Stress Path Tests Research Articles

Investigation of the particle crushing characteristics of stacked stone materials under different stress path conditions

Stone fill materials are widely used in engineering projects such as earth-rock dams, railways, and highway embankments. Nevertheless, they are susceptible to particle breakage, which can lead to a decline in engineering performance. A complete set of studies on crushed stone aggregates was carried out to investigate particle crushing characteristics under various stress path conditions. To quantify the amount of particle crushing, the relative particle crushing rate was utilized as an indicator. The plastic work performed during the trials was compared across different stress routes, and the relationships between particle crushing and average stress p and generalized shear stress q were investigated. In triaxial testing, the differences in particle crushing behavior under different stress routes were thoroughly examined. The results show that particle crushing is substantially lower in the modelling of a rockfill dam constructed with a core wall and subjected to reservoir water pressure pathways than in conventional triaxial tests. A clear relationship was discovered. Across several stress routes, a clear link between plastic work and the relative particle crushing rate was detected, which was successfully characterized by a power function. Notably, the contributions of different stress components to the overall plastic work varied significantly. The plastic work exerted by the generalized shear stress q accounted for a large percentage of traditional triaxial testing, although the proportion of plastic work given by p and q changed significantly in complex stress path tests. This research enhances the understanding of the particle breakage characteristics of stone fill materials under complex stress paths.

Elastoplastic constitutive model for overconsolidated clays with an advanced dilatancy relation

AbstractThe dilatancy behavior of overconsolidated (OC) clays is a key factor in determining their strength and deformation characteristics. Recognizing the limitations of previous dilatancy relations for OC clays, a novel dilatancy relation is proposed that can effectively capture the changes in dilatancy point, volume dilatancy and contraction with the overconsolidation ratio (OCR). As OC clays revert to the normally consolidated (NC) state, the proposed dilatancy relation smoothly transitions to that of the modified Cam‐clay (MCC) model, ensuring a unified description of the dilatancy relation between OC and NC clays. The dilatancy relation can be easily incorporated into the constitutive model of different theoretical frameworks. Subsequently, this advanced dilatancy relation is integrated into a new elastoplastic constitutive model for OC clays within the framework of bounding surface and generalized plasticity theory. The validity of the proposed model is confirmed through drained triaxial compression and extension, undrained triaxial compression and extension, as well as complex stress path tests for clays with various OCRs, and the simulation results of the proposed model are compared with those of the SANICLAY model. The comparative analysis demonstrate that the model performs well in simulating the behavior of OC clays.

Study on the Unified Mechanical Properties of Ili Undisturbed Loess under the Influence of Soluble Salt

Through three stress path tests of unsaturated Ili undisturbed loess, the effect of soluble salt content on the deformation of net mean stress, suction, and deviated stress were investigated. The mechanical properties of the normalized compression curve, soil water characteristic curve and critical state line were revealed. The test results indicate that: in the isotropic compression test, the normal compression curves controlling different suctions can be characterized by using the initial void ratio and the yield net mean stress, and can be described as a two-parameter exponential function. In the triaxial shrinkage test, the soil water characteristic curves controlling vary net mean stresses are dimensionless by using saturated moisture and air entry value, and the normalization formula can be characterized by a single parameter exponential function. In the consolidation shear test, the corresponding effective net mean stress is calculated by suction and saturation. The critical state lines under the unsaturated condition controlling vary suctions can be described as the critical state line under the saturated condition on the plane of effective net mean stress and deviator stress. When the effective net mean stress is constant, the critical state lines under the unsaturated condition can be characterized by the degree of gas saturation and the ratio of unsaturated void ratio to saturated void ratio. The research will offer reference pointing at the regulation and utilization of water and salt in the loess region of Central Asia, so as to better guarantee the sustainable development of saline-alkali land project construction.

