures and the microscopic characteristics of the gravels (e.g., changes in internal structures) under different deviator stress levels were discussed. The changes in particle-level responses were linked to those in macroscopic responses. From both macroscopic and microscopic analyses, some insights into the underlying mechanism of the stress-strain relationships were provided.

Large-scale triaxial compression tests of geocell-reinforced sand

A series of large-scale triaxial compression tests were performed on sandy soil confined by a circular geocell in this study. The effects of the relative density of the infill sand, the stiffness and strength of the geocell, and the aspect ratio (height-to-diameter ratio) of the specimen on the overall behavior of the geocell-soil composite were investigated. Furthermore, the measured confining pressure increment and apparent cohesion induced by the geocell membrane confinement were compared to those predicted by existing equations. The study results indicated that the end effects were very small in this experimental study. The effect of geocell stiffness on the stress-strain behavior of geocell-soil composites is closely related to the relative density of the soil. The measured confining pressure increments and the apparent cohesion are generally in good agreement with those predicted by Bathurst and Karpurapu's equations. The effect of the axial strain at failure on the apparent cohesion is discussed, and a method is proposed to determine the axial strain at failure for medium dense sand and dense infill sand on the basis of the hyperbolic nonlinear elastic model.

Triaxial compression behaviour of gangue solid wastes under effects of particle size and confining pressure

Underground coal mining leads to environmental problems such as gangue pollution, surface subsidence and soil erosion, etc. Solid backfilling coal mining (SBCM) can control the strata movement, reducing gangue discharge and environmental pollution in mining areas. Gangue solid wastes (GSW) are backfilled into the goaf space as the supports for the overburden strata in solid backfilling coal mining. In this case, GSW are under triaxial compression. The deformation characteristics of the GSW under this loading condition are the key factors determining the control efficiency of strata movement and surface subsidence. The influence of the particle size grade and the confining pressure condition on the deformation and breakage characteristics of the GSW is studied based on large-scale triaxial compression tests in this paper. Also, the effect of particle breakage on the deformation of the GSW is revealed. The results indicate that in the triaxial compression process, the GSW exhibits strain hardening characteristics and volume decrease under compressive loading. The maximum load-bearing stress is more sensitive to the confining pressure condition but less sensitive to the particle size grade. The maximum load-bearing stress increases linearly with the confining pressure. Particle breakage of the GSW is notably influenced by the particle size grade but almost independent of the confining pressure condition. The S1 specimen with a reasonable proportion has the minimum relative breakage index, while the S3 specimen that mainly contains large particles has the maximum relative breakage index. The research outcomes are significant to the understanding of the mechanical characteristics of GSW, the selection and preparation of the backfilling materials in the field practices and the ensurence of the backfilling efficiency.

Estimations of Three Characteristic Stress Ratios for Rockfill Material Considering Particle Breakage

The particle breakage during specimen shearing has a significant influence on the critical-state line (CSL) of the rockfill material. A series of large-scale triaxial compression tests on the rockfill material from Henan Province (HPR) were conducted in a wide range of initial void ratios and confining pressures. The influences of the particle breakage on the critical-state stress ratio $$M_{\mathrm{c}}$$ , the peak stress ratio $$M_{\mathrm{p}}$$ and dilatancy stress ratio $$M_{\mathrm{d}}$$ were investigated. The deviatoric stress and particle breakage of the HPR at the critical state increase with the increase in confining pressure, while the influences of the initial void ratio on these behaviors are too little to be considered. The gradient of the CSL in the $$q\hbox {-}p$$ space of the rockfill, $$M_{\mathrm{c}}$$ , was found to be passively correlated with the particle breakage index $$B_{\mathrm{r}}$$ , rather than being a constant. Additionally, the observed values of $$M_{\mathrm{c}}$$ at low confining pressures (low particle breakage occur) will be substantially undervalued if $$M_{\mathrm{c}}$$ is estimated as a constant. In the critical-state-theory-based constitutive models, $$M_{\mathrm{p}}$$ and $$M_{\mathrm{d}}$$ are estimated as the combinations of $$M_{\mathrm{c}}$$ and state parameter $$\psi $$ . It is believed that the simulations of $$M_{\mathrm{p}}$$ and $$M_{\mathrm{d}}$$ when $$M_{\mathrm{c}}$$ is correlated with $$B_{\mathrm{r}}$$ are obviously more favorable than those when $$M_{\mathrm{c}}$$ is constant.

