Monotonic Triaxial Tests Research Articles

Evaluation of monotonic and dynamic shear modulus of sand using on-sample transducers in cyclic triaxial apparatus

This paper highlights the importance of the measurement of local strains in triaxial test using on-sample transducers. Specimens of sandy soil, prepared at two different relative densities (30% and 90%), were subjected to different effective confining stresses, and sheared monotonically as well as cyclically. Local strain measurements are obtained using Hall-type on-sample transducers attached at the mid-height of the specimen. The measurements from the on-sample transducers were further used to determine the modulus degradation curve for low-strain (less than 0.01%) to high-strain (greater than 1.0%) levels. Maximum shear modulus estimated from the cyclic tests is found to have an agreeable match with the same obtained from the monotonic tests corresponding to very low strains, as well as with those obtained from standard correlations. The experimental study indicates that for both monotonic and cyclic triaxial tests, the on-sample transducers can be effectively used to measure local strains in the soil specimen and evaluate the corresponding strain-dependent dynamic shear modulus.

Long-term behaviour and degradation of calcareous sand under cyclic loading

Calcareous sand has been used as the main filling material in numerous reef constructions. Therefore, understanding its long-term dynamic engineering geological characteristics under cyclic applications of traffic or wave loading is crucial to the design and prediction of the serviceability of reef infrastructures. Considering this, seven monotonic triaxial tests and 33 high-cycle (25,000 cycles) drained triaxial tests were conducted on calcareous sand with various mean effective stresses (p’) and cyclic stress ratios (ζ). The results show that the development of the accumulated axial strain (ε1acc) has a strong correlation with the positional relationship between the cyclic stress path and critical strength line (CSL) or static strength envelope (SSE) in the p’-q plane. The critical cyclic stress ratio ζc, that represents the cyclic stress path reaching the CSL, is proposed. When ζ>ζc, the growth rate of ε1acc with ζ increases drastically. Moreover, when the cyclic stress path approaches the SSE, ε1acc enters an incremental collapse state. The increase in p’ diminishes the dilatancy of the volumetric strain εv. However, this response of εv reverses from decreasing to increasing with increasing ζ. The resilient modulus (Mr) increases with the number of cycles (N) due to the initial densification. If εv dilation occurs, Mr will decrease with N as particles lose contact. A single parameter for the cyclic stress path ratio (ψ) considering the effects of p’, ζ, and SSE is proposed. Additionally, empirical formulas for calculating the ultimate ε1acc and Mr are established. Furthermore, particle degradation significantly worsens ε1acc and εv, but has a more profound effect on the compressibility of εv. This degradation is also detrimental to Mr. A certain deviation trend of the predicted values from the measured values is found to correspond with the relative particle breakage.

Monotonic and dynamic properties of riverbed sand and hill-slope soils of seismically active North-east India for ground engineering applications

The stress–strain characteristics of soil under monotonic loading conditions and the dynamic properties of soils under cyclic loading are necessary for foundation engineering, ground response analysis and soil structure interaction studies. In the absence of site/region-specific properties, it is a common practice to use representative parameters based on the index properties of soil and available standard correlations. Such correlations might be inadequate for regional soils of varying index properties. Based on monotonic and cyclic triaxial tests, this study reports the monotonic and dynamic soil properties of the prevalent soils from the northeastern region of India, namely the cohesionless, cohesive and silty-sandy soils. It is observed that monotonic response of cohesionless soil is mostly influenced by the initial relative density, while the response of cohesive and silty-sands are primarily influenced by the effective confining pressure and degree of saturation. The cyclic triaxial tests revealed that, irrespective of the soil type, the strain-dependent modulus reduction and especially the damping ratio curves are noticeably different from available standards. The study highlights the importance of identifying the site/region specific soil properties for their usage in GRA and SSI applications.