DEM simulations of critical state behaviour of granular materials under various drained triaxial stress path tests

DEM simulations of critical state behaviour of granular materials under various drained triaxial stress path tests

Mechanical behavior and strength criterion of frozen silty clay under complex stress paths

Frozen soil masses under various geologic conditions are usually in complex stress states and may carry complex loads with different stress paths. Thus, a comprehensive understanding of the mechanical behaviors of frozen soil under different stress paths is crucial to guide frozen soil engineering and develop the frozen soil mechanics. In this work, triaxial compressive tests under 32 stress paths were conducted on frozen silty clay, taking both global and local strain measurements. The test stress paths were composed of nine initial confining pressures and 14 stress increment direction angles, whose influences on the mechanical behavior were investigated. The coupling effects between them on the strength and deformation characteristics were revealed: the lower the initial confining pressure was, the stronger the effects of the stress increment direction angle on the strength and deformation were, and the larger the stress increment direction angle was, the stronger the effects of the initial confining pressure on the strength and deformation were. This coupling indicated that the influence of the stress path on the stress–strain response might depend on the change of density during loading: the denser the specimen was, the less the anisotropic it was. The uniqueness of the strain increment directions for a certain stress state reached through different stress paths was found, when assuming the direction of the strain rate coincided with that of the stress. An asymmetric failure function in the meridian plane was proposed, and then a three-dimensional strength criterion was constructed by combining the deviatoric-plane failure function of the unified strength criterion. The validity of the proposed criterion was examined by using stress-path test results of three types of frozen geomaterials in the literature.

Energy Evolution and Damage Characteristics of Rock Materials under Different Cyclic Loading and Unloading Paths

In order to study the deformation and failure characteristics of rocks under different cyclic loading and unloading paths, three stress path tests were conducted, and acoustic emission (AE) monitoring was conducted simultaneously. The mechanical characteristics and AE characteristics under different stress paths were analyzed, and the influences of the different stress paths on the energy dissipation and deformation damage were investigated. The law of energy evolution considering viscoelasticity under different stress paths was obtained. The concept of ultimate damage energy and its calculation method was proposed. The results show that the “hardening effect” of sandstone and granite under the constant lower limit (CLLCL) is the most significant in maximizing the mechanical property. The CLLCL imparts a stronger elastic property to rocks than the variable lower limit (VLLCL) does, while the VLLCL causes more damage to rocks than the CLLCL. A significant linear relationship between the proportion of damage energy and the proportion of elastic energy was discovered. Based on this linear relationship, the ultimate damage energy can be calculated for sandstone and granite. The evolution of the damage variable based on damage energy was compatible with the real damage condition, which validates the ultimate damage energy calculation method. The research results lay a theoretical foundation for the design and construction of geotechnical engineering.

Using true-triaxial stress path to simulate excavation-induced rock damage: a case study

This study presents an example illustrating the role of in situ 3D stress path method in simulating the roof damage development observed in the Mine-by tunnel at Underground Research Laboratory (URL) located in Manitoba, Canada. The 3D stress path, at the point 1 cm in the crown of the Mine-by tunnel, was applied to a cubic Lac du Bonnet (LdB) granite sample to further understand the roof damage process and the associated seismicity. After careful calibrations, a numerical model was used to reproduce the experiment, which produced similar seismicity processes and source mechanisms. Acoustic emission (AE) events obtained from laboratory and numerical modeling were converted to locations in relation to the tunnel face and were compared to the field microseismicity (MS) occurring in the upper notch region of the Mine-by tunnel. The crack development and damage mechanism are carefully illustrated. The difference between tests and field monitoring was discussed. The intermediate principal stress (σ2) unloading process was carried out in numerical simulation to investigate its role in rock damage development. The results clearly showed σ2 could play a significant role both in damage development and failure mode. It should be considered when predicting the damage region in underground excavations. This study highlights the potential role of laboratory and numerical stress path tests to investigate fracture processes and mechanisms occurring during engineering activities such as tunnel excavation.

Experimental quantification of 3D deformations in sensitive clay during stress-probing

Unique four-dimensional (4D) deformation data are collected during drained triaxial tests on intact specimens of a natural sensitive clay. This requires the development of a miniature triaxial cell for advanced stress path testing, specifically designed for X-ray computed tomography. Salient features include the omission of a membrane, and a mounting procedure that minimises disturbance by the experimenter. Three distinct drained stress ratios are studied for pseudo-isotropic, K0, and highly deviatoric loading paths. The results indicate that the K0path shows the most uniform deformation mechanism, where the measured ratio of deviatoric and volumetric strain increments reach the stress ratio applied at boundary value level for large magnitudes of total strain. The pseudo-isotropic test also reaches a strain ratio close to η at large total strain levels; however, the deformation field is less uniform. Furthermore, the highly deviatoric stress path shows the most heterogeneous deformation fields commensurate with the applied stress ratio, although the ratio of deviatoric and volumetric strain increments falls above the η applied. The mean value of the three-dimensional spatial fields of strain corresponds well with the changes observed at boundary level, supporting prior research on drained stress-probing on clays for which there are no 4D deformation data available.

