Triaxial Cyclic Loading Tests Research Articles

Correlating ballast resilient modulus with particle movement through SmartRock sensing

The resilient modulus of ballast is a key parameter in assessing track quality and is influenced by various loading conditions, such as deviator stress, bulk stress, loading frequency, and confining pressure. These factors, when incorporated into different models, can sometimes complicate their application in track maintenance planning. Therefore, a simplified model that collectively considers these factors would be beneficial in practical applications for estimating the ballast resilient modulus. This study utilizes smart sensing technology, SmartRock, to track particle movement during large-scale triaxial cyclic loading tests, exploring the potential correlation between ballast resilient modulus and particle motion. The findings indicate that the settling acceleration of ballast particles significantly impacts the ballast resilient modulus. This effect is dependent on the applied cyclic loading pulses, an aspect previously overlooked in research. Consequently, an index termed the Cyclic Loading and Acceleration Index (CLAI) has been introduced. CLAI integrates the effects of deviator stress, loading frequency, loading pulses, and settling acceleration. Based on CLAI, a simplified model for ballast resilient modulus has been developed and validated.

Behaviour of Model Stone Column Subjected to Cyclic Loading

Behaviour of saturated soft clay deposits subjected to cyclic stresses is of considerable importance in the design of railway infrastructure, since these deposits are characterised by very low bearing capacity and excessive settlement. Stone column is a well-established method that used to improve both bearing capacity and reduce settlement of such soil. However, much of the work to date has dealt with the behaviour of stone columns under static loading. Little is known about their response to cyclic loading. A laboratory based study was undertaken to investigate the response of both column and the surrounding soft soil to cyclic loading using a series of static and cyclic loading triaxial tests under undrained condition. Testing was performed on normally consolidated specimens of soft clay (undrained shear strength of ≈ 12 kPa, diameter 100 mm, and 200 mm height) reinforced with 28 mm diameter stone columns. Effects of loading frequencies at range of cyclic stress ratios were examined. It was found that the cumulative strain developed was dependent on the cyclic stress ratio, but independent of frequency from 0.5 to 3 Hz.Overall, stone columns have shown the potential to strength subgrade when exposed to cyclic loading, and hence reduce both capital costs (reduce the ballast needed for subbase), and operational cost through extended maintenance period.

The cyclic loading resistance of old railway track sub-ballast materials at different water contents

Water content is one of the most important factors influencing the performance of earth structures. It is well known that the water content of sand has a significant effect on its shear strength and its resistance to cyclic loading. In this study, the loading resistance of four different sand materials was investigated in a cyclic load triaxial test at different water contents. Three of the four material samples were taken from the sub-ballast layer of an existing Finnish railway track. The laboratory tests confirmed that the resistance to cyclic loading decreased with an increasing saturation level for all tested materials. The weakest material in the cyclic loading triaxial test had the most uniform grain size distribution with a small median grain size. The calculation of the vertical stress increase in the sub-ballast layer revealed that even a fine-grained sand can withstand the train load well when the rolling stock is mostly passenger traffic with an axle load of 160 kN and the sub-ballast layer is not completely saturated. However, heavy freight traffic with axle loads of 225 kN and 250 kN can approach the bearing capacity of a weak sub-ballast material if the water content of the upper part of the sub-ballast layer is high. This study has shown that the top part of the sub-ballast layer should be kept below 60–70% saturation level to avoid the development of excessive permanent deformation.

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.

Influence of true triaxial stress paths on mechanical properties of marble

The rock brittle failure is the result of the stress redistribution caused by the excavation of engineering construction, which can be estimated by cyclic loading tests. In this study, a series of true triaxial tests with different stress paths were conducted to investigate the influence of the variations in three principal stresses on the mechanical properties of Jinping marble. The complete stress-strain curves can be obtained due to the strain control in the monotonous loading tests whereas only the pre-peak stress-strain curves can be acquired using the stress control under cyclic loading conditions. In the true triaxial cyclic loading tests, the irreversible strains can be obtained accurately and used to describe the damage evolution of Jinping marble. Test results show that the irreversible strains have the different change trends in the different stress paths and their changes are most sensitive to the minimum principal stress. In addition, Jinping marble in the cyclic loading of the minimum principal stress test has the largest failure angle and numbers of the failure fragments, which indicates that Jinping marble becomes more brittle under this condition.

