Large-scale Triaxial Apparatus Research Articles

Large-Scale Triaxial Test on Mechanical Behavior of Coral Sand Gravel Layered Samples

Layered structures comprising coral sand and gravel have been observed in hydraulic filled foundations in the coral reefs in the South China Sea, leading to anisotropy in their physical and mechanical properties. However, the effect of a layered structure on the strength and deformation of the coral soil foundation remains unclear. In this study, a series of large-scale triaxial compression tests and step-loading tests were carried out on four types of samples, i.e., clean coral sand, clean coral gravel, sand-over-gravel layered sample, and gravel-over-sand layered sample, to investigate the impact of confining pressure and the layered structure on the strength and failure modes of these soils. The results indicate that the stress–strain relationships of all samples predominantly exhibit strain hardening under drained conditions. Under identical confining pressures, the peak strength of clean coral sand is the lowest, while that of coral gravel is the highest. The peak strengths of the two layered samples fall between these extremes, with the gravel-over-sand layered sample exhibiting higher strength. All four samples have similar peak friction angles, slightly exceeding 40°. The difference in peak strength among the four types of samples is attributed to the variations in cohesion, with the cohesion of clean coral gravel being up to four times that of clean sand, and the cohesion of layered samples falling between these two. Both clean sand and clean gravel samples exhibit a bulging phenomenon in the middle, while the layered samples primarily exhibit bulging near the coral gravel layer. In the step-loading tests, the bearing capacity of the layered samples falls between those of clean coral sand and coral gravel, with the gravel-over-sand layered samples demonstrating higher strength. Moreover, the p-s curve of the gravel-over-sand layered samples obtained from the large-scale triaxial apparatus under a confining pressure of 400 kPa resembles that from the plate load tests on the same samples.

Sustainable Performance of Recycled Rubber and Mining Waste Utilized for Efficient Rail Infrastructure

AbstractUtilizing waste byproducts from mining industries and recycled rubber as alternate materials in railway tracks promotes sustainability of transportation infrastructure, while also increasing track longevity by reducing ballast degradation. This paper provides an overview of two such applications including (i) rubber-intermixed ballast stratum (RIBS) by replacing 10% ballast aggregates with granulated rubber particles with the particle sizes carefully selected according to Australian Standards, (ii) synthetic energy absorbing layer using a mixture of steel furnace slag, coal wash and rubber crumbs to replace traditional capping layer. These materials when tested using large-scale triaxial apparatus and field trials proved that tracks with waste materials performed better than the conventional ballasted tracks by reducing ballast breakage and exploiting the higher damping potential of these materials. Though the vertical deformations of the track slightly increased by using these materials albeit within the specified standards, the overall stability improved by reduced dilation and track vibrations. Increasing the life of ballast layer can lead to long-term cost benefits by saving millions of dollars in track maintenance and provide environment benefits through minimizing quarrying of natural rock aggregates and reducing the carbon footprint of mining industries.

Discrete Element Simulation of Macro and Micro Mechanical Properties of Round Gravel Material under Triaxial Stress

In this study, the effects of relative density and confining pressure on the shear characteristics of round gravel are investigated using a large-scale triaxial apparatus and the discrete element method. A simple and efficient numerical method for simulating flexible membranes is introduced. The results show that the stress-strain curves develop from hardened to softened type with increasing relative density, while the stress-strain curves develop from softened to hardened type with increasing confining pressure. As the axial strain increases, the strong contact force chains are vertically distributed, and the larger the relative density and confining pressure, the greater the number and thickness of the strong contact force chains. In the shear process, the distribution of average normal and tangential contact forces show “peanut-shaped” and “petal-shaped”, respectively. The increase in relative density increases the anisotropy of the specimen, while the increase in confining pressure results in a decrease. A linear relationship exists between the macroscopic stress ratio and the anisotropy coefficient. The anisotropy coefficient of the normal contact force provides the greatest contribution to the macroscopic shear strength (about 55%), followed by the anisotropy coefficient of the contact normal (about 26%) and that of the tangential contact force (about 19%).