Effect of strain rate on triaxial extension behavior of silty clay

Traditional methods of suction caisson foundation design only consider the soil is under either purely drained or purely undrained conditions and ignore the effects of the excess pore water pressure (EPWP). A series of triaxial extension stress path tests were conducted in this article to evaluate the effects of the loading rate on the development of EPWP and soil strength. The soil specimens were consolidated under pressures of 60, 80, and 100 kPa and then unloaded at seven different loading rates. It is found that the EPWP and soil strength are heavily influenced by the loading rate. The EPWP tends to be negative at small axial strain and then increases to be positive with the increase of axial strain. The normalized deviatoric stress versus loading rate curves are in the backbone curve. The minimum deviatoric strength happens when the normalized loading rate V ref=5 and linearly increases with the increase of consolidation pressure with the slope of −0.715. The magnitude of the slope of the critical state line in p′ – q space is heavily influenced by the loading rate.

Stress Path Tests with Local Deformation Profile in Flexible Boundary Plane Strain Device

AbstractStress path tests in plane strain (PS) conditions with varying stress ratios have been carried out using a newly developed flexible boundary (FB) multiaxial testing device. It employs elect...

Deformation characteristics of medium-dense sand-clay mixtures under a principal stress rotation

A moving wheel load induces a principal stress rotation (PSR) in pavement foundation geomaterials including subgrade/subbase soils. Simulating such a stress condition is not possible with stress path tests conducted with conventional cyclic triaxial (CT) equipment. More complex stress paths such as a heart-shaped stress path are required to determine the deformation characteristics of these under a PSR. A heart-shaped stress path can be simulated on a soil specimen in cyclic hollow cylinder (CHC) tests via user-defined waveforms for its stress components (axial stress, and torsional shear stress). In this study, a series of CT and CHC tests were performed to analyze the impact of a PSR on strain behaviors of medium-dense sand-clay mixtures. The specimens contained 0%, 5%, 10%, and 20% clay by weight and were prepared at an initial relative density of 50%. All specimens were anisotropically consolidated under K0≈0.5. It was determined that all CT specimens underestimated the strain performances (both axial strain and shear strain) of the sand-clay mixtures. On the other hand, a heart-shaped stress path was simulated successfully in CHC tests and thus, all specimens yielded more accurate strain results. At low clay content (≤10%), the impact of a PSR on strain performances of the sand-clay mixtures was observed to be less (axial strain (εz) < 0.12%, and shear strain (γzθ) < 0.8% after number of load cycles (N) = 5000) due to the low stress ratios (CVSR = 0.15 and η = 1/3). On the other hand, despite the low stress ratios, a PSR caused a rapid increase in axial strain and shear strain (εz = 5%, and γzθ > 0.8%) of the specimen containing 20% clay, which resulted in the failure of the specimen at N = 478. Results of this study clearly indicated that the effect of a PSR should be taken into consideration while evaluating the strain characteristics of the sand-clay mixtures that contain clay particularly at high contents (≥20%) under traffic loads.

Development of K0 Coefficient Measurement and Stress Path Test Function of Triaxial Apparatus

Development of K0 Coefficient Measurement and Stress Path Test Function of Triaxial Apparatus

Research on Deformation Mechanism of Underground Chamber Surrounding Rock Based on the Influence of Surrounding Rock Stress Path

During the construction of the pumped-storage power station, problems such as the stability of the surrounding rock of the underground chamber of the powerhouse appeared, which put forward higher requirements for the safety of the construction of the project. How to reasonably analyze and evaluate the stability of the surrounding rock of the underground chamber became a constraint to the smooth construction of the project Obstacles to progress. In response to this problem, a typical underground powerhouse chamber of a pumped storage power station was selected, and for the problems caused by the stress unloading of the surrounding rock during the excavation of the powerhouse chamber, field sampling, indoor triaxial unloading test, and numerical analysis were adopted. The stress path test results are applied to the numerical simulation analysis of different types of surrounding rock and different excavation conditions. The calculation analysis shows that the types of surrounding rock and the excavation conditions affect the arches, arch abutments and spans of underground chambers. Key positions such as the intersection of the arch bottom and the side wall have a significant impact. When the surrounding rock grade is the same, the displacement field distribution characteristics and maximum displacement of the two step-by-step excavation schemes are not much different; when the surrounding rock grade changes from type II to type III, the maximum displacements of the arch top and the bottom of the surrounding rock increase doubled.

Small-strain shear modulus and yielding characteristics of compacted high-plasticity clay

This experimental study describes the elastic and yielding response of compacted high-plasticity clay in conjunction with deformation characteristics at relatively larger strains for different stress paths and stress states. K0 consolidated undrained triaxial compression and extension stress path tests were conducted on compacted Nagpur cohesive soil with bender element measurements to explore the elastic and yielding response of soil. The experimental results showed that the stress path direction had a significant impact on the stress–strain–strength properties of soil as the peak deviatoric stress was found to increase with the increase in stress path angle. The measured small-strain shear modulus (Gmax) values were observed to be a function of the mean effective stress (p′) with slight dependency on deviatoric stress (q). Gmax was found to reduce with increase in strain level during shearing. An attempt was made to formulate an empirical correlation for compacted cohesive soil based on the elastic shear stiffness in pre-yield conditions. This correlation was further used to determine the yielding criteria of compacted cohesive soil along with the other two well-established criteria of stress–strain and strain energy.

Investigation of Lade‐Kim Plastic Potential Applicability under Various Stress Paths for Rockfill Materials

The dilatancy behavior of rockfill materials shows obvious stress path dependence. Lade‐Kim plastic potential equation has been proposed for a long time to model the mechanical behavior of sand and concrete materials. However, it lacks the verification of rockfill materials, especially under various stress paths. In this paper, the dilatancy performance of coarse‐grained materials under various stress paths is investigated, and then the dilatancy equation description and verification method based on Lade‐Kim plastic potential are given. The applicability of Lade‐Kim plastic potential for different stress path tests, such as conventional triaxial tests, constant P tests, and constant stress (increment) ratio tests, are verified and evaluated. It is found that Lade‐Kim plastic potential is difficult to consider the influence of stress path. Finally, the Lade‐Kim plastic potential, together with nonlinear dilatancy equation, is evaluated by changing the dilatancy equation in the framework of generalized plasticity. Lade‐Kim plastic potential is suitable for constant stress increment ratio loading experiments and special care should be taken when applied to other stress paths. These works are helpful to understand stress path dependence of dilatancy behavior for rockfill materials and is beneficial for the establishment of stress path constitutive model.

Discrete element modelling of strength and critical state characteristics of granular materials under axial compression and axial extension stress path tests

The critical state soil mechanics (CSSM) framework has been widely used across a range of problems in geomechanics involving complex loading conditions. However, the uniqueness of the critical state has been disputed for many years and it remains a controversial issue. Motivated by previous investigations, a series of discrete element method (DEM) simulations were performed under both axial compression (AC) and axial extension (AE) stress paths. All samples were isotropically compressed at varying mean normal effective stresses (confining pressures) and sheared to a large axial strain of approximately 60%. It is found that there exist unique values of critical void ratios and stress ratios under critical state, which are independent of the samples’ initial packings but dependent on stress paths. And the critical strength (stress ratio) for the AC stress path tests is higher than that for the AE stress path. The critical state lines (CSLs) are found to path-dependent but unique for each stress path. A unique linear relationship between the critical coordination numbers and critical void ratios is identified under the AC and AE stress paths respectively, but such a relationship depends on the stress paths. It is also found that there exist unique values of microscopic parameters in terms of deviator fabric under critical state, which are independent of the samples’ initial packings but dependent on stress paths. All these simulation results lead to the conclusion of non-uniqueness of CSLs from both macroscopic and microscopic viewpoints.

Experimental Study on Mechanical Properties of Soft Clay under Different Stress Paths

In actual engineering, when the soil element is subjected to load, the stress state will change accordingly. In this paper, undrained triaxial stress path tests for Ningbo reshaped saturated soft clay under different confining pressures are carried out. The test uses the GDS bidirectional vibrating triaxial instrument. The stress path dependence of saturated soft clay is studied. The stress-strain relationship and undrained shear strength under different stress path conditions are mainly analyzed. The test results show that the stress-strain relationship of the stress path test is obviously non-linear, basically strain hardening. The stress-strain curves of saturated remolded soft clay under different stress paths and consolidation pressures can be expressed in hyperbolic form. The initial stiffness E0 basically increases with the increase of the deflection angle of the stress path. The test results have certain reference significance for foundation pit and tunnel engineering.

Experimental study on the anisotropy of the effective stress coefficient of sandstone under true triaxial stress

The effective stress principle plays an important role in the study of the permeability evolution and mechanical behavior of coal and rocks. A key to evaluating the effective stress is determining the evolution of the effective stress coefficient α. However, conventional triaxial stress-path tests are not suitable for reproducing the actual stress state of coal and rocks and thus determining the effective stress coefficient. Therefore, this study was conducted to investigate the anisotropic characteristics of the effective stress coefficient for sandstone under different true triaxial stress and pore pressure conditions. The results show that the effective stress coefficient exhibits anisotropy due to the different principal strains in three directions and the anisotropy of pore structure, and it is closely linked with the pore pressure and principal stress. An increase in pore pressure causes sandstone particles that block the seepage channel to migrate, which reduces the sensitivity of permeability to principal stress, increasing the effective stress coefficient. When principal stress increases, pores and fractures are compressed, which reduces the sensitivity of permeability to pore pressure, resulting in a decrease in α. However, as principal stress continues to increase, effective stress coefficients can increase in the directions of major and intermediate principal stresses, while an α3 beyond unity appears in the direction of minor principal stress. This is related to the pore fluid flowing through highly compressible clay aggregates, in which the pore area is large, and the sensitivity of permeability to pore pressure is enhanced. The effective stress coefficient increases with an increase in permeability due to the effect of porosity. A new formula for calculating the volumetric strain of a linear elastic isotropic porous medium is established based on the anisotropy of the effective stress coefficient, which can be applied to experimental results.

Quantitative shear strength–consolidation stress–void ratio interrelations for reconstituted clays

Isotropically and anisotropically consolidated undrained triaxial compression shear tests are conducted on 87 specimens of eight kinds of clays reconstituted over a wide spectrum of initial water contents. These data in conjunction with those of 55 specimens previously published by the authors and their research team are used to investigate the shear strength–compressibility interrelation for reconstituted clays. The effects of the initial void ratio and the consolidation stress path on both the consolidated undrained shear strength–consolidation effective stress relationship and the compression curves in terms of the void ratio against the consolidation effective stress are investigated. It is found that the consolidated undrained shear strength–void ratio relationship is independent of the initial void ratio and the consolidation stress path, and a quantitative expression is proposed for determining the relationship as a function of the void ratio at the liquid limit. Meanwhile, the compression curves of reconstituted clays at different values of the initial void ratio and the void ratio at the liquid limit from different consolidation stress path tests can be normalised into a unique line with the application of the intrinsic compression framework. Accordingly, a quantitative expression can be established to determine the relationship between the consolidated undrained shear strength and the effective consolidation stress. The validity and accuracy of the proposed equations are discussed based on data from independent researchers and from the research team.

Microstructure investigation of soft clay subjected to triaxial loading

The microstructure properties of soft clay in Shanghai were discussed in light of experimental observations from a series of triaxial stress path tests and field emission scanning electron microscope (FESEM) tests performed on undisturbed soft clay specimens. The microstructure changes of soft clay were observed during different stress paths tests. The microstructure parameters were analysed by fractal geometry theory. The results show that the mechanical properties of soft clay are subject to the effect of stress paths, variations in initial confining pressure, which are controlled by its microstructure. Shanghai soft clay is manifested by a typical skeleton microstructure with large pores and flat laminated clay particles. The total volume and number of pores increase, and the distributions of soil aggregated particles and pores become looser due to loading; thus, the void ratio and fractal dimensions of the pores and soil particles are positively correlated with the axial strain. They reveal that cementation between the soil particles decreases and that the microstructure of soft clay is damaged by not only pore enlargement, but also degradation of the soil particles. Damage onset, damage development and failure occur during the entire process of soft clay subjected to triaxial loading. The microstructure parameters affected by reduced triaxial compression (RTC) tests change to a larger degree than those affected by conventional triaxial compression (CTC) tests, indicating that there is weaker cementation between the soil particles in the RTC tests due to the stress path and negative pore pressure generation. The microstructure evolution of natural soft clay during loading can be used to analyse the macro-mechanical behaviours.