Geotechnical characteristics of lateritic clay admixed with biomass silica stabiliser

This paper examines the effectiveness of using varying proportion (2%, 4% and 6% by dry weight of soil) of a newly formulated biomass silica soil stabiliser (BSS) on tropical lateritic clay which has deleterious effects on construction materials due to its high amounts of SiO2 and Fe2O3. A series of laboratory experiment testing methods: i.e. particle size distribution, Atterberg Limits, linear shrinkage, pH value, compaction, California Bearing Ratio, unconfined compression test, and cyclic load triaxial test were conducted in accordance to BS1377:1990 and AASHTO T307–99:2003 to examine the geotechnical properties of the BSS structured lateritic clay subjected to 0, 7, 14 and 28 curing days. Physicochemical tests such as X-ray fluorescence (XRF), X-ray diffraction (XRD) and field-emission scanning electron microscope (FESEM) were conducted to further support the mineralogical composition and microstructure evidence of the structured soil and substantiate the interpretation of the mechanical strength data. The test results reveal that the notable changes in the properties and behavior of structured lateritic clay was governed by the BSS proportion and curing period. There is an immediate enhancement in the geotechnical properties of the 2% BSS structured lateritic clay, altering soil type from CH to MH before curing begin. The unconfined compressive test results indicate a distinct effectiveness in stabilizing lateritic clay, from 400 kPa to 3675 kPa for 6% BSS structured lateritic clay cured to 28 days. Both soaked and unsoaked California Bearing Ratio (CBR) values were 12.4 and 5.8 folds higher than that of the virgin lateritic clay. The cyclic load triaxial test reveals that the resilient modulus, MR of the structured soil was significantly increased. For full-scale applications, the association between the mechanical properties and linear shrinkage test results deduce that the optimum proportion of BSS required for an effective stabilisation performance is 6%. Finally, the mechanisms of the microstructural changes in the stabilisation process were mainly attributed to the agglomeration and aggregation of the cemented particle through composition of calcium silicate hydrate cementitious products evidenced in the physicochemical tests.

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.

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.

Geomechanical assessment of an inert steel slag aggregate as an alternative ballast material for heavy haul rail tracks

The geomechanical behaviour of railway ballast is a key aspect in the performance of ballasted tracks, mainly under increased axle loads and/or train speeds. Recently, and driven by the circular economy paradigm, some researchers have evaluated the use of alternative ballast materials as replacement for traditional natural crushed rocks. This work presents a comparison between the geomechanical behaviour of a granite aggregate and the one of an electrical arc furnace steel slag aggregate, with the trade designation of ‘Inert Steel Aggregate for Construction’ (ISAC). Both materials were designed for heavy haul railway track applications and showed similar particle size distribution (PSD) curves established by the standard gradation AREMA N. 24. Laboratory tests were performed on scaled down ballast specimens to evaluate both the macro-structural behaviour under cyclic loading triaxial tests (long-term permanent deformation and resilient modulus under various stress paths) and the micro-structural behaviour (particle breakage in cyclic loading and single-particle crushing strength). With a view to analise the structural response of the railway track and to compare the influence of the two aggregates, numerical simulations by FEM are presented, using static loading and non-linear elastic material models for both the ballast and the sub-ballast layers. The models were calibrated on basis of laboratory test results, to obtain the elastic vertical displacement on the rail top level and the stress distribution on the track foundation layers. The results suggest that the use of steel slag aggregate as ballast material is promising. When compared with the granite aggregate, the ISAC demonstrated to have a higher crushing strength of its particles; a greater tendency to stabilize the permanent deformation (PD); a lower particle breakage after PD tests; and a higher resilient modulus. The results of the numerical modeling showed that the rail presented less maximum elastic vertical displacements (approximately 3% on average) and that the foundation soils underwent less maximum vertical stresses (approximately 4% for a 32.5 t/axle loading and 9% for a 40 t/axle loading), in the model with ISAC in the ballast layer.