Characterization of mechanical behavior of binary granular assemblies through the equivalent void ratio and equivalent state parameter

The proposed research of this experimental investigation aims to study the effects of fines content, fabrics type and confining pressure on the undrained shear strength response of Chlef sand-silt mixtures under static loading conditions using the equivalent void ratio (e*) and equivalent state parameter (ψ*) concepts. For this purpose, a series of undrained monotonic triaxial tests was carried out on reconstituted saturated sand-silt mixture samples with fines content ranging from 0 to 40% (Ip=5%) at an initial relative density (Dr=52%) and subjected to different confining pressures (σ’3 = 100 kPa, 200 kPa and 300 kPa). The different samples were prepared in such a way that two fabrics types were produced: dry funnel pluviated fabrics and wet deposited fabrics. The obtained results show that the wet deposited fabrics exhibits a higher state parameter (ψ) than the dry funnel pluviated fabrics. The state parameter confirms that the wet deposited fabrics were more vulnerable to liquefaction phenomenon than the dry funnel pluviated ones. Moreover, the obtained data confirm the existence of a linear relationship between the dry funnel pluviation and wet deposition in terms of equivalent void ratio (e*) and exponential relationship in terms of equivalent state parameter (ψ*).

Influence of non-plastic fine on static iquefaction and undrained monotonic behavior of sandy gravel

Many geological hazards such as landslide, liquefaction, slope failure and rockfall occur in mixed gravelly alluvium or colluvium deposits, highlighting the importance of studying gravel–sand–silt mixtures. Although many natural strata are built using these mixtures, they are not entirely understood. A fundamental experimental study was designed to explore the influence of non-plastic fine content on the undrained behavior of gravel-sand-silt mixtures as common geomaterials in nature. Undrained monotonic triaxial tests were carried out. Three types of sandy gravels with different sand-to-gravel ratios were selected as base gravels. The impacts of the non-plastic fine content on the stress–strain behavior, static liquefaction potential, steady sate line, residual strength and critical state friction angle were studied. The laboratory results show that both sand-to-gravel ratio and non-plastic fine content have significant impacts on the undrained response of gravelly mixtures. It was observed that mixtures with zero non-plastic fines content exhibit the non-flow liquefaction behavior. However, adding silt content up to 45% shows the limited liquefaction or flow behavior, depending on the base gravelly soils and fine content percentage. A threshold for non-plastic fine content was found to be 35%–45%. Below this threshold, the steady state line shifts downward as silt content increases, and then shifts upward with further increase in the silt contents. The identical trend was also noted for residual strength and critical state friction angle. For silt contents below the threshold, the equivalent intergranular void ratio concept (modified with a new logical assumption) was used to analyze the behavior of gravelly mixtures. The results clearly indicate that the accurate value of equivalent intergranular void ratio can fully explain the behavior of sandy gravel-fine mixtures, similar to sand-fine mixtures. From a microstructure perspective, both sand and silt content act as fine-grains in gravelly mixtures and should be considered when studying the mechanical properties of theses mixtures.

Calibration of a Hypoplastic Constitutive Model with Elastic Strain Range for Firoozkuh Sand

A series of laboratory experiments on Firoozkuh sand were conducted followed by a set of numerical simulations to determine the parameters of a general hypoplastic constitutive model for sand. This model describes inelastic and incrementally nonlinear material properties without a yield surface and detached strain increments in the elastic and plastic portions that is used to accurately simulate soil behavior for small and large strains and unloading–reloading paths. The general models are salient and the model requires unique constitutive constant values for all densities of a particular soil at all pressures. The 13 hypoplastic parameters used were calibrated through standard laboratory monotonic and cyclic triaxial and oedometer tests. Additional undrained triaxial and cyclic triaxial testing data that had not been used in the calibration process were used to validate the parameters.