Influence of Particle Breakage on Critical State Line of Rockfill Material

AbstractThe influences of particle breakage on the position of the critical state line (CSL) were systematically investigated in this paper through a series of large-scale triaxial compression tests on Tacheng rockfill material (TRM). It was found that the critical-state stress ratio of TRM (i.e., the gradient of the CSL in the p−q space) was approximately regarded as a constant. In the e−logp space, the CSL of TRM descended with a decrease in the initial void ratio, whereas the gradient of the CSL was constant. A procedure was established for evaluating the critical state point at a same particle breakage, which comprised the breakage critical state line (BCSL). An increase of the particle breakage led to not only a vertical translation but also a rotation on the BCSL of TRM in the e−logp space, which was similar to the observation of Dog’s Bay sand. Consequently, the initial gradation (or the corresponding initial void ratio) was the dominant factor that affected the position of the CSL of TRM in the e−...

Strength and Deformation of Rockfill Material Based on Large-Scale Triaxial Compression Tests. I: Influences of Density and Pressure

AbstractA series of large-scale triaxial compression tests were conducted to investigate the strength and deformation behaviors of Tacheng rockfill material (TRM) in relation to the initial void ratio and initial confining pressure. The critical state friction angle of TRM was expressed as a linear function of the logarithm of the initial confining pressure. The excess peak state friction angle and excess characteristic state friction angle of TRM were formulated as linear equations of a revised relative dilatancy index to capture the influences of density and pressure on the peak state and characteristic state friction angles. The initial elastic modulus, tangent modulus, and secant modulus of TRM were dependent on the initial void ratio and initial confining pressure. In addition, a formulation incorporating density and pressure was proposed to simulate the initial elastic modulus of TRM. The volumetric and deviatoric strains of TRM at the critical state were also dependent on density and pressure. The ...

Strength and Deformation of Rockfill Material Based on Large-Scale Triaxial Compression Tests. II: Influence of Particle Breakage

AbstractThe amount of particle breakage of Tacheng rockfill material (TRM) in the process of shearing was quantitatively measured by a relative breakage index. The fractal dimension of TRM at the final state of the tests was found to be linearly related to the void ratio and logarithm of the confining pressure. The influences of the relative breakage index on the friction angle, modulus, and deformation were investigated based on a series of large-scale triaxial compression tests on TRM. An increase in the relative breakage index leads to an increase in the volumetric strain and to decreases in the peak state friction angle, critical state friction angle, deviatoric strain, and critical state void ratio. The test results were used to propose simple formulations pertaining to the relative breakage index for the strength and deformation indexes (e.g., peak state friction angle, critical state friction angle, initial elastic modulus, secant modulus at 50% of the peak strength, volumetric strain, deviatoric s...

A constitutive model for the state-dependent behaviors of rockfill material considering particle breakage

The particle breakage during specimen compaction had more significant influence on the position of the breakage critical-state line (BCSL) of Tacheng rockfill material (TRM) in the e-lnp′ plane than the particle breakage during shearing, based on the large-scale triaxial compression tests on TRM in a wide range of densities and pressures. The state-dependent dilatancy and the plastic modulus were correlated to the breakage index, based on the formulations of the BCSL of TRM in the e-lnp′ plane. The state-dependent model considering particle breakage was proposed for TRM within the framework of the generalized plasticity theory. The proposed model contained fourteen material constants. The test data of TRM from Group A were adopted to determine these material constants, while the test data from Group B were used independently to validate the model predictive capacity. The comparisons between model simulations and test data illustrated that the model with consideration of particle breakage could well represent the stress-strain behaviors of TRM, e.g., the strain hardening and volumetric contraction behaviors at a loose state and the strain softening and volumetric expansion behaviors at a dense state, and also the particle breakage behaviors of TRM.