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.

Investigation on the mechanical properties of a calcareous sand: the role of the initial fabric

This study examines the effect of the initial fabric on the coefficient of the lateral earth pressure at rest K0, the undrained monotonic loading strength and the liquefaction resistance of a calcareous sand from a reclamation land in Persian Gulf. K0 consolidation, undrained compression and extension and cyclic undrained loading triaxial tests are performed on laboratory samples reconstituted by air and water pluviation, dry and moist tamping and dry funnel deposition methods. Test results show that the air pluviation and moist tamping samples own the highest and the lowest K0 respectively. All the samples at medium loose and medium dense states show strain hardening when subjected to undrained monotonic loading. The air pluviation samples are more contractive than the other samples at the phase transformation points and the moist tamping samples exhibit an over-consolidated behavior. In the cyclic loading tests, the air pluviation and the moist tamping samples exhibit the lowest and the highest liquefaction resistance. It is shown that the evaluation and comparison of the cyclic undrained strength of various sands should take the sample preparation method into consideration scrupulously. The test results also indicate that although the mechanical property of soil sample is influenced by the fabric differences induced by sample preparation method, other factors such as homogeneity and stress history should not be neglected.

A new evaluation index for reliquefaction resistance of Toyoura sand

In 2011 off the Pacific coast of Tohoku Earthquake, liquefaction and reliquefaction were observed along 600 km of the Northeast Coast of Japan and the infrastructure in this area experienced severe damage. During the earthquake, the fact that reliquefaction in aftershocks may also cause severe damage to infrastructure draws much attention of the researchers specialized in liquefaction. Researches in the literature indicated that if a sand experienced liquefaction, strong stress-induced anisotropy would occur, which could greatly influence the reliquefaction behavior of sand in aftershocks. Nevertheless, the influence of stress-induced anisotropy on mechanical behavior of sand subjected to cyclic loading has not yet been investigated in laboratory tests. For this reason, a systematic test program was conducted with sequential undrained triaxial cyclic loading tests on Toyoura sand, in which, all the samples were firstly subjected to the same undrained cyclic loading until liquefaction, after which the liquefied samples were fixed at different residual strain and consolidated. Then, the consolidated samples were again subjected to the cyclic loading under undrained condition. It is found that the residual strain will greatly influence the reliquefaction behavior of sand, which clearly indicates the importance of the stress-induced anisotropy formed during the first liquefaction. Based on the testing results, a new index for evaluating the reliquefaction resistance was proposed.

A dynamic strength criterion for frozen sulfate saline silty clay under cyclic loading

The strength properties of frozen saline soils under cyclic loading are more complex than those of unfrozen soils and frozen salt-free soils due to the existence of salt crystals in soil matrix. In this paper, a serious of tri-axial cyclic loading tests are conducted for frozen sulfate saline silty clay (FSSSC) with different Na2SO4 contents under different confining pressures and loading dynamic stress ratios at the temperature of −6 °C, respectively. The experimental results illustrate that the dynamic strength of FSSSC decreases with the increase of axial strain. The critical state line of FSSSC under cyclic loading is moving constantly. The dynamic critical state lines of FSSSC with different salt contents are different under same axial strains. The dynamic friction angle of FSSSC is related to salt content, loading dynamic stress ratio and confining pressure. Based on the experimental results, a dynamic strength criterion for FSSSC under cyclic loading is proposed, and the method of determining dynamic strength parameters is given. This strength criterion not only can reflect the effect of loading dynamic stress ratio and salt content on dynamic strength, but also can describe the influence of high confining pressure on dynamic strength.