Dynamic properties of sand-bitumen mixtures as a geotechnical seismic isolation material

Geotechnical seismic isolation (GSI) has been emerged as an economical and alternative method in recent years that reduces earthquake that will affect the superstructure. Crumb rubber-granular soil mixtures (SRM) are generally used as damping geomaterial in this method. However, the major disadvantage of SRM is that the addition of rubber leads to significant reduction in stiffness of the granular soil. In this study, dynamic properties (secant shear modulus (Gsec) and damping ratio (D)) of sand and bitumen mixtures (SB) which can be used as damping materials in GSI systems were determined by cyclic triaxial tests. In addition, monotonic triaxial tests were performed to determine the pre- and post-cyclic behavior of the mixtures. In the cyclic tests, the effect of bitumen penetration and content on the energy absorption capacity of the bituminous specimens was examined depending on cyclic stress ratio (CSR), cell pressure and loading frequency. According to the findings, it was determined that higher bitumen penetration and (CSR) result in decrease of Gsec and increase of D, yet the increase in confining pressure and the increase in loading frequency have the opposite effect. For the specimens prepared with both bitumen types, it was specified that the D increased depending on the increase of bitumen content, and the highest Gsec value was observed to be bitumen content of 8%. Moreover, the findings obtained from the monotonic triaxial tests showed that after a high number of cyclic loading, the peak deviatoric stresses of the SB samples increase slightly.

Small to large strain mechanical behaviour of an alluvium stabilised with low carbon secondary minerals

Deep dry soil mixing is a popular ground improvement technique used to strengthen soft compressible soils, with Portland cement being the most popular binder. However, its continued use is becoming less sustainable given the high CO2 emissions associated with its manufacture. Alkali-activated cements are considered to be viable low carbon alternative binders, which use industrial waste products such as blast furnace slag. This study focusses on the stabilisation of a potentially liquefiable soft alluvial soil using a dry granulated binder comprising sodium hydroxide-activated blast furnace slag (GGBS-NaOH). This binder has previously been demonstrated by the authors to have potential as a replacement for Portland cement due to its excellent engineering performance, positive contributions towards the circular economy, reducing energy usage and CO2 emissions in the construction sector. A detailed comparison in mechanical behaviour is presented between the soil in its reconstituted, undisturbed and cemented states after 28 days curing through the use of advanced monotonic triaxial testing techniques, including small strain measurements. Mechanical behaviour was specifically analysed regarding peak deviatoric strength, pore pressure response, stress – volumetric dilatancy, shear stiffness degradation over small to large strain ranges, critical state and failure surfaces. Using 7.5% GGBS-NaOH increased the stiffness and shear strength of the soil significantly, whereby the shear strains at which initial shear stiffness degrades is three times higher than the untreated undisturbed soil. As a result, larger amounts of dilation was observed during shearing of the material and resulted in an upward shift of the soil’s original critical state line due to the creation of an artificially cemented soil matrix through the precipitation of C-(N)-A-S-H gels.

Triaxial test behaviour of silty sands treated with agar biopolymer

ABSTRACT In the current research, the study of undrained behaviour of agar biopolymer treated natural silty sand obtained from Kalpetta, Kerala was done. An experimental investigation was carried out to study the undrained behaviour of silty sand and biopolymer treated silty sand for examining the efficiency of agar biopolymer additive on undrained shear strength improvement. The practical implications of biopolymer treatment have also been evaluated. A series of consolidated undrained monotonic triaxial tests were performed at varying confining pressure (50 kPa, 100 kPa and 150 kPa), agar dosages (0.5%, 1% and 2%) and curing time periods (5 hours, 1 day, 3 days, 7 days and 28 days) to evaluate the efficiency of treatment. The treatment method displayed a considerable enhancement in shear strength behaviour (about 106% increase) mainly due to the significant amount of cohesion developed in the cohesionless soil under study.