State-Dependent Constitutive Model for Rockfill Materials

AbstractA series of large-scale triaxial compression tests on Tacheng rockfill material (TRM) showed that the dilatancy and stress-strain behaviors were significantly influenced by density and pressure. The critical state friction angle of TRM decreased with an increase in the initial confining pressure. In addition, the critical state line of this material in the e-log⁡p′ plane ascended with an increase in the initial void ratio. On the basis of the specific critical state behaviors of TRM, a state-dependent constitutive model was slightly adapted in the triaxial stress space. The model, with only a set of model parameters, could capture the state-dependent behaviors of the dilatancy, stress-strain, and mobilized friction angle of TRM at various densities and pressures. The peak friction angle of TRM was found to be linear, with a coefficient of 0.21, in relation to the maximum dilatancy angle. The coefficient for rockfill materials was smaller than that for sands, indicating that the influence of dilata...

Strength and deformation characteristics and small strain properties of undisturbed gravelly soils

A series of medium-scale and large-scale triaxial and unconfined compression tests was conducted in order to evaluate the strength and deformation characteristics and small strain properties of undisturbed well-graded gravelly soils retrieved from three tunnel excavation sites in Toyama prefecture, Japan. Undisturbed gravelly soils were taken by means of a new sampling method using thick water-soluble polymer solutions. The strength and deformation characteristics were evaluated mainly by performing sustained loading and large amplitude unloading and reloading cycles during otherwise monotonic loading at a constant strain rate in drained triaxial compression tests. During isotropic consolidation and shearing, at several stress states, eleven very small vertical cycles were applied to evaluate the quasi-elastic deformation property at small strain levels around 0.001% by static measurement. Dynamic measurements using a pair of accelerometers attached to the side surface of the specimen and wave sources attached to the top cap were also conducted at the same stress levels as static measurements in a single test. Several effects including grading characteristics and pressure level on the difference between the moduli measured statically and dynamically were discussed. The relationship between the small strain and strength properties of undisturbed gravelly soils was evaluated. The small strain properties of air-dried dense Toyoura sand in large-scale triaxial compression tests were also investigated in this study to compare the results of undisturbed gravelly soils.

Simulating Ballast Shear Strength from Large-Scale Triaxial Tests

The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.

STRENGTH CHARACTERISTICS OF SAND REINFORCED WITH FULL SCALE REINFORCING MATERIALS IN LARGE SCALE TRIAXIAL COMPRESSION TEST

A reinforcing mechanism, and strength and deformation characteristics of the reinforced soil have been investigated by a laboratory element test using a small size specimen. However, the properties of soil that reinforced with a full scale reinforcing material can not be studied by the laboratory test as the size of specimen is very small.In this paper, a series of large scale triaxial compression tests were performed in order to investigate the strength and deformation characteristics of sand that reinforced with full scale reinforcing materials (geo-grid, non-woven and metal strip for Terre armee). The tests were carried out on a dry sand sample, the results were represented by comparing the stress-strain relation of reinforced sand with that of unreinforced sand.

Strength of Sand Reinforced with Full Scale Reinforcement Material in Large Scale Triaxial Compression Test

A reinforcing mechanism, and strength and deformation characteristics of the reinforced soil have been investigated by a laboratory element test using a small size specimen. However, the properties of soil that reinforced with a full scall reinforcing material can not be studied by the laboratory test as the size of specimen is very small.In this paper, a series of large scale triaxial compression tests were performed in order to investigate the strength and deformation characteristics of sand that reinforced with full scall reinforcing materials (geo-grid, non-woven and metal strip for terre armee). The tests were carried out on a dry sand sample, the results were represented by comparing the stress-strain relation of reinforced sand with that of unreinforced sand.