Undrained Cyclic Laboratory Behavior of Sandy Soils

The complex cyclic shear stress path experienced by the soil during an earthquake, which could also induce liquefaction phenomena, can be approximated in the laboratory only by using sophisticated testing apparatuses. Cyclic triaxial tests have been widely used, especially for coarse grained soils, as in this study. In the framework of the design for the seismic retrofitting of the ‘‘Ritiro viaduct’’ foundations along the A20 motorway connecting Messina with Palermo (Italy), a soil liquefaction study was also carried out. With this aim, a detailed geological and geotechnical characterization of the area was performed by in situ and laboratory tests, including seismic dilatometer Marchetti tests (SDMTs), the combined resonant column (RCT) and cyclic loading torsional shear tests (CLTSTs), and undrained cyclic loading triaxial tests (CLTxTs). In particular, the paper presents the results of cyclic triaxial tests carried out on isotropically consolidated specimens of a sandy soil. The seismic retrofitting works include the reinforcement of the foundation and replacement of the decks with newly designed type and structural schemes, mixed steel, and concrete with continuous girder. During the investigation, data were acquired for the characterization of materials, for the definition of degradation phenomena with the relative identification of possible causes, and for the estimation of the residual performance characteristics of the building. The structural campaign of investigations necessary to determine all of the key parameters useful for a correct definition of the residual performance capabilities of the work was divided into two phases: One in situ and one in the laboratory.

A constitutive model for rock based on energy dissipation and transformation principles

The study on constitutive relations is of great significance to the understanding of the complex mechanical properties and failure criteria of rock. The establishment of a scientific rock constitutive model can provide theoretical support for engineering safety and geological disaster prevention. From the view point of energy, a new rock constitutive model under triaxial compression was developed by connecting the law of energy transformation with the mechanical properties of rock in the paper. First, the concepts of elastic energy conversion rate and elastic energy conversion function were proposed to describe principles of energy dissipation and energy transformation in the whole stress-strain process of rock. And, a mathematical description was made of the work of external force transforming into elastic energy. Second, assumptions were proposed to describe the evolution laws of some important mechanical parameters. Finally, the quasi-static constitutive model for rock under triaxial compression based on conservation of energy was established, combining theories of damage mechanics and thermodynamic, and the fitted results of this constitutive model were compared to experimental data of sandstone under triaxial cyclic loading tests. The rationality of the constitutive model for rock was proved by describing the whole stress-strain process of triaxial compression under different confining pressures. And, the model can describe the initial compaction stage and residual stress of stress-strain curve, which can reflect the nonlinear characteristics of rock.

Experimental investigation of the mechanical properties of fine-grained sandstone in the triaxial cyclic loading test

Fine-grained sandstone is widely distributed in the Xiangjiaba Hydropower Station. To investigate the mechanical properties of the sandstone, a series of triaxial cyclic loading tests were conducted under the confining pressures of 5 MPa, 15 MPa, and 20 MPa. According to the mechanical characteristics of the sandstone, a typical stress–strain curve can be divided into five stages: initial compression stage, linear elastic stage, strain hardening stage, strain softening stage and residual strength stage. Initial yield strength, peak strength, and residual strength linearly increase with the increasing of confining pressure, respectively. The initial yield stresses $$ {\sigma_{\text{y}}} $$ are approximately 69.0–74.0% of the peak strengths; while the residual strengths $$ {\sigma_{\text{s}}} $$ are 39.5–43.5% of the peak strengths under various confining pressures. Based on the experimental results, the strength and deformation parameters are obtained and analyzed. The strength parameters are calculated according to the Mohr–Coulomb criterion. The cohesion increases at first and then decreases with the increasing of internal variable $$ \kappa $$ . The friction angle first gradually decreases, then it increases to the maximum and finally decreases to a stable value with the increasing of internal variable $$ \kappa $$ . The maximal variations of friction angle and cohesion are approximately 29.1% and 62.5%, respectively. Variations in elastic modulus decrease, respectively, 10.3%, 6.1%, and 6.8% pre-peak strength under different confining pressures, and the variations in Poisson’s ratio are, respectively, 71.1%, 36.2%, and 63.5%. The nonlinear mechanical behaviors of fine-grained sandstone are the result of the accumulation of irreversible deformation and damage evolution. Moreover, the hysteresis loop is obvious with the accumulation of irreversible deformation and damage evolution. Damage starts to evolve when the stress level is more than the damage threshold. The experimental investigation can enhance the knowledge of the nonlinear behaviors of rock material.