Study on the shear and deformation characteristics of geogrid-reinforced gravelly soils based on large-scale triaxial tests

Geogrid reinforcement has a limiting effect on the lateral deformation and thus improves the shear strength of the soil, the overall strength of the soil and the overall stability of the corresponding geotechnical structure. In this study, large-scale triaxial tests without and with geogrid reinforcement were conducted on three typical gravelly soils in Xinjiang using a large-scale triaxial apparatus. The shear strength and deformation characteristics of gravelly soils with different particle shapes and the stress-strain relations, strength characteristics, damage patterns, and reinforcement effects of gravelly soils with and without reinforcement were investigated. Geogrid reinforcement effectively enhances the strength of the soil; the internal friction angle remained relatively constant with and without reinforcement, whereas the cohesive force increased significantly. The reinforcement effects interpreted from the results obtained from the triaxial tests were discovered when a certain deformation or relative displacement with the reinforcement materials of the soil occurred. Under uniform test conditions, the volumetric strain of the samples of gravelly soil with reinforcement significantly decreased with increasing confining pressure, and the difference in volumetric strains with and without reinforcement was greater when the confining pressure was higher. The highlight of this study is its significance in explaining the reinforcement mechanism in gravelly soils and in selecting engineering design parameters.

Applicability evaluation of a two-dimensional piezoelectric transducer to monitor dynamic soil stress in unbound granular materials of road engineering

In road engineering, the applications of piezoelectric ceramics on road energy harvesting or electronic sensing have drawn many attentions. However, the application potential of piezoelectric sensor on UGMs (unbound granular materials) in road is unclear. The main problem is the applicability of piezoelectric equation considering that the soil structure of UGMs and the working environment that the transducer may suffer may influence the transferred stress on the piezoelectric transducer and then the electricity generation. In this study, a two-dimensional piezoelectric transducer made by PZT-5H piezoelectric ceramics was installed in the sample of UGMs, and a series of cyclic tests were performed on the UGMs samples by a large-scale triaxial apparatus. This test method is specially designed for the case that the piezoelectric transducer is installed in UGMs of pavement base/subbase layers and then undergo traffic loading under complex working environment. Test results show that the electricity generation is primarily dependent on the magnitude and frequency of cyclic loading, while the influences of other factors are limited. The piezoelectric equation to calculate the open-circuit voltage is deduced and verified under the complex working environment. This indicates that the piezoelectric equation is applicable to the case that the applied stress on the piezoelectric transducer is transferred from the soil medium, and the two-dimensional piezoelectric transducer has a sensor potential to monitor the dynamic vertical and horizontal soil stresses under traffic loading.

Thermal Effects on Shear Characteristics of Unbound Granular Materials under Drained Conditions

In this study, the thermally induced changes in the shear characteristics of unbound granular materials (UGMs) were investigated. A series of drained shear tests were conducted using a large-scale triaxial apparatus under controlled temperatures. The thermal dependence of the peak strength, critical strength, internal friction angle, and dilatancy of UGMs were investigated under different temperatures, initial mean effective stresses, and relative compaction. Dense UGMs tended to expand during heating and slightly contract during cooling. In the heating and cooling process, a nonlinear relationship was observed between thermal deformation and temperature. Thermal deformation was attributed to the deformation, movement, and rotation of soil particles and closely related to the initial mean effective stress and relative compaction. A thermal softening effect was observed during the shear test, and the peak strength decreased with increasing temperature. However, negligible thermal effects were observed relative to residual strength. The effect of temperature on the shear characteristics of UGMs was primarily related to the thermal expansion of pores in the soil skeleton. The relationship between peak strength and initial thermal volumetric strain was found to be exponential. Considering the thermal effect, a normalized semi-empirical model of the parabolic Hvorslev envelope for dense UGMs was proposed to supply guidelines for road base design.

Influence of Coarse Grain Content on the Mechanical Properties of Red Sandstone Soil

Coarse-grained red sandstone soil is often used as embankment filling material but is prone to being broken by extrusion, which lowers the stability of the roadbed. This paper aimed to clarify the influence of the variation in coarse-grain content on the mechanical properties of coarse-grained red sandstone soil. Soil with a grain size greater than 5 mm is regarded as coarse-grained soil, and coarse-grained red sandstone soils with different contents of coarse grains were prepared as cylindrical specimens with a diameter of 300 mm and a height of 600 mm. Under three different confining pressures, a large-scale triaxial apparatus was used to carry out triaxial shear tests. The results showed that as the content of coarse grains of red sandstone (denoted as p) increased, the deviation stress of static failure increased, showing a hyperbolic relationship. The internal friction angle also increased hyperbolically, while the cohesion reached a peak value and then decreased, and the maximum value of 133.8 kPa was reached at p = 30%. As the content of coarse grains increased, the maximum dilatancy increased. The maximum amount of shrinkage reached a peak value and then decreased, and the maximum value was reached when p = 30%. A coarse grain content p equal to 30% was the optimum value when coarse-grained red sandstone soil was used as a filling material.

Experimental study on the road energy harvesting of piezoelectric ceramic in unbound granular materials based on a large-scale triaxial apparatus

Experimental study on the road energy harvesting of piezoelectric ceramic in unbound granular materials based on a large-scale triaxial apparatus

Influence of rockfill particle breakage on long-term settlement of embankment dams

The current research was implemented to monitor and rehabilitate the structural health of Masjed-E-Soleyman dam (MESD). With a height of 177 m, the MESD is one of the highest rockfill dams in Iran, and suffers from extensive and unpredictable settlements and related cracks. To investigate the non-homogeneous (conglomerate) rockfill's brittle behaviour in before–after and during triaxial tests, a series of laboratory tests under conventional stress paths were carried out using a large-scale triaxial apparatus. A numerical time-dependent model was used to find a relationship between particle breakage and the time-dependent settlement of the embankment dam. The core aim of this work was to investigate particle breakage of the conglomerate material in the MESD and to identify the non-homogeneous relative particle breakage index (RPBI) by establishing a relationship between the RPBI for this material and plastic work. The research was divided into three significant steps. The breakage index (before–after tests) was calculated in the first step. The relationship between the breakage index and plastic work (during tests) was then introduced. Finally, the future crest settlement of the MESD was predicted using a numerical viscoelastic Burgers model that was calibrated using accurate monitoring data of the dam.

A gradation-dependent particle shape factor for characterizing small-strain shear modulus of sand-gravel mixtures

In this study, a series of shear wave velocity tests were conducted on two types of sand-gravel mixtures with a wide range of particle size distributions. The effect of the particle shape on the small-strain shear modulus (Gmax) was investigated using a customized, large-scale triaxial apparatus. The results showed that Gmax increases with the decrease in particle roundness when the uniformity coefficient (Cu) is less than a certain value. However, this increasing trend gradually flattens with the further increase of Cu. Thus, the effect of particle shape on the Gmax varies with the particle gradation. The performances of six relationships proposed in the literature for predicting Gmax were compared and evaluated in terms of their predictive capabilities. An empirical Gmax model based on the Hardin and Black equation was proposed by including a Fourier-based shape factor modified by Cu. The model can characterize the difference in the variation of Gmax with Cu due to different particle shapes and was thereafter validated. Furthermore, the model serves as a reference guide for estimating the Gmax of soils with various particle size distributions through their particle characteristics.

Development of an empirical hyperbolic material model for TDA utilizing large-scale triaxial testing

ABSTRACT There is a growing interest in reusing scrap tires in civil engineering applications. Previous studies have shown that there are many promising applications in civil engineering. In most of these applications, scrap tires are not used directly but are shredded to form a product referred to as tire-derived aggregate (TDA). Unlike most other civil engineering materials, TDA has not yet been subjected to sufficient experimental scrutiny to identify its physical properties. Most of the experiments conducted on TDA have been limited to small TDA shreds with no steel wires. This study aims to address this problem through a series of consolidated drained triaxial tests on TDA using a large-scale triaxial apparatus. The results of the tests are presented, discussed and fitted to empirical equations. In addition, an empirical hyperbolic material model that represents the test results has been developed based on the hyperbolic model of Duncan and Chang. .

Effects of rock block content and confining pressure on dynamic characteristics of soil-rock mixtures

Soil-rock mixture (S-RM) is a complex geo-material characterized by competent rock inclusions floating in a weaker soil matrix structure. The mechanical performances of it are significantly controlled by the interaction and properties of rock blocks and the external environment. In this study, experiments on S-RM were conducted through a large-scale triaxial apparatus for investigating the effects of rock block content and confining pressure on the dynamic characteristics of them. The features of the axial stress and strain time-histories of S-RM were analyzed. The reductions of the maximum shear modulus and the dynamic shear modulus ratio were investigated. The increase in the damping ratio was explored. And the effects of rock block content and confining pressure on the dynamic characteristics of S-RM were discussed in detail. Results indicate that there is an approximately linear relationship between the maximum shear modulus and the rock block content while an approximately exponential relationship between the maximum shear modulus and the confining pressure. The distribution of the dynamic shear modulus ratio of S-RM takes on a low dispersion at the same normalized shear strain. The unnormalized/normalized damping ratio of S-RM is increasing with the shear strain and takes on a feature of zonal distribution as a whole. Both the dynamic shear modulus ratio and the normalized damping ratio of S-RM could be evaluated by a function of normalized shear strain. The four-parameter formulas for the dynamic characteristics of S-RM were introduced for all practical purposes in the final. The results of this study can provide a reference to further understand the dynamics performance and the parameter determination of S-RM and other similar geo-materials.

