Deviatoric Stress Levels Research Articles

Deformation characteristics of cement mortar under triaxial cyclic loading: An experimental investigation

The deformation behavior of cement mortar was examined using triaxial monotonic loading and triaxial cyclic loading tests. Triaxial cyclic loading tests were carried out with an increasing, decreasing, and constant deviatoric stress lower limit. The deformation and strength characteristics of cement mortar samples under different loading patterns were discussed. The elastic modulus and Poisson's ratio of each deviatoric stress level were analyzed and compared. Scanning electron microscope and energy dispersive spectroscopy tests were conducted on the failed cement mortar samples to examine the mechanical properties and failure mechanism for the various loading patterns.

DEM study of creep and stress relaxation behaviors of dense sand

The creep and stress relaxation behaviors under the triaxial shearing of an idealized granular soil were studied using the three-dimensional discrete element method, into which the rate process theory was incorporated to enable simulation of these two time-dependent physical processes by a high-performance computing facility. The model was validated by comparing the simulation results with published experimental data. The study is focused on the effects of deviatoric stress during triaxial shearing of the granular soils on their creep and stress relaxation behaviors. In particular, to gain insights into their micromechanical mechanisms, a careful examination was made of the relationship between the shear-strain localizations occurring during creep and triaxial shearing, and the evolution of the contact number, average contact forces, and shear-induced anisotropies during these two time-dependent processes. The results indicate that the deviatoric stress level strongly affects the rates of creep strain and stress relaxation and dictates whether creep rupture will occur. These two time-dependent processes of the granular soil are also found to be isotach, which is evidenced by a unique normalized stress–strain relationship derived from the creep and stress relaxation tests.

Experimental study of the mechanical behavior of calcareous sand under repeated loading-unloading

Understanding the mechanical behavior of calcareous sand under repeated loading-unloading is important to engineering design for island-reef underground caverns and pile foundation projects. In this study, we conducted conventional consolidated-drained triaxial shear tests on calcareous sand collected from the South China Sea under different effective confining pressures, and then, we performed multiple sets of triaxial repeated loading-unloading tests on calcareous sands with equivalent physical properties under different deviatoric stress levels and effective confining pressures. Based on the results, we conducted an in-depth investigation of the effects of the loading mode and effective confining pressure on the strain softening behavior and shear strength angle of calcareous sand. We studied the variation in the unloading rebound modulus as the effective confining pressure and deviatoric stress level changed, verified the applicability of the Dunkan-Chang model’s estimation equation for the unloading rebound modulus to calcareous sand, and established the relationship among the volume contraction during unloading, the effective confining pressure, and the deviatoric stress level. The results of this study provide an important basis for selecting appropriate parameters for island-reef construction engineering.

Laws of volumetric deformation and particle breaking of calcareous sand under cyclic loading

In order to study the change law of volume deformation and particle breakage of calcareous sand under cyclic loading, this paper carries out both conventional triaxial consolidated drained shear tests (the conventional tests) and triaxial cyclic loading tests on calcareous sand. The test results under these two stress paths were compared and showed that, compared with the stress path of the conventional tests, the stress path of cyclic loading aggravates the softening features of the calcareous sand. In each cycle, there is a hysteresis loop in the stress-strain curve, whose area is proportional to the deviator stress level at the initial stage of unloading. The stress path of cyclic loading suppresses the dilation trend of the sample under low cell pressure; the contraction amount is positively correlated with the deviator stress in the unloading phase. The two stress paths bring similar particle breaking law. Under the same cell pressure, the shear moduli of calcareous sand particles decrease with the growing cycle number. The manuscript clarified the law of deformation and particle breakage of calcareous sand under cyclic loading, which provided a theoretical basis for practical engineering.

Permanent deformation performance and a nonlinear constitutive model with thermal-mechanical coupling of PCPMA mixture

Polymers cracking products modified asphalt (PCPMA) pavement is a new warm mixing pavement technology. The permanent deformation properties of three types of PCPMA mixture were studied using repeated load triaxial (RLT) tests at different temperatures and deviator stress levels. The half sine wave intermittent load was adopted to simulate the vehicle load on the actual pavement. The test results indicate that the permanent deformation of the PCPMA mixture is increased gradually with the increase of temperature and stress level at the same load times. A robust skeleton structure can improve a specific resistance to the deformation of the PCPMA mixture. A ‘three-stage’ deformation characteristics, which are transfer section, stabilization section, and destruction section in proper order, exhibited under high temperature and significant stress level. For the condition of lower temperature and smaller stress levels, the permanent deformation only accumulates in the first two stages. The PCPMA was only damaged at 50 °C and 0.7 MPa, which has better durability than the matrix asphalt mixture. Accumulative residue viscoelastic strain and viscoplastic strain are derived from RLT test results. And then, a new viscoplastic mechanical model is proposed to predict the permanent deformation of the PCPMA mixture at different conditions, considering the nonlinear characteristics. Furthermore, this new mechanic model, which realized the unification of three and two stages deformation law of PCPMA mixture, may be effectively used for mechanical analysis and permanent deformation calculation of PCPMA pavement.

Influence of Deviatoric Stress on the Deformation and Damage Evolution of Surrounding Rock under Unloading Conditions

The study of the deformation and damage evolution behaviour of surrounding rock under excavation unloading conditions is of vital importance for a deep understanding of the mechanism of roadway failure. In this study, unloading testing using a partially hollow thick-walled cylinder cement mortar specimen with dimensions of 280 mm (height) × 200 mm (outer diameter) × 60 mm (inner diameter) and a solid height of 60 mm at the bottom was performed to investigate the deformation response and damage failure evolution characteristics of the surrounding rock. The experimental results showed that the higher deviatoric stress level accelerated the damage development caused by the unloading effect and improved the expansion rate of the internal cracks, which led to a higher radial strain rate, total strain, and acoustic emission hits. When deviatoric stress increased to a relatively higher level, the radial strain rates were highly unstable, and the surrounding rock near or at the opening free surface was damaged locally and regionally. During the failure process of the specimen, the generation of the deformation and damage in the unloading stage was more alive (as indicated by the growth rate). Nevertheless, the main deformation and damage to the surrounding rock were generated and accumulated in the maintaining stage after unloading.

Performance evaluation of recycled granular material as sustainable sub-base material

With the rapid increase in the population and rapid urbanization, the usage of the recycled aggregates for the civil engineering applications gained prime importance. One of the important steps for achieving sustainability is using recycled aggregates for pavement construction. However, it requires a comprehensive understanding regarding the characterization, applications, and limitations of those materials. Present study focusses on determining the permanent deformation behavior of the artificially prepared sub-base material under six deviatoric stress levels and three confining pressures. A comparative study was carried out for the sub-base material, which consists of natural aggregate and recycled aggregate. Also, the resilient behavior was examined for the complete characterization of the material. Results represented the suitability of recycled aggregate as an efficient sub-base material. VTT model and Power-law models suggested in literature were used to fit the experimental data obtained from cyclic triaxial test, and it displayed good fit.

Mechanical strength characteristics of fiber-reinforced pond ash for pavement application

The pavement construction sector throughout the world is facing a major challenge of non-availability of suitable construction materials. On the other side, many waste/sub-products are being generated from manufacturing/production units causing environmental pollution. One of such waste materials is pond ash produced from thermal electric power plants, which can be utilized as an alternative construction material in place of traditional materials. In this context, an attempt was made on fiber-reinforced pond ash as subbase layer of flexible pavements to understand its engineering behavior by conducting static (California bearing ratio, CBR) and dynamic (repeated load triaxial, RLT) tests. Randomly distributed polypropylene fibers of various proportions were used in pond ash as reinforcement inclusions. The test results were analyzed to address the influence of fiber on conventional parameters of pond ash such as CBR, resilient modulus (MR), and permanent strain (ϵp). Also, the influence of confining (σc) and deviatoric stress (σd) levels on MR as well as ϵp with load cycles have been investigated for both unreinforced and reinforced pond ash. The results obtained have shown that the inclusion of fiber improves the CBR, MR and ϵp characteristics of the pond ash. With an increase in stress levels (σc and σd), the MR of reinforced pond ash increased compared to non-reinforced ash. Irrespective of the fiber content in pond ash, permanent strain values were increased with an increase in the number of loading cycles and cyclic deviatoric stresses. However, with the introduction of fiber, the rate of strain development in pond ash was reduced significantly. Furthermore, the experimentally obtained MR and ϵp values were validated against existing models established by previous researchers. From the above findings, it can be concluded that fiber-reinforced pond ash can be used effectively in pavement layer applications.

Mechanical, acoustic, and fracture behaviors of yellow sandstone specimens under triaxial monotonic and cyclic loading

The mechanical, acoustic, and fracture behaviors of sandstone play important roles in design and stability analysis in rock engineering applications. In this research, a series of triaxial monotonic and cyclic tests was conducted on yellow sandstone. The results of triaxial monotonic experiments were used to analyze the influence of the confining pressure on the mechanical parameters of the sandstone. The strength and deformation parameters depended strongly on the confining pressure. The triaxial strength of the sandstone specimens under cyclic loading was approximately equal to that under monotonic loading, whereas the fatigue loading led to a reduction in triaxial strength. Under lower deviatoric stress levels, the specimen was highly resistant to fatigue loading; however, higher deviatoric stress level lead to fatigue failure of the specimen even though after large cycle numbers. The influence of the cycle number on the deformation and crack damage parameters of the sandstone was also investigated. The internal damage of the sandstone specimens were characterized on basis of the recorded P-wave velocity and acoustic emission (AE) counts during triaxial monotonic loading, and the effect of the cycle number on the AE behaviors was explored under triaxial cyclic loading. Finally, the fractured yellow sandstone specimens were further analyzed by using X-ray micro computerized tomography (CT) scanning. The internal fracture characteristics of the sandstone specimens were investigated by cross-section analysis and three-dimentional (3D) reconstruction of the CT scanning, which revealed the fracture mechanism of the sandstone.

The 3‐D Spatial Distribution of Shear Strain Energy Changes Associated With the 2016 Kumamoto Earthquake Sequence, Southwest Japan

AbstractShear strain energy is an essential physical quantity governing earthquake generation. To calculate the shear strain energy changes due to an earthquake, we need both coseismic stress changes and background crustal stress. The orientation of the background stress can be estimated from earthquake focal mechanisms, but its absolute level is still uncertain. Assuming the level of the background deviatoric stress to be frictional strength in the crust, we evaluated the three‐dimensional distribution of shear strain energy changes associated with the 2016 Kumamoto earthquake sequence, southwest Japan. Its spatial patterns strongly depend on the background stress level. From the energy balance of shear faulting, we proposed that the volume integral of the shear strain energy changes could constrain the background deviatoric stress level. It should be >14 MPa at 10‐km depth at the very least. We showed that approximately 75% of aftershocks occurred where the shear strain energy increased.

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.

Permanent deformation behaviour of black cotton soil treated with calcium carbide residue

Uncontrolled dumping of huge quantities of calcium carbide residue (CCR) which is an industrial by product of acetylene gas industry is hazardous to environment due to its high alkalinity. Utilization of calcium carbide residue for stabilization of subgrade soils is a cost effective and sustainable practice, which can be adopted in design and construction of flexible pavements. In the present work, the efficacy of treatment with CCR in reducing the accumulation of permanent deformations in black cotton soil (BC soil) subgrade subjected to repeated loading is investigated. Untreated black cotton soil samples and CCR stabilized samples are subjected to repeated load triaxial tests for determining the accumulation of plastic strain with repeated loading cycles. Effect of cyclic deviatoric stress levels and moulding water content on the development of permanent deformation is examined. The accumulation of permanent deformation is quantified using the critical cyclic stress levels based on the shake down theory. The experimental data for the permanent deformation of treated as well as untreated soil samples is found to fit well with two the models (VTT model and power law model) reported in literature. The efficacy of CCR stabilization in reducing the permanent deformations of black cotton soil subjected to repeated loading is confirmed based on the mineralogical and morphological studies carried out on the treated samples.

The role of pore water plays in coal under uniaxial cyclic loading

Geofluids are common in underground coal mines and have a significant influence on the deformation of coal rock, where the pore pressure of geofluids varies during swelling or shrinkage processes. In order to characterize the role of pore water in coal, the uniaxial behavior of saturated coal under cyclic compression was investigated considering different loading rates, amplitudes, and numbers of cycles. The results show that Young's modulus (E) increased in the first cycle and then stabilized in the remainder of loading cycles. Also, the increment of E in the first cycle was inversely correlated with the loading rate. For the first time, it is found that the radial residual strain gradually decreased in each subsequent loading cycle. This is different from that of water weakening property of coal by immersion of the coal in water without pressurization. In the unloading phase, the pore pressure decayed more slowly than the stress, leading to a high level of deviatoric stress. As a result, the acoustic emission events appeared even at the minimum of cyclic loading. The transverse relaxation time (T2) distribution shows that the pore water pressure mainly changed the primary pores, while its influence on the secondary pores was negligible. From a poromechanical perspective, high pore pressure developed in pores of poor connectivity during initial cyclic loading, causing stress orientation rotation and tensile damage. With further cyclic loading, the pore water was continuously discharged as the pores were compacted into flat cracks. As a result, the onset stress of dilatancy gradually increased in the subsequent loading cycles due to the enhanced frictional resistance to crack slip.

Long-Term Evolution of the Deviatoric Background Stress in the Focal Region of the 2011 Tohoku-Oki Earthquake

On 11 March 2011, a giant interplate earthquake of magnitude Mw 9.1 struck Tohoku-Oki area in eastern Japan. The earthquake is acknowledged to have permanently altered the stress field in and around the focal region. To capture the temporal change of the overall stress field, I examined 1251 focal mechanisms, from the NIED MT catalog, that occurred before and after the Tohoku-Oki earthquake from January, 2006 to January, 2017 in the focal region near the subducting slab. The examined observations were grouped into six periods, and based on the selected NIED MT catalog, the stress regime in each period was obtained by using the damped stress tensor inversion method. Based on the temporal evolution of stress rotation, the corresponding deviatoric stress level was estimated using a simplified 2D model. Results of the 10-year seismic stress cycle show that several years before the Tohoku-Oki earthquake, the stress accumulation level seems to have experienced an acceleration process. Studies suggest that this increasingly critically stress state combined with the sufficiently reduced coupling rate off the Tohoku area finally resulted in the unprecedented megathrust event. The coseismic process was violent and released almost all of the deviatoric stress that presented before the main shock. The resultant stress state even reached frictional overshoot. Thus, the postseismic stress pattern in the source region was reshaped significantly, especially for the upper plate and updip portion of the lower plate. After the main shock near the rupture surface, a surprisingly rapid and high-level stress reloading occurred within several postseismic years. To reconcile the classical subduction zone earthquake generation cycle model, the event may be described as an instantaneously decoupled stress state between the upper and inner plates.

Creep Behaviors of Methane Hydrate-Bearing Frozen Sediments

Creep behaviors of methane hydrate-bearing frozen specimens are important to predict the long-term stability of the hydrate-bearing layers in Arctic and permafrost regions. In this study, a series of creep tests were conducted, and the results indicated that: (1) higher deviator stress (external load) results in larger initial strain, axial strain, and strain rate at a specific elapsed time. Under low deviator stress levels, the axial strain is not large and does not get into the tertiary creep stage in comparison with that under high deviator stress, which can be even up to 35% and can cause failure; (2) both axial strain and strain rate of methane hydrate-bearing frozen specimens increase with the enhancement of deviator stress, the decrease of confining pressure, and the decrease of temperature; (3) the specimens will be damaged rather than in stable creep stage during creeping when the deviator stress exceeds the quasi-static strength of the specimens.

A rheological model to simulate the shear creep behavior of rockfills considering the influence of stress states

Rockfills have been extensively used in the construction of high-fill embankments and dams, which exhibit complex time-dependent behavior. This paper presents a rheological model to simulate the shear creep behavior of rockfills based on the classic Burgers model, in which a nonlinear elasto-damage unit, a modified Newton unit, and a modified Kelvin unit are connected in series. By analyzing the results of large-scale triaxial creep tests on different rockfills, the influence of stress states (i.e., the deviator stress level and confining pressure) on the shear creep behavior of rockfills is revealed. A method to modify the key model parameters is proposed to consider such influence and is incorporated into the proposed model. Furthermore, the accelerated creep behavior of rockfills is reproduced by a nonlinear elasto-damage unit. Despite its simplicity, the model can simulate the essential aspects of the shear creep behavior of rockfills, and reasonable consistency is found between the simulation results and the large-scale triaxial creep test results.

Mechanical Behavior of a Granite from Wuyi Mountain: Insights from Strain-Based Approaches

A series of monotonic and cyclic loading tests were conducted on granite samples from Wuyi Mountain to investigate the mechanical behavior under different confinements and damage states. The irreversible axial and radial strains were computed to characterize the development of the damage state, and three characteristic stages were detected according to the average gain rate. The evolution of the apparent deformation parameters with the deviator stress level was investigated to determine the linear elastic stage, but no substantial progress was achieved. In addition, two typical approaches (transitional region and same gradient region of hysteresis) were also found to be unsuitable for determining the elastic parameters to complete the cyclic loading test. Therefore, a secant approach was proposed based on volumetric strain to identify the lower bound of the nominal linear elastic stage with the upper bound established by linear regression. The results indicated that both elastic parameters and stress thresholds showed positive correlations with confinement. With the accumulation of the damage state, the peak stress and crack initiation threshold decreased, crack damage threshold and elastic parameters first increased and then decreased, and the crack closure threshold showed a tendency to decrease–increase–decrease. In addition, the strain softening stage showed a higher slope and minor stress drop value under larger confinement, and a transition from plastic compaction to plastic dilatancy was also detected.

A Straightforward Procedure to Characterize Nonlinear Viscoelastic Response of Asphalt Concrete at High Temperatures

This paper proposes a straightforward procedure to characterize the nonlinear viscoelastic response of asphalt concrete materials. Furthermore, a model is proposed to estimate the nonlinear viscoelastic parameters as a function of the triaxiality ratio, which accounts for both confinement and deviatoric stress levels. The simplified procedure allows for easy characterization of linear viscoelastic (LVE) and nonlinear viscoelastic (NVE) responses. First, Schapery’s nonlinear viscoelastic model is used to represent the viscoelastic behavior. Dynamic modulus tests are performed to calibrate LVE properties. Repeated creep-recovery tests at variable deviatoric stress levels (RCRT-VS) were designed and conducted to calibrate the nonlinear viscoelastic properties of four types of mixtures used in the Federal Aviation Administration’s National Airport Pavement and Materials Research Center test sections. The RCRT-VS were conducted at 55°C, 140 kPa initial confinement pressure, and wide range of deviatoric stress levels; mimicking the stress levels induced in a pavement structure under traffic. Once calibrated, the model was validated by comparing the model predictions and experimental measurements at different deviatoric stress levels. The predictions indicate that the proposed method is capable of characterizing NVE response of asphalt concrete materials.

DEM study on the macro- and micro-responses of granular materials subjected to creep and stress relaxation

The behavior of a typical crushable granular material, i.e., rockfill, during creep and stress relaxation is studied using the discrete element method. Rockfill particles are modeled as breakable agglomerates with time-dependent bond strengths. Parameters are derived from experimental results and reasonable macro-responses are reproduced. The deviator stress level is found to have significant influence on the evolution of the agglomerate breakage number and mechanical coordination number during creep, whereas these two numbers increase slightly during stress relaxation in all situations. The relationship between the macro- and micro-responses is explored. The micro-mechanism of the observed nonisochronous phenomenon is also investigated.

Resilient and permanent deformation behaviour of clayey subgrade soil subjected to repeated load triaxial tests

Mechanistic-empirical design of flexible pavement AASHTO practice requires resilient characteristics of sub grade soil. Though the resilient modulus is assumed to account for plastic strain accumulation, some subgrade soils with more fine content undergo excessive plastic deformation under repeated loads even though they have remarkable resilient properties. Hence permanent deformation is also an important parameter which should be considered in the design process. In the present study, permanent deformation behaviour of clayey subgrade soil with intermediate compressibility is studied using repeated load triaxial tests. The influence of confining pressure, moisture content and deviator stress levels on the permanent deformation behaviour is examined. The transition of the soil sample from stable to unstable state can be observed and critical stress levels established with the help of shake down theory. Two regression models reported in the literature, Power law model and VTT model, are used to predict the plastic strains with a number of load cycles and stress levels as the variables.