Cyclic load triaxial test procedures for soils in transportation engineering

Unbound granular base course is built in pavement structures between subgrade and bound base course. Its main function is bearing capacity - load transfer from upper layers to subgrade. For its successful design, one of the required parameters is resilient modulus. Resilient modulus describes behaviour of unbounded materials at various load conditions, and is defined as the ratio of the applied cyclic stress to the elastic strain. It can be determined with cyclic triaxial test that gives good simulation of stresses and strains which are result of traffic loads. In this paper resilient modulus testing procedure will be described with the emphasis on European Standard (EN 13286-7). Furthermore, it will be given comparison of European standard with other test procedures.

Influence of Load Frequency on Cohesive Soil Respond

The mechanism of cohesive soils response to cycling loading is less investigated compared to cohesionless soils. Multiple load-unload cycles cause significant changes in the structure of cohesive soils, which result in complex behaviour under the given load. The aim of the paper was to investigate and study the influence of load frequency on cohesive soil reaction. In order to obtain results, tests were conducted using the cyclic triaxial apparatus. Three cyclic tests were carried out, each for different frequency −0.5 Hz, 1.0 Hz, 2.0 Hz and one static triaxial test. The maximal value of deviator stress qmax, used in the cyclic tests, was set to 40 kPa. Afterwards samples were unloaded to qmin = 30 kPa. Cyclic loading triaxial tests were performed in a consolidated-undrained (CU) one-way loading manner, a sinusoidal waves were used. After the cycling loading was completed, a static triaxial shear test was conducted. Changes in the cohesive soil responses depending on cycling load frequency were presented in the paper. Differences in the accumulation of plastic strains were noticed, as well as changes of degradation index values, resilient degradation index values and differences in the excess pore water pressure development.

Absence of Stress‐Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite

AbstractKnowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress‐induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behavior is elastic up to high stress ( of rock strength) and nondilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at 90–95% of their compressive strength. Microcrack damage is extremely localized near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by “mode II” shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite, is comparable to that in granite. The mechanical behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.

A simple permanent deformation model of rockfill materials

Existing experimental results have shown that using a semi-log linear relationship between the permanent volumetric strain and cyclic number underestimates the volumetric deformation of rockfill materials with a large cyclic number, and that the error increases with the confining pressure. The existing permanent deformation models are not suitable for the seismic safety analysis of high dams during strong earthquakes. In this study, a series of large-scale triaxial cyclic loading tests of rockfill materials were performed, and a new permanent deformation model of rockfill materials was developed and validated with three kinds of rockfill materials. The results show that the proposed model can properly reflect the general features of the permanent deformation of rockfill materials. The main features of the model are as follows: (1) relations between the cyclic number and permanent volumetric/shear strain are described by hyperbolic functions, which can avoid underestimating the volumetric deformation occurring during strong earthquakes; (2) the model can capture the effect of the mean effective stress on the permanent volumetric strain, with greater confining pressure correlating to greater permanent volumetric deformation, and the permanent volumetric strain under low confining pressure near the dam crest can be well represented; and (3) the model can reflect the effect of the consolidation stress ratio on the permanent shear strain.