Triaxial Tests Research Articles

Mechanical Properties of Rock Salt from the Kłodawa Salt Dome-A Statistical Analysis of Geomechanical Data.

Rock salt is a potential medium for underground storage of energy resources and radioactive substances due to its physical and mechanical properties, distinguishing it from other rock media. Designing storage facilities that ensure stability, tightness, and safety requires understanding the geomechanical properties of rock salt. Despite numerous research efforts on the behaviour of rock salt mass, many cases still show unfavourable phenomena occurring within it. Therefore, the formulation of strength criteria in a three-dimensional stress state and the prediction of deformation processes significantly impact the functionality of storage in salt caverns. This article presents rock salt's mechanical properties from the Kłodawa salt dome and a statistical analysis of the determined geomechanical data. The analysis is divided into individual mining fields (Fields 1-6). The analysis of numerical parameter values obtained in uniaxial compression tests for rock salt from mining Fields 1-6 indicates an average variation in their strength and deformation properties. Upon comparing the results of Young's modulus (E) with uniaxial compressive strength (UCS), its value was observed with a decrease in uniaxial compressive strength (E = 4.19968·UCS2, R-square = -0.61). The tensile strength of rock salt from mining Fields 1-6 also exhibits moderate variability. An increasing trend in tensile strength was observed with increased bulk density (σt = 0.0027697·ρ - 4.5892, r = 0.60). However, the results of triaxial tests indicated that within the entire range of normal stresses, the process of increasing maximum shear stresses occurs linearly ((σ1 - σ3)/2 = ((σ1 + σ3)/2)·0.610676 + 2.28335, r = 0.92). A linear relationship was also obtained for failure stresses as a function of radial stresses (σ1 = σ3·2.51861 + 32.9488, r = 0.73). Based on the results, the most homogeneous rock salt was from Field 2 and Field 6, while the most variable rock salt was from Field 3.

Mechanical Properties and Constitutive Model for Artificially Structured Soils under Undrained Conditions

Triaxial tests are performed for remolded, artificially isotropic, and anisotropic structured samples under undrained conditions at confining pressures of 25, 100, and 200 kPa. Based on these test results, a binary-medium constitutive model is formulated based on homogenization theory and a breakage mechanism to describe the behaviors of structured soils. In this model, the binary-medium material is idealized as a representative volume element (RVE) composed of bonded elements, whose mechanical behaviors are expressed by the linearly elastic model, and frictional elements, whose mechanical behaviors are described by the double-yield surfaces constitutive model. The parameters of the bonded and frictional elements are determined from the test results of structured and remolded samples, respectively. The expressions for the breakage ratio and local stress coefficient matrix are introduced, and their parameters are provided. The computed results are compared with the test results, demonstrating that the model can reflect the main deformation features of structured soil relatively well, including the influence of anisotropy, gradual damage to particle bonding, and pore development.

Evolution of Soil Pore Structure and Shear Strength Deterioration of Compacted Soil under Controlled Wetting and Drying Cycles

This study investigates the evolution of soil pore structure and shear strength deterioration in compacted clayey soil under controlled wetting and drying (wd) cycles, which are expected to become more frequent due to climate change. Thirty soil samples were compacted at optimal moisture content and 90% maximum dry density. These samples were then subjected to 0, 1, 5, 10, and 15 controlled wd cycles from saturation to the wilting point, and volumetric changes were recorded during each cycle. After the wd treatment, the soil samples were scanned using X-ray computed tomography (CT) at 50 μm resolution and then sheared under unconsolidated–undrained and consolidated–undrained conditions in a triaxial test. Significant shrinkage and swelling of soil samples were observed during wd cycles, with average volumetric strain fluctuating between +12% at saturation and −5% at the wilting point. X-ray CT visualisation and analysis revealed higher porosity, more prominent pores, and increased pore length in soil samples with increasing wd cycles. Both undrained and effective soil shear strength markedly decreased with increasing wd cycles. CT-derived macroporosity and pore length were significant predictors of the soil’s undrained and effective shear strength when exposed to wd cycles. The findings emphasise the considerable impact of climate change, specifically wd cycles, on clayey soil, highlighting the need for consideration in the design of earth-based infrastructure.