Packing Density and Overconsolidation Ratio Effects on the Mechanical Response of Granular Soils

Liquefaction of sandy soils is associated with a sudden loss of the shear strength due to a dramatic increase of the excess pore water pressure in undrained conditions. Its occurrence usually causes significant damages to buildings and infrastructures during earthquakes. Therefore, the proposed laboratory research investigation aims to study the effects of the combined effects of sample depositional methods, initial relative density and overconsolidation ratio on the undrained shear strength of Chlef sand samples under static loading conditions. For this purpose, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated Chlef sand samples and subjected to different overconsolidation ratios (OCR = 1, 2, 4 and 8). The initial confining pressure for all tests was kept constant (σ′3 = 100 kPa). The samples were prepared using two depositional methods named: dry funnel pluviation (DFP) and wet deposition (WD) at three selected relative densities (Dr = 18%, 52% and 88%). The obtained data confirm the existence of a linear relationship between the undrained peak shear strength of overconsolidated Chlef sand samples (qpeak_OC) and undrained shear strength of normally consolidated Chlef sand (qpeak_NC) for the two sample preparation techniques (DFP and WD) and range of the initial relative density (18% ≤ Dr ≤ 88%) under consideration. The overconsolidation (OCR) and initial relative density (Dr) increase the dilativeness of the normally consolidated and overconsolidated sandy samples for the two sample reconstitution techniques. The dry funnel pluviated sandy samples exhibit higher undrained shear strength (liquefaction resistance) than the wet deposited sandy samples. Moreover, the obtained data reveal that the wet deposited sandy samples were more vulnerable to liquefaction resistance than dry funnel pluviated sandy samples for the range of the initial relative density (18% ≤ Dr ≤ 88%) and lower overconsolidation ratios (1 ≤ OCR ≤ 4).

Settlement analysis using a generic ballasted track simulation package

Uneven track settlement inevitably occurs for ballasted track and eventually results in poor track geometry and support stiffness leading to considerably high maintenance cost. Considerable in situ and laboratory experiments have been carried out and empirical formulas have been proposed in order to predict track settlement. Nevertheless, laboratory tests are usually restricted in size for financial reasons and the site characteristics vary significantly to fully understand influential parameters. Therefore, the main aim of the present work is to develop an efficient model capable of replicating localised track settlement for different circumstances. A generic ballasted track simulation package BaTrack is introduced combining the Finite Element (FE) software Abaqus, Python, and Fortran. The three-dimensional (3D) FE model includes rail, sleepers, rail-pads, under sleeper pads (USPs), ballast and foundation layers. An advanced non-linear ballast material model is introduced using porous material properties and extended Drucker-Prager model with hardening and is able to account for different confining pressure values. The model is firstly used for comparison against a series of monotonic triaxial tests and has shown good agreement. It is then validated against a series of full size tests carried out at the Southampton Railway Testing Facility (SRTF). A number of settlement analyses are carried out and characteristics of the stress, contact pressure distribution and void evolution from different track configurations are discussed in detail.

Inspection of three sophisticated constitutive models based on monotonic and cyclic tests on fine sand: Hypoplasticity vs. Sanisand vs. ISA

The prediction quality of three of the most sophisticated constitutive models for sands has been examined based on a comparison of results from element test simulations and laboratory tests. Hypoplasticity with intergranular strain, the Sanisand elastoplastic model in the version of 2004 and the recently proposed ISA (intergranular strain anisotropy) model have been inspected. The laboratory tests performed on Karlsruhe fine sand used in the present study are freely available on the homepage of the first author. The model predictions have been inspected for drained and undrained monotonic triaxial tests with various densities and pressures and for monotonic tests with a few number of un- and reloading cycles. The main focus, however, lies on undrained cyclic triaxial tests with either stress or strain control and with different densities, initial stresses and stress/strain amplitudes. The strengths and weaknesses of the different constitutive models for various test conditions are worked out.

Microscopic Mechanism Affecting Shear Strength in Lignin-Treated Loess Samples

In China, engineers have worked to create additional usable land for building construction by flattening the ridges of hills and filling in the adjacent valleys. China’s Loess Plateau comprises a type of soil (loess) with a large pore structure that can collapse and become unstable when exposed to groundwater. Conventional valley fill materials include remolded loess or remolded loess treated with cement, lime, gypsum, or other stabilizing additives. These stabilizers are often detrimental to the surrounding environment. Moreover, loess treated with conventional stabilizers exhibits excessive brittleness, which is not suitable for building foundations. Adequate stability of the building foundations in the filled valleys is required to ensure public safety. In this study, we tested 50 remolded loess samples treated with a lignin polymer compound to determine its potential as a valley fill material. Triaxial tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to study the mechanical characteristics of each sample, determine the effects of the lignin treatment on the loess, and identify the microscopic mechanism affecting shear stress in the lignin-treated loess. The corresponding development of excess pore pressure and volumetric responses under monotonic triaxial testing were also considered. Based on this study’s results, the optimum lignin content in the treated loess samples was 4%; lignin contents exceeding 4% decreased axial stress and increased dilation after saturation. The shear strength and strain-hardening phenomenon of the lignin-treated loess samples increased as the lignin content increased, while the excess pore water pressure decreased. Microscopically, the addition of lignin increased cohesion in the loess samples, while slightly contributing to the internal friction angle. The use of lignin as a stabilizing additive for valley fill material shows potential for controlling building foundation deformation by increasing soil strength and minimizing environmental impacts by maintaining the soil pH and limiting pollutant production.

Mechanical behavior of inert steel slag ballast for heavy haul rail track: Laboratory evaluation

Abstract The ballast mechanical behavior is one of most important aspects governing the performance and safety conditions of railway track lines. Usually comprised of natural crushed rock, the research for alternative ballast materials becomes interesting both environmentally and economically. This paper presents a comparison between aggregates of steel slag and granite by means of monotonic and cyclic triaxial tests carried out on scaled down ballast specimens. The monotonic triaxial tests were conducted at three different confining pressures to failure and to critical state. The long-term cyclic tests (at least 106 cycles) were conducted in two stress paths compatible with heavy haul loadings. The results showed that the steel slag aggregate has higher values of strength parameters compared to the granite aggregate and better long-term deformability behavior, expressed by lower degradation and particle breakage and a faster tendency for stabilization the permanent deformation.

Liquefaction and post-liquefaction assessment of lightly cemented sands

Post-liquefaction response of lightly cemented sands during an earthquake may change and become similar to uncemented sands due to bonding breakage. In the current study, the effect of degree of cementation on liquefaction and post-liquefaction behaviour of lightly cemented sands was studied through a series of cyclic and monotonic triaxial tests. Portland cement with high early strength and Sydney sand were used to reconstitute the lightly cemented specimens with unconfined compression strength ranging from 25 to 220 kPa. A series of multi-stage soil element tests including stress-controlled cyclic loading events with different amplitudes and post-cyclic undrained monotonic shearing tests were carried out on both uncemented and cemented specimens. Furthermore, a series of undrained monotonic shearing tests without cyclic loading history on different types of specimens was conducted to investigate the effect of cyclic loading history on the post-cyclic response of the specimens. The results show that residual excess pore-water pressure is correlated to the cyclic degradation of lightly cemented sands during cyclic loading. In addition, optical microstructure images of the cemented specimens after liquefaction showed that a major proportion of cementation bonds remained unbroken, which resulted in a superior post-liquefaction response with respect to initial stiffness and shear modulus in comparison to the uncemented sand.

Thermo-hydro-mechanical behaviour of cold reclaimed asphalt aggregates without binder addition

This paper details a cold recycling evaluation of 100% reclaimed asphalt aggregates without any binder addition. In this context, a series of triaxial laboratory tests was performed. Monotonic triaxial tests were conducted to determine failure resistance at 20°C, 50°C and with a specimen preheated at 50°C then tested at 20°C. Repeated Load Triaxial (RLT) tests were performed to analyse the permanent and resilient strains at 20°C, at three frequencies and with two water contents. It is found that increasing temperature causes a drop of failure resistance while cohesion is increased. RLT tests reveal that higher water content implies higher strains, as well that water content presents a higher impact on resilient behaviour than low frequencies. Finally, the non-linear elastