Cyclic Triaxial Compression Tests Research Articles

Factors influencing static and cyclic behaviour of polymer-grouted sands

ABSTRACT In this study, thorough laboratory research was conducted in order to increase knowledge of the beneficial effect of water-soluble epoxy resin on the monotonic or cyclic triaxial strength of different sands. Seven sand fractions (with different particle sizes, uniformity coefficients and mineralogical compositions) were injected with solutions of epoxy resin and water, at ratios of 3.0, 2.0 and 1.5. To evaluate the influence of chemical treatment on the static strength behaviour of grouted sands, a series of monotonic triaxial tests on dry specimens and isotropically consolidated undrained (CIU) triaxial tests, were conducted on samples cured for 90 days. CIU cyclic triaxial compression tests were also performed, to assess improvements in the cyclic resistance of treated sands. The experiments showed that this resinous grouting resulted in an appreciable increase in cohesion values, particularly for fine sands and well-graded sands, ranging from 0.15 to 0.66 MPa. Despite the fact that friction angle values were reduced, the overall shear strength increased over a wide range of stresses. Moreover, all grouted sands exhibited a substantial improvement in cyclic properties, with failure occurring at significantly elevated stress levels and after enduring a larger number of loading cycles, compared to their untreated counterparts.

Energy Storage and Release of Class I and Class II Rocks

As underground excavations become deeper, violent rock failures associated with the sudden release of elastic energy become more prevalent, threatening the safety of workers and construction equipment. It is important to figure out the energy-related failure mechanisms of rocks. However, the energy evolution across the complete deformation of different types of rocks and the effect of high confinement on energy storage and release are not well understood in the literature. In this study, a series of cyclic triaxial compression tests were conducted for Class I and Class II rocks to investigate the confinement-dependent characteristics of energy evolution. The results showed that three types of energy evolution were identified as the rock behavior changed from brittle to ductile. The energy storage limit was linearly enhanced by confinement. The nonlinear increase in dissipated energy at peak stress with increasing confinement was suggested to indicate the start of the brittle–ductile transition. The post-peak fracturing process was characterized using the ratio of the local withdrawn elastic energy and fracture energy, and a novel energy-based index was proposed to quantify the failure intensity of the rock. This paper presents a complete investigation of the energy conversion characteristics of the rock, which may shed light on the failure mechanisms of violent rock failures in underground projects.

Linear energy storage and dissipation laws and damage evolution characteristics of rock under triaxial cyclic compression with different confining pressures

Triaxial cyclic compression tests with different confining pressures were conducted to explore the energy distribution characteristics of rock and their responses to confining pressure. The pre-peak input strain energy, pre-peak elastic strain energy, and pre-peak dissipated strain energy at different stress levels were computed based on the resultant stress−strain curves. The results indicated that under different confining pressures, both the pre-peak elastic strain energy and pre-peak dissipated strain energy were linearly related to the pre-peak input strain energy, and thus the linear energy storage and dissipation laws in triaxial cyclic compression were confirmed. The energy storage coefficient in triaxial compression of the rock was positively correlated with the confining pressure, whereas the corresponding energy dissipation coefficient exhibited an opposite correlation. Using the linear energy storage law, the peak elastic strain energy and peak dissipated strain energy of rock in triaxial compression were deduced. Furthermore, the damage evolution characteristics of rock under different confining pressures were analysed by considering the linear energy dissipation law.

Investigation of the Usability of Foamed Bituminous Mixtures as Bituminous Base Course

Materials recycled with foam bitumen are promising materials for pavements economically and environmentally. However, there are different opinions in the literature about how these materials are positioned in the pavement. It is clear that their performances will not be suitable for the wearing course, but that they are used for the plant mixture base course because they contain bitumen, and that these materials will be neglected in the evaluation of these materials. In the study, 4 different foam bituminous mixes prepared with a recycled pavement material and a standard bituminous base course mix were compared in terms of the resilient modulus obtained from the uniaxial indirect tensile resilient modulus test. Gradation is the same for three of the mixtures, two of them use cement as a mineral binder, but 70/100 grade bitumen in one of these two productions and 50/70 grade bitumen in the other. In the third mixture, 70/100 grade bitumen and hydrated lime (HL)+fly ash (FA) were used. In the final mixture, 70/100 grade bitumen and cement were used as mineral binders, but gradation was changed. In addition, permanent deformation control for the final production was carried out with a triaxial cyclic compression test. It was concluded that all these variables affect the results, but the resilience modulus values obtained for all were quite close to those obtained from the bituminous base sample. Considering that the foamed bituminous mixtures will be thicker than the bituminous base course in practice, it is considered structurally appropriate to use this layer instead of the bituminous base course.

Development of low-strength stabilization method for fouled ballasted tracks

In ballasted tracks on railways, fine fraction content (hereinafter referred to as “Fc”) mixed in ballast increases due to repeated train loads or track repair work using ballast tampers. When the Fc increases and becomes “fouled ballast”, settlement of ballasted track tends to increase even after track repair, which results in a rise in maintenance frequency. In such cases, ballast replacement is the standard measure applied to reduce frequency of maintenance. However, ballast replacement is costly, and railway operators therefore need countermeasures to prevent large settlement which are both low cost and quick enough to implement during periods of track work carried out at night. This study describes the development of a new repair method which reduces sleeper settlement by mixing super-quick hardening cement and polymeric stabilizer (hereinafter referred to as “stabilizing materials”) with fouled ballast. This method uses a ballast tamper and weakly stabilizes fouled ballast to allow tamping even after stabilization, hereinafter referred to as the “low-strength stabilization method”. This study investigates the relationship between sleeper settlement and amount of stabilizing materials needed for this low-strength stabilization method, and examines the settlement reduction mechanism through cyclic loading triaxial compression tests. Confirmation of settlement reduction was conducted through full-scale cyclic loading tests.

High temperature effect of foamed concrete under equal displacement increment triaxial cyclic compression

Foamed concrete (FC) has been extensively used for its lightweight, good thermal insulation, and sound insulation properties. It has gained broad investigations in the past decades, but the data of triaxial cyclic compression tests are lacking. In this study, triaxial cyclic tests were conducted under different confining pressures (0 MPa, 0.3 MPa, 0.5 MPa, 0.8 MPa) with four temperature treatments (25 °C, 100 °C, 200 °C, 300 °C) to obtain the failure modes and stress-strain curves, analyze the compressive strength, residual strength, elastic modulus and brittleness index, explain the fracture mechanism and damage evolution of FC from the perspective of energy dissipation. Furthermore, the strength prediction model of FC was proposed and its yield criterion was verified. The results show that the damage pattern of FC was significantly influenced by confining pressure and the mechanical parameters were more sensitive to temperature. The energy density and damage increased linearly with the number of cycles, and the dissipated energy accounted for a large proportion under low confining pressure or high temperature. The Crushable-foam yield criterion had good applicability to FC, and thermal damage decreased the hydrostatic yield strength of FC.

Effect evaluation of drainage condition and water content on cyclic plastic deformation of aged ballast and its estimation models

With the ballast aging, the changes in the size and shape of the ballast particle reduce the drainage capacity, as well as cause a greater permanent deformation of the railroad ballast. Therefore, it is meaningful to investigate the effect of aging on the mechanical behavior of unsaturated ballast, and to estimate the cyclic plastic deformation by considering the aging effects. Here, “aging effect” means the increase in fine fraction content and the particle shape becomes rounded and smooth as compared with fresh ballast. In this study, the influence of aging on the cyclic plastic deformation of unsaturated ballast was evaluated through a series of cyclic loading triaxial compression tests. Test results indicate that the cyclic plastic deformation of ballast is seriously affected by water content, fine fraction content and drainage condition, and the increasing trend becomes more remarkable at the water-rich aged ballast under the fully undrained condition since the effective confining pressure decreases due to the generation of excess pore water pressure. Furthermore, the applicability of two types of estimation models (i.e., a semi-empirical model named University of Illinois at Urbana-Champaign (UIUC) model, and an elasto-plastic model named subloading surface extension (SSE) model) to the prediction for cyclic plastic deformation of unsaturated ballast is also verified in this study by comparing with results of cyclic loading triaxial compression tests. As the result, it is revealed that the UIUC model is suitable for predicting the cyclic plastic deformation of fresh ballast (or slightly aged ballast) with different water contents under the fully drained condition, and the SSE model shows good potential to estimate the cyclic plastic deformation of fresh and aged ballasts by considering the effects of water content and drainage condition. The findings of this study indicate that drainage conditions have a significant effect on predicting the cyclic plastic deformation of aged ballast, and appropriate test conditions for triaxial compression tests (i.e., CD and CU tests) should be selected according to hydraulic properties (i.e., permeability and water retentivity) of the aged ballast.

Pore water pressure model for sands reinforced with randomly distributed fibers based on cyclic triaxial tests

In this study, the pore water pressure accumulation laws in sand reinforced with randomly distributed fibers was investigated through cyclic triaxial compression tests. The effects of relative density, cyclic stress ratio, fiber content, and fiber length were examined. The test results showed that reinforcing the sand with randomly distributed fibers effectively delayed the pore water pressure accumulation and significantly increased liquefaction resistance. When the test conditions remained unchanged, the pore water pressure accumulation rate of the fiber-reinforced sand decreased as the fiber length and fiber content increased, and reached the lowest when fiber content reached 0.6% and fiber length attained 12 mm. Whereas, the accumulation rate of fiber-reinforced sand increased as the cyclic stress ratio increased, and reached the highest when relative density reached 30%, followed by 50% and 70%. Based on the test results, a three-parameter pore water pressure model that accounts for the effects of fiber content, fiber length, relative density, cyclic stress ratio, and sand particle diameter was established, and a method for estimating the parameters a and b was recommended. The predictions agreed relatively well with the test results, suggesting that the model is applicable to analysis the pore water pressure development laws in fiber-reinforced sands.

Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests

Cyclic loading–induced deformation of soil is a common problem in engineering practice. In current practice, however, monotonic triaxial tests are more commonly used due to the availability of apparatus and ease of operation. Thus, it would be very useful and practical if monotonic triaxial tests could be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress–strain surface in a three-dimensional p′–q–εa space (p′, effective mean principal stress; q, deviatoric stress; εa, axial strain). It was also found that it is possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure is proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against additional test data from the previous literature.

Mechanical Behavior of Berea Sandstone under Cyclic Loading: An Application to Dynamic Loading of a Wellbore

Summary During the drilling process, many operations will cause cyclic loading of the wellbore, including surge/swab pressure generated by tripping operations and pressure fluctuations induced by shutdowns/startups of the mud pumps. With cyclic loading, the wellbore rock may fail even if the stress level is lower than the predetermined static rock strength because of fatigue. In this paper, comprehensive triaxial cyclic compression tests have been conducted under different maximum cyclic loading levels, confining pressure, amplitude, and frequency. New methods have been proposed and successfully applied to predict the monotonic strength of the rock samples. These approaches overcome the uncertainty caused by the standard average method. Using these experimental results, some empirical models (amplitude and fatigue life, frequency, and fatigue life) are proposed. Also, the evolutional laws of rock parameters (strain, residual strain, energy dissipation, etc.) during the cyclic loading stage are generated. A clear three-stage behavior is observed: early stage, steady stage, and late stage. Finally, the experiments are combined with two field-drilling cases. Results show to avoid rock fatigue because of tripping operations and shutdowns/startups of the mud pump, the maximum loading level should be less than 95% of the static strength for Berea Sandstone used in this study. Therefore, the maximum equivalent loading stress should avoid being close to the failure boundary. A safe buffer zone should be designed between the wellbore collapse boundary and actual bottomhole pressure.

Kaiser Effect in Sandstone in Polyaxial Compression with Multistage Rotation of an Assigned Stress Ellipsoid

The authors examine Kaiser effect in sandstone in nonproportional triaxial cyclic compression tests with stage rotation of an assigned stress ellipsoid through an angle of 90°. The load program consists of three pairs of cycles such that the maximal nominal stress of the second cycle exceeds the first cycle stress by 20 MPa at the constant side support. Cyclic loading is applied to sandstone in three orthogonal directions, with two cycles in each direction. Kaiser effect only appears in the second loading in the same direction, and activation of acoustic emission upon the change in the active loading direction is independent of the earlier reached stress level. This fact points at the orientation-driven nature of Kaiser effect, which means the material remembers its lattermost internal damaged structure.

An anisotropic bounding surface elastoplastic constitutive model for frozen sulfate saline silty clay under cyclic loading

To study the mechanical properties of frozen saline soil under cyclic loading, the cyclic tri-axial compressive experiments of 36 samples are performed for frozen saline silty clay with 0.0, 0.5, 1.5 and 2.5% of Na2SO4 contents at the temperature of −6 °C, respectively. The experimental results illustrate that the effect of the salt contents and confining pressures on the strength and deformation properties of frozen sulfate saline soils under cyclic loading is significant. To understand the deformation characteristics of frozen sulfate saline silty clay (this kind of saline soil), a bounding surface elastoplastic constitutive model is presented based on the test results and bounding surface plasticity theory. The proposed model includes the effects of salt contents and confining pressures on dynamic characteristics, therefore, it can suitably describe the dynamic stress-strain response of frozen saline soil under cyclic dynamic loading. This proposed model includes: (1) Critical stress ratio calculation formula, considering the effect of salt contents and confining pressures, is proposed. Critical stress ratio and bounding surface shape parameter varying with confining pressures are introduced into plastic potential surface function and bounding surface function to describe pressure melting and crushing characteristics of frozen soils under high confining pressures. (2) The influence of the initial anisotropy and the induced anisotropy by dynamic loads on bounding surface of rotation hardening is considered. A non-associated flow rule together with a mixed hardening rule is used to describe the plastic flow rule and hardening rule of soils during cyclic loading, respectively. (3) A radial mapping rule with mobile mapping origin is employed to simulate elastoplastic properties of soils during cyclic loading. By comparing its simulated results with the test results of cyclic triaxial compression tests, it is found that the proposed model in this paper can well predict deformation characteristics of this kind of saline soil under cyclic loading.

Mechanical properties and stress\u2013dilatancy relationships of unsaturated soil under various cyclic loading conditions

Most studies investigating the effect of cyclic loading on soil properties have been conducted for saturated soils. Embankments such as fill dams, roads and railways are usually constructed by unsaturated geo-materials and retained under unsaturated conditions during their in-service periods. Then when the stabilities of the embankments against dynamic motions such as earthquakes and traffic loads are evaluated, it is necessary to account for the cyclic properties of unsaturated soils. However, there are few studies investigating mechanical properties of unsaturated soils under cyclic loadings. There are two objectives in this paper. One objective is to investigate cyclic properties of an unsaturated silt under various cyclic loading conditions, while the other is to investigate the stress–dilatancy relationships; the relation of plastic strain increment ratio, − dεvp/dγp, versus stress ratio, q/p′; and to derive the plastic potential function of the unsaturated silt. Cyclic triaxial compression tests under various loading conditions were performed using the unsaturated silt. The material used is an artificial silty soil named DL clay. It was found from the series of the tests that the stiffness of the soil increased with an increase in suction and the number of cyclic loadings. The total volume reductions in the specimens decreased with an increase in suction. When the numbers of cyclic loadings and suction increased, the dilation also increased. Each unique stress–dilatancy relationship could be found in both loading and unloading processes. The relationships were similar to those of saturated soils under cyclic loadings. A unique plastic potential function could also be derived from the stress–dilatancy relationships.

Evaluation of effect of limited pore water pressure development on cyclic behavior of a nonplastic silt

In this study, an experimental procedure was adopted to evaluate cyclic behavior of silts of pore water pressure ratios less than 50%. A commercially available nonplastic silt was used to eliminate the effects of plasticity. All the specimens were isotropically consolidated under 100 kPa and later subjected to cyclic triaxial compression tests. In this regard, dependence of cyclic stress ratio (CSR), initial relative density (Dr) and cyclic axial strain (εsa) on pore water pressure generation pattern – either at small or large deformation levels – were evaluated. It was inferred that, volume changes in silt due to the reconsolidation after cyclic loading with an induced pore water pressure ratio (Ru) less than 50% shows limited liquefaction.

Research on Characteristic Stress and Constitutive Equation of Confined Sandstone during Damage Evolution Based on Energy Evolution Analysis

This study implements the cyclic loading-unloading triaxial compression tests with confining pressures of 5, 10, 20, and 30 MPa for determining the characteristic stress and constitutive equation of sandstone during damage evolution. The energy evolution characteristics and transformations are analyzed, the energy proportion evolution law is defined and analyzed, and its process is divided into five stages according to the whole stress-strain curve. Then, the dissipation energy proportion method for determining the characteristic stress is proposed and compared with the lateral strain method and the volumetric strain method, and the differences of characteristic stress determined by the three methods are within the acceptable error range, i.e., the proportion of crack initiation stress and crack damage stress to peak stress are all about 40% and 70%, respectively, and least affected by the confining pressures. The new method has strong maneuverability, clearer response to the damage evolution, better coordination with the damage evolution, and less subjective initiative. In addition, the dissipation energy proportion behavior is captured by fitting function and established a relationship with elastic modulus of sandstone, and then the damage evolution constitutive equation of sandstone is established based on energy proportion. Finally, the theoretical curve and experimental curve of damage evolution are compared according to the five stages of dissipation energy proportion evolution, and the result shows that the damage evolution constitutive equation is reasonable.

Determination of Holmquist–Johnson–Cook Constitutive Parameters of Coal: Laboratory Study and Numerical Simulation

The main sensitivity parameters of the Holmquist–Johnson–Cook constitutive model for coal were obtained from a variety of tests such as uniaxial compression, uniaxial cyclic loading, splitting and triaxial compression tests, as well as the indirect derivation equation of a briquette. The mechanical properties of briquettes under dynamic impact were investigated using a split Hopkinson pressure bar experiment. Based on the experimental measurement of the Holmquist–Johnson–Cook constitutive model, the numerical simulation of briquette was performed using ANSYS/LS-DYNA software. A comparison between experimental and simulation results verified the correctness of simulation parameters. This research concluded that the failure of briquette at different impact velocities started from an axial crack in the middle of the coal body, and the sample was swollen to some extent. By the increase of impact velocity, the severity of damage in the coal body was increased, while the size of the coal block was decreased. Moreover, there was good compliance between experimental and simulated stress wave curves in terms of coal sample failure and fracture morphology at different speeds. Finally, the parameters of the validated Holmquist–Johnson–Cook constitutive model were applied to the numerical simulation model of the impact damage of heading face and the process of coal seam damage in the roadway was visually displayed. The obtained results showed that the Holmquist–Johnson–Cook constitutive model parameters suitable for the prominent coal body were of great significance for the improvement and exploration of the occurrence mechanism of coal and rock dynamic disasters.

Dynamic behavior of soil and rock mixture using cyclic triaxial tests and X-ray computed tomography

This paper aims at investigating the meso-scale mechanisms that result in damage evolution in soil and rock mixtures (SRM). Although the macroscopic cyclic stress–strain response of SRM has been well investigated, the mesoscopic damage cracking mechanisms are still not well understood. In this work, artificial SRM sample with a rock block percentage (RBP) of 30% (mass ratio) was investigated by carrying out cyclic triaxial compression tests with a constant stress amplitude and low strain level, under tomographic monitoring using a 450 kV industrial X-ray computed tomography (CT). A series of 2D CT images were obtained at different loading stages throughout the test and from different positions in the sample using CT scanning. The results show that the hysteresis loop presents a sparse–dense–sparse pattern caused by the accumulation of plastic strain during the test. Besides, parameters of damping ratio and resilient modulus vary as increasing cycles, much energy loss, and decreasing resilient ability occur with large plastic strain. The study also finds that a linear relationship exists between the hysteresis loop area and total crack area extracted from the CT images. Volumetric dilatancy caused by the damage cracking behavior has closed link with the evolution of hysteresis loop, which are controlled by the meso-structural changes during sample deformation.

Dynamic properties of frozen silty soils with different coarse-grained contents subjected to cyclic triaxial loading

In cold seismic regions, the dynamic properties of frozen silty soils with different coarse-grained contents have great influence on the stability of core-wall dam in hydraulic engineering subjected to seismic/repeated loading. In order to investigate their dynamic behaviors, the materials were prepared with silty soils incorporating different percentages of coarse grains, and the mass ratios of coarse particles to silty soils for the samples are 0%, 20%, 40%, 60% and 80%, respectively. A series of cyclic triaxial compression tests at the temperature of −6°C were carried out with 1.0 Hz frequency by employing the modified MTS-810 apparatus with confining pressures ranging from 0.3 to 4.0 MPa. Laboratory results demonstrate that the coarse-grained contents have great effect on the static and dynamic behaviors of frozen soil mixtures. With the increasing coarse-grained contents, the static strength first decreases to a smaller value and then increases slightly. When subjected to cyclic loading, the resilient modulus increases all the time, whereas the damping ratio presents conversely a decreasing tendency. As the loading cycles increase, the accumulative plastic strain presents an accelerating development at the virgin stage and then gradually approaches the failure strain, whose evolving rule can be well predicted by the modified Monismith model, and the rate of accumulative plastic strain descends first and then reaches a stationary value. The dynamic strength increases with the increase of confining pressure, and the damage evolution can be denoted by the axial strain method in the process of cyclic loading.

Undrained dynamic behaviour of peaty organic soil under long-term cyclic loading, Part I: Experimental investigation

To investigate the undrained cyclic behaviour of peaty organic soil under long-term cyclic loading, a series of undrained cyclic triaxial compression tests was conducted. The permanent axial strain, accumulated excess pore pressure as well as secant shear modulus during the loading cycles were studied and their dependencies on variables such as confining pressure, cyclic deviator stress and static deviator stress were summarized. Distinctive characteristics of the peaty organic soil under long-term cyclic loading were presented in comparison to a soft clay from Wenzhou. For a given stress state, the permanent strain over the initial limited loading cycles was smaller for the peaty organic soil than for the inorganic soft clay for the higher static strength of peaty organic soil. The peaty organic soil displayed distinct viscous nature and higher permanent strain rate under long-term cyclic loading. Due to the highly decomposed and often amorphous organic components in the peaty organic soil, which results in a weak structure strength, the development of permanent strain was found to be insensitive to confining pressure. In addition, for a given shear strain, the secant modulus of peaty organic soils increased with decreasing organic content and degree of decomposition or with increasing confining pressure. While the equivalent damping ratio was found to generally decrease as the confining pressure, organic content or degree of decomposition increased.

Characterization of the deformation behavior of fine-grained sandstone by triaxial cyclic loading

In order to study the deformation behavior of fine-grained sandstone subjected to cyclic loading, a series of triaxial cyclic compression tests were performed under different confining pressures in the laboratory. Each of the tests combined constant-amplitude cyclic loading and increasing-amplitude cyclic loading. The experimental results showed that the evolutions of the elastic modulus and Poisson’s ratio with cycles had 3 variation modes according to the applied deviatoric stress. Within a given stress level, the axial and lateral irreversible strain initially increased with a decreasing rate, followed by steady-state increases before failure. When the specimens approached failure, an accelerated increase in the irreversible strain occurred. Under lower stress levels, the volumetric strain was characterized by compaction-dominated behavior. However, it changed from compaction to dilation within each loading cycle under higher stress levels. SEM (scanning electron microscope) tests were conducted on unruptured specimens to study the deformation mechanism of the fine-grained sandstone during cyclic loading. This marks the beginning of a study that aims to provide a reference to the design, construction, and operation of hydropower stations in China.