Multi-angle property analysis and stress–strain curve prediction of cementitious sand gravel based on triaxial test

In order to further promote the application of cementitious sand gravel (CSG), the mechanical properties and variation rules of CSG material under triaxial test were studied. Considering the influence of fly ash content, water-binder ratio, sand rate and lateral confining pressure, 81 cylinder specimens were designed and made for conventional triaxial test, and the influence laws of stress–strain curve, failure pattern, elastic modulus, energy dissipation and damage evolution of specimens were analyzed. The results showed that the peak of stress–strain curve increased with the increase of confining pressure, and the peak stress, peak strain and energy dissipation all increased significantly, but the damage variable D decreased with the increase of confining pressure. Under triaxial compression, the specimen was basically sheared failure from the bonding surface, and the aggregate generally did not break. Sand rate had a significant effect on the peak stress of CSG, and decreased with the increase of sand rate. Under the conditions of the same cement content, fly ash content and confining pressure, the optimal water-binder ratio 1.2 existed when the sand rate was 0.2 and 0.3. After analyzing and processing the stress–strain curve of triaxial test, a Cuckoo Search-eXtreme Gradient Boosting (CS-XGBoost) curve prediction model was established, and the model was evaluated by evaluation indexes R2, RMSE and MAE. The average R2 of the XGBoost model based on initial parameters under 18 different output features was 0.8573, and the average R2 of the CS-XGBoost model was 0.9516, an increase of 10.10%. Moreover, the prediction curve was highly consistent with the test curve, indicating that the CS algorithm had significant advantages. The CS-XGBoost model could accurately predict the triaxial stress–strain curve of CSG.

On the influence of the shearing rate on the monotonic and cyclic response of Malaysian kaolin

This paper experimentally evaluates the influence of shearing rate on the monotonic and cyclic response of isotropically consolidated samples of Malaysian kaolin. On the one hand, a series of undrained monotonic triaxial tests were performed with varying shearing displacement rate. On the other hand, undrained cyclic triaxial tests were conducted considering different deviatoric stress amplitudes and loading frequencies. The well-known soil rate dependency under monotonic loading was confirmed up to a displacement rate threshold. The experimental results under cyclic loading suggest that for the given loading frequency, the variation of the deviatoric stress amplitude remarkably influences the strains and pore water pressure accumulation rates. In addition, the results suggest that, depending on the loading frequency, different shapes of mobilised effective stress loops are obtained. Larger loading frequencies lead to banana-shaped effective stress loops, while smaller frequencies reproduce eight-shaped effective stress loops. Furthermore, higher loading frequencies result in a larger number of cycles required to reach failure conditions. The reasons for the observed differences in the behaviour are thoroughly analysed and discussed.

A Time-Dependent Damage Constitutive Model for Brittle Rock Materials Based on Subcritical Damage Theory.

In the domain of geotechnical engineering, a profound understanding of the long-term mechanical deformation characteristics of rocks is indispensable for the design and construction of structures, dams, tunnels, and various engineering projects. The deformation behavior of rocks under long-term loads directly impacts the stability and safety of engineering structures. This study employs micromechanical methods to investigate the subcritical extension of microcracks under stress corrosion. By examining the accumulated damage resulting from this phenomenon, the research explores the patterns of aging damage development and establishes a constitutive model for aging that incorporates cumulative damage over the stress history. The accuracy of the proposed model is evaluated through a comprehensive comparison of numerical results with experimental data. The experimental data set encompasses traditional triaxial compression tests, single-stage creep, multistage creep, and single-stage relaxation tests conducted under varying confining pressures. The predicted results exhibit strong consistency with the entire data set. Furthermore, this paper employs crack damage stress as an indicator characterizing the long-term strength of rock. Through frictional damage coupling analysis and derivation, an analytical expression for the long-term strength of rock materials containing microcracks is provided, serving as a theoretical basis for investigating the long-term mechanical performance of brittle rock materials and ensuring the long-term stability of large-scale rock engineering projects.