Predicting the small strain shear modulus of coarse-grained soils

Coarse-grained soils are important geotechnical materials widely used in construction. The contribution of particle size distribution on the small strain shear modulus Gmax of coarse-grained soils has not yet been fully recognized. This paper presents an experimental investigation into the effects of the uniformity coefficient Cu and the mean particle size D50 on the small strain shear modulus of coarse-grained soils. To this end, a series of shear wave velocity tests were conducted at different confining stresses and void ratios on reconstituted specimens covering a wide range of particle size distributions. All tests were performed by taking advantage of a custom-developed measurement device in a large-scale triaxial apparatus. The results demonstrate that Cu and D50 have pronounced but opposite effects on Gmax. Specifically, Gmax decreases with Cu and increases with D50. The effects of Cu and D50 were then quantified to generate two types of empirical equations, type-A and type-B, that are representative of a broad family of Gmax equations accounting for the influence of particle size distribution. The equations are tuned based on model parameters A, x and n. In both equations, n is a function of Cu. The difference between the two types of equations is that x is the function of Cu and D50 and A is the function of Cu in the type-A equation, while x is constant and A is the function of Cu and D50 in the type-B equation. The proposed equations for predicting Gmax were used to produce acceptable estimates for both coarse-grained soils in this study and from existing literature. The findings acquired from this study are expected to provide a reference for estimating the small strain shear modulus of coarse-grained soils at the beginning of the project when the test data have not yet been obtained.

Experimental studies of deformation anisotropy of gravel soils

When gravel-pebble soil is laying and compacting into the dam body, particles are reoriented to form a layered body (transversely isotropic soil), in which not only the filtration but also the deformation properties are different in directions along and across the layering. To analyze the differences in the gravel-pebble soil deformations in correspondence with the particles’ orientation we have performed experimental researches on the river pebble gravel using odometer, vacuum triaxial compression machine, and a large scale triaxial apparatus. To obtain the deformation characteristics of the soil in different directions we’ve carried out experiments according to two schemes with gravel particles (fr. 10-60 mm) laid horizontally and vertically. More deformations occur in the cross-layerage axis of the soil rather than in the parallel direction (εy > εx), so for the soil in the dam body Ex > Ey. The experiments showed that for gravel-pebble soil the strain-induced anisotropy coefficient η is dependent on its loading conditions and changes in the range from 1 to 1.8. The change of anisotropy degree is bound with the main tension value σ1 with the increase in which the anisotropy decreases.

Experimental study to evaluate the effect of particle size on the small strain shear modulus of coarse-grained soils

Abstract Coarse-grained soils are found commonly in nature and are extensively used for a wide range of engineering applications, such as slopes, earth dams, and railway foundations. This study presents an experimental investigation on the effect of mean particle size D50 on the small strain shear modulus Gmax of coarse-grained poorly-graded and well-graded soils. Combined with particle morphology analysis, a series of shear wave velocity tests were performed on pure sand, pure gravel and sand-gravel mixture specimens by taking advantage of a torsional vibration measurement device in a large scale triaxial apparatus. The results demonstrate that Gmax is insensitive to D50 for uniform sand and gravel specimens. On the other hand, Gmax increases gradually with D50 for well-graded sand-gravel mixture specimens. The discrepancy in the relationship between Gmax and D50 for poorly- and well-graded soils is related to the degree of size polydispersity in the specimens.

Resilient and permanent deformation of unsaturated unbound granular materials under cyclic loading by the large-scale triaxial tests

The road base and subbase are normally situated above the underground water table and thus in the unsaturated state. To study the cyclic behavior of unbound granular materials (UGMs) in the road base and subbase courses under controlled matric suction conditions, a series of cyclic tests on the unsaturated UGMs were conducted by the large-scale triaxial apparatus. The large-scale triaxial apparatus was upgraded using an unsaturated module to control the matric suction by the axis-translation technique. Emphases were placed on the effects of matric suction and cyclic stress amplitude on the accumulated axial strain and resilient modulus of UGMs. Test results showed that the increase in matric suction reduced the development of accumulated axial strain exponentially under different cyclic stress amplitudes. Based on the shakedown theory, both the plastic shakedown and plastic creep limits were determined, which tended to grow linearly with the increase in matric suction. A new empirical model was proposed to predict the accumulated axial strain of unsaturated UGMs. Moreover, the increase in both matric suction and cyclic stress amplitude led to the increase in resilient modulus, and linear relationships existed between the resilient modulus and matric suction.

Effects of moisture content on the cyclic behavior of crushed tuff aggregates by large-scale tri-axial test

The crushed rock aggregates are widely used as road base and subbase fillings of road. The moisture content of the crushed aggregates changes due to rainfalls and water infiltration from the pavement cracks during the road service period, and this usually brings detrimental effects to the performance of road base and subbase. To investigate the effects of moisture content on the resilient modulus and accumulated strain of crushed aggregates under cyclic loadings, comparative experiments on samples under optimum moisture condition and saturated condition were conducted through a large-scale tri-axial apparatus. Different cyclic stress amplitudes, confining pressures and initial deviatoric stresses were also considered in the experiments. Test results show that the change from optimum to saturated moisture content aggravates the accumulated axial strain and reduces the resilient modulus of the aggregates under cyclic loading. The influence of moisture content on the cyclic behavior of CTAs is related to the factors such as cyclic stress ratio and initial deviatoric stress.

Equipamento triaxial cíclico de grande escala para análise mecânica de lastro ferroviário

The ballast layer usually is responsible for giving support to the railway superstructure and is loaded and unloaded repeatedly due to the passage of trains. In this context, the comprehension of the stress-strain characteristics of this noncohesive material used in that layer is important for optimizing maintenance operations, thus ensuring a safe and efficient transport. The material used in ballast layer is usually composed of highly coarse-graded gradation, hence large-scale laboratory tests are more difficult to conduct. The purpose of this paper is to evaluate the applicability of a new Brazilian large-scale triaxial apparatus (400 mm x 800 mm) that allows testing large-size particles such as in the field, without the need of scaling aggregates and ensuring more representativeness. Thus, it is possible to simulate loading that occurs due to the passage of trains, in order to observe the mechanical behavior of ballast in terms of resilient and permanent deformations. A cyclic triaxial test carried out with that apparatus showed the availability of preparing large-size samples and obtaining results of resilient modulus and permanent deformation compatible with what was being expected.

Triaxial testing and discrete-element modelling of geogrid-stabilised rail ballast

Geogrids have been found to improve the performance of unbound aggregate layers in transportation applications by providing confinement and arresting movement through interlock between individual aggregate particles and their apertures. Geogrid reinforcement offers an effective remedial measure when railway structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength and permanent deformation behaviour of geogrid-stabilised railroad ballast. Geogrids with triangular, rectangular and square apertures were tested in the laboratory experiments. Cylindrical ballast specimens were prepared and tested with geogrids placed at different heights within the specimen using a large-scale triaxial apparatus. An imaging-based discrete-element-method approach was developed to model triaxial test results and investigate geogrid-reinforcement mechanisms. With the capability to create actual ballast aggregate particles as three-dimensional polyhedron elements having the same particle-size distributions and imaging quantified average shapes and angularities, the modelling was able to capture the ballast behaviour with and without geogrid reinforcement reasonably accurately.

Acoustic Emission Characteristics of Sericite Schist Coarse Aggregates under Consolidated Undrained Triaxial Tests

In order to study the acoustic emission characteristics of coarse aggregates, improved of large-scale triaxial apparatus is used to do the consolidated undrained triaxial tests of sericite schist coarse aggregates, with the acoustic emission signals monitoring. The test results show that a large number of AE signals of sericite schist coarse aggregates are generated in the initial stage. Before the axial strain reach 10%, AE counts are relatively quiet. But after 10%, they become anomalous and emerge obvious leap values. When the confining pressure up to 200kPa, AE signals are mainly generated by sliding friction. With confining pressure increasing, the proporation of rolling friction and particle breakage is more and more obvious.