A simulation approach for quantifying ballistic impact damage in ultra-high-performance concrete

In this work, we provide a hydrocode simulation model for high-velocity projectile impact against ultra-high-performance concrete targets and establish a methodology to extract damage quantities from the simulation results. In the parameter derivation process, published and own data stemming from material experiments, such as uniaxial, triaxial, and planar plate impact tests, are used as a starting point. To fill the systematic gaps of strength data for pressures of 1 GPa to 5 GPa and for fractured concretes, residual velocities of projectiles and qualitative target damage information from ballistic experiments with high-hard steel spheres are additionally used as a reference in parametric simulations. All criteria from the comparatively broad data basis are successfully reproduced by the simulation model simultaneously. The simulated damage quantities derived by the proposed extraction procedure are reasonable counterparts to the corresponding experimental measures from earlier published works, allowing a new quality of comparison between both worlds.

A new macro- and micro- coupling model of barrier dam soil

Barrier dams are widely distributed worldwide. It is highly important to accurately evaluate the safety and emergency response of barrier dams. However, the stressstrain relationship of barrier dam soil, which is an important issue for the safety evaluation of barrier dams, has not been elucidated clearly because of its complex composition and wide particle gradation. Based on the proposed macro- and micro- coupling modeling scheme, a series of CT triaxial tests were carried out on barrier dam soil to observe the macroscopic and microscopic behavior. Based on the particle displacement distribution, a fabric evolution rate was proposed as an index to describe the evolution of fabric behaviors. A significant correlation was detected between the shear band fabric evolution rate and the macroscopic mechanical properties of the soil. The fabric evolution equation, modulus evolution equation and Poisson's ratio evolution equation were proposed to set up a new macro- and micro- coupling model for barrier dam soil. The model has only five parameters that have definite physical meanings and can be easily determined by a series of triaxial tests. The validity of the proposed model was verified by different types of tests of barrier dam soil and soilrock mixtures. The model can reasonably describe the fundamental properties of barrier dam soil with wide particle gradation. The proposed model established by macro- and micro- coupling modeling considers the relationship between macroscopic mechanical properties and microscopic fabric behavior, which is suitable for barrier dam soils with wide particle gradations.

Evaluating the impervious performance of hydraulic asphalt concrete in embankment dams: A study of crack evolution at different temperatures

Hydraulic asphalt concrete (HAC) is extensively utilized as impervious interior core walls or upstream facings in embankment dams. This study examines the evolution of crack growth in HAC and its impact on its impervious performance under varying external loadings and temperatures. By employing X-ray computed tomography (CT) scanning in-situ triaxial compressive tests, the behavior of HAC specimens at 5 °C and 20 °C was analyzed. Statistical methods enabled the categorization of cracks by their equivalent diameter, revealing their evolution from shrinkage and initiation to expansion, interconnection, and convergence with increasing deformation. The fractal dimensions of these cracks initially decreased gradually, then stabilized, before increasing sharply. Furthermore, a logarithmic relationship between the fractal dimensions of cracks and the crack ratio was established. At equivalent deformation levels, crack development was more pronounced at 5 °C than at 20 °C. Notably, the specimens did not exhibit complete external interconnection of cracks at either temperature near the peak stress, highlighting HAC's robust impervious qualities at the meso scale. Compared to clay and concrete, HAC demonstrated superior deformation resistance and impervious capabilities in the anti-seepage structure of embankment dams. This research provides valuable insights for the application of HAC as a water barrier in complex geological settings.

The modified Mohr-Coulomb model considering softening effect and intermediate principal stress

Based on the Mohr-Coulomb model, a modified Mohr-Coulomb model that can consider both softening effect and intermediate principal stress effect was formed by introducing plastic internal variables that characterize material damage and a modified stress Rhodes angle. The modified model is numerically integrated by implicit algorithm, the physical meaning of the model parameters of the modified model is clarified, and the multivariate analysis and sensitivity analysis of the model parameters are carried out. Combined with the results of the gypsum triaxial test, the model parameters of the modified model were determined by the equation solving and RBF neural network optimization method, and the validity of the model was verified. The research results showed that: The modified model established can consider both the material softening effect and the influence of the intermediate principal stress. Compared with the traditional Mohr-Coulomb model, the calculation results of the modified model are more consistent with the test results qualitatively and quantitatively. The comprehensive use of direct determination, equation solving and RBF neural network optimization methods can effectively determine the model parameters of the modified model, and the determination process is relatively simple. It is suggested that this combined method can be used for subsequent determination of more complex constitutive model parameters. Compared with the traditional Mohr-Coulomb model, the modified model only needs one more true-triaxial test to determine the model parameters. In addition, the experience of using the parameters of the traditional Mohr-Coulomb model in a large number of engineering practices can be used to improve the model, so the modified model has great potential for engineering applications.

Dynamic deformation characteristics of lightweight soil based on Davidenkov model

The dynamic deformation characteristics of lightweight soil were studied using consolidated-undrained dynamic triaxial tests to predict its non-linear stress and strain behaviour under a traffic load. The results showed that the influence of confining pressure on the attenuation of the dynamic shear modulus ratio became significant with increasing expanded polystyrene (EPS) particle content or cement content. The attenuation curve of the dynamic shear modulus ratio was evaluated using Davidenkov model parameters, A, B and γ 0. It was proved that the parameter A was mostly affected by EPS particle content and confining pressure, with its value ranging between 1.1 and 1.85. The parameter γ 0 was affected by cement content and confining pressure, and its value ranged between 0.0007 and 0.0016. The parameter B was found to range between 0.38 and 0.55, in general. The relationship curves between damping ratio and dynamic shear strain revealed two forms, S shaped and bell shaped. The verification tests showed that the Davidenkov model could predict accurately the dynamic deformation characteristics of lightweight soil both under a changing stress state and under constant gradient loading conditions. These results provide a theoretical basis for determining the deformation of a lightweight soil subgrade under a traffic load.

Triaxial failure behavior and microfracture source time–frequency characteristics of marble under in situ stress conditions at different depths considering engineering disturbance

The safety of the surrounding rock in deep underground engineering is crucial. To understand the surrounding rock hazards at different depths of the Jinping underground cavern, triaxial compression tests were conducted, and the acoustic emission (AE) of Jinping marble samples was monitored while considering the influence of engineering disturbances and in situ stress conditions at different burial depths (100 m, 1000 m, 1400 m, 1800 m, and 2400 m). The results revealed that the AE events were dominated by small and medium-sized energy releases of magnitude 0–2, accounting for approximately 90 % of all AE events. The dominant frequency range of the microfracture sources was 0–450 kHz. As the burial depth increases, the AE frequency band aggregation time tended to stabilise at the average frequency during the loading process, and the failure mode trended towards ductile failure. This study offers theoretical support for assessing the operational security of deep underground caverns.

Effect of the cyclic loading on the yield surface of a chemically stabilised soil

When a soil is subjected to cyclic loading, there are changes in the material's geomechanical behaviour that need to be characterised before safely designing any future projects. In terms of cyclic loading, it is important to characterise not only the failure of the soil but also its behaviour before failure, in particular the yield point and the elastic behaviour of the material. This study examines the effects of the number of loading cycles on the behaviour of a chemically stabilised soft soil with a particular focus on the yield surface. To this end, a series of triaxial tests were performed on specimens, previously or not subjected to a different number of loading cycles (1000–100 000). The results were analysed in terms of the evolution of accumulated permanent axial strain, the yield surface and stress–strain behaviour. It was observed that an increase in the number of loading cycles promoted: an increase in the permanent axial strain, an increase in the undrained resilient modulus, a shrinkage of the yield surface but its shape is maintained, and there is a small increase in the peak strength of the stabilised soil explained by the strain-hardening effect induced by the cyclic loading.

Study on the Permanent Deformation and Dynamic Stress–Strain of Coarse-Grained Subgrade Filler under Cyclic Loading

Using coal gangue as a subgrade filler will produce good benefits, and its application prospects are very broad. It is of great engineering and scientific value to study the improvement method and dynamic characteristics of coal gangue subgrade filler under traffic load. Combining the properties of coal gangue material, fly ash and lime and soil were added to improve the bearing behavior of coal gangue subgrade filler. Then, a compaction test was carried out using the principle of orthogonal experimental design. By analyzing the compaction test results, the optimal proportion of each additive was obtained. A large-scale dynamic triaxial test was carried out with the proportion of each admixture in the maximum dry density group in the compaction test. Based on the dynamic triaxial test results, the effect of confining pressure on the permanent strain was analyzed, the analysis model of permanent deformation and cycle number of traffic loading was proposed, and the correctness of the model was verified. In addition, a modified Hardin–Drnevich model was established, which can describe the dynamic stress–dynamic strain curve of coal gangue subgrade filler under traffic load, and then, the dynamic modulus and damping ratio were analyzed.

Can rubberized-asphalt comprising tire-derived aggregates produced through dry process be an alternative subsurface road material?

The major objective of this study was to investigate the suitability of utilizing tire-derived aggregates (TDA) at varying contents for the design of rubberized-asphalt base / subsurface layers through the dry process, along with (i) understanding the cracking potential and mechanical stress-strain response of the different materials under cyclic and confining stresses through triaxial resilient modulus (MR), dynamic modulus |E*|, and static semi-circular bend test, and (ii) assessing bonding strength of TDA with binder-aggregate matrix through moisture susceptibility test. The scope encompassed preparation of ten base course mixtures: one unbound granular base (UGB), three cement treated base (CTB), two asphalt treated base (ATB), and four rubberized-asphalt treated base (RATB) and obtaining mechanical performance at varying temperature-frequency combinations totaling 2636 data points. Triaxial MR tests indicated that CTB mix displayed highest modulus followed by ATB and RATB mixes, which was then followed by the UGB mix mainly ascribed to cement / asphalt treatment of the unmodified control subsurface material. Further, a finite element modeling study helped optimize the confinement levels to obtain realistic |E*| of ATB and RATB mixes. |E*| test results depicted that RATB mixtures would be rut-resistant at higher temperatures as they performed similar to the ATB mixes as well as crack-resistant at low to intermediate temperatures with significantly lower |E*| than ATB mixes, attributed to slower rate of change of viscosity with changing temperature in conjunction with enduring viscoelastic property (1.5–3 times higher phase angle than ATB) ascribed to high asphalt binder content and resilient nature of the rubber particles. Albeit the fracture toughness of ATB was twice higher than RATB, the latter had higher fracture energies, mainly due to the resiliency rendered by the rubber inclusions in concert with the extra viscous effect provided by the rubber-asphalt binder conglomerate. Based on the performance-based ranking, RATB mixes performed better than control mixes, and thus were adjudged as the most suitable alternative subsurface / base course materials in a pavement system. It is envisioned that this research will advance the current understanding of utilizing TDA in asphalt base / subbase courses using the dry process, and further assist in judging the viability of using rubberized-asphalt as potential sustainable alternatives to conventional systems to develop futuristic resilient pavement infrastructures with a strong emphasis on recycling of end-of-life tires, conservation of materials, and circular economy.

Engineering properties of pozzolanic cement-stabilized organic soil

In this study, the engineering properties of organic soil (OS) stabilized with pozzolanic cement (PC) were investigated. In tests, 10, 15 and 20% proportions of PC were added to organic soil. Consistency limit, Proctor, unconfined compressive strength (UCS), triaxial (TA), swelling and compressibility tests were performed on the OS stabilized with PC. The addition of PC to OS increased the maximum dry density (MDD) and optimum moisture content (OMC) values. The UCS values for the OS stabilized with PC increased up to day 56 and then there was a reduction in the UCS after day 56. The internal friction angle value increased by 26% and the cohesion intercept value increased by 86% for the OS stabilized with PC. With the increase in the cement content in the OS stabilized with PC, the UCS, internal friction angle and cohesion intercept values increased. Swelling of the OS stabilized with PC rapidly reduced up to day 10, and this reduction slowed after day 10. The compressibility of the OS stabilized with PC increased up until day 10, and this increase slowed after day 10. With an increase in the cement content in the OS stabilized with PC, swelling and compressibility values reduced. As a result of the study, it appears that pozzolanic cement is an effective stabilizer for organic soil.

Experimental study on the dynamic properties of soft clay with sand interlayers under cyclic loading

Unlike uniform soils, soft clays with sand interlayers in coastal soft clays, can affect their mechanical properties, potentially impacting underground-construction safety and stability. Consolidated undrained cyclic triaxial tests were conducted to study the dynamic properties and deformation behavior of clay, focusing on how the thickness ratio between the sand and clay layers and the cyclic-stress ratio influence the pore pressure, axial strain, shear-modulus ratio, and normalized damping ratio. The results indicate that higher thickness ratios and cyclic-stress ratios lead to a faster decay of the shear-modulus ratio, quicker increases in pore pressure, faster strain accumulation, and fewer cycles to failure. The normalized damping ratio has three different forms: decreasing, decreasing then increasing, and increasing. However, at a cyclic-stress ratio of 0.2 and thickness ratio of 0.25, the samples exhibit better dynamic characteristics. Soft clay with sand layers exhibits characteristics in line with the stability theory. At low thickness and cyclic-stress ratios, purely elastic and elastically stable phases are observed. As the thickness and cyclic-stress ratios increase, it transitions to plastic stability and incremental failure.

Experimental investigation on mechanical behavior of natural soft clay by true triaxial tests

Laboratory tests on undisturbed and remolded Shanghai clay were conducted with a novel mixed boundary true triaxial apparatus. The strength and deformation behaviors of the structured soft clay at different Lode angles were carefully investigated. The relationship between the shear stress ratio and the dilatancy ratio of the clay in three different stress spaces, that is p–q space, TS stress space and tij modified stress space, are discussed in detail. In particular, the influences of the stress path and the structure of non–disturbed clay on the strength, deformation and failure criterion of Shanghai clay were quantitatively evaluated. The shear strength and dilatancy of undisturbed /remolded clay decreases with the increase of the Lode angle. It is also found that the shear strength and dilatancy of the undisturbed clay are larger than those of the remolded clay; while the influence of the structure on the dilatancy decreases with an increase in the Lode angle. In the π–plane, the maximum strength of Shanghai clay under different stress paths generally obeys the SMP failure criterion. The coincidence of the stress–dilatancy relationship given by the TS and tij stress spaces with the test results is much better than that given by p–q stress space. Furthermore, the linearity of the plastic potential curves of the undisturbed/remolded clay is more evident in tij stress space.

Research on the bearing creep characteristics and constitutive model of gangue filling body

The creep characteristics and potential deformation patterns of gangue backfill material are crucial in backfill mining operations. This study utilizes crushed gangue from the Gangue Yard in Fuxin City as the research material. An in-house designed, large-scale, triaxial gangue compaction test system was used. Triaxial compaction creep tests were conducted on gangue materials with varying particle size distributions. Analysis was performed based on different particle sizes, stresses, and confinement pressures. The study investigates the creep characteristics of the gangue under different conditions and explores the underlying causes. It reveals the relationship between the creep deformation of gangue materials and the passage of time. Mathematical methods are applied to develop a triaxial compaction creep power law model for gangue backfill materials. Finally, the creep results are fitted using an empirical formula approach.

Estimating Hardening Soil-Brick model parameters for sands based on CPTU tests and laboratory experimental evidence

The Hardening Soil-Brick model for soils is designed to carry out complex numerical analyses of soil-structure interaction problems taking into account strong stiffness variation in the range of small strains. However, to calibrate its parameters advanced triaxial and oedometric tests are required. In case of uncemented sands laboratory testing is usually difficult. Therefore, to facilitate calibration procedures, a CPTU based method, enhanced by an experimental evidence derived from advanced triaxial drained and oedometric tests, has been proposed and verified. It is shown in the paper that using exclusively the CPTU test results one can calibrate most important model parameters for sands with accuracy that is sufficient for solving real life problems. The major goal of this paper is to identify correlations between all reference stiffness moduli, then verify them, and finally link with the CPTU based identification procedures. It is shown in the paper that such correlations exist and they exhibit very high coefficients of determination. Moreover, as the seismic version of the CPTU test is not often available in the practice, an enhanced procedure for identification of very small strain shear stiffness modulus has been proposed and then verified, using set of the SCPTU tests conducted in Gdańsk sands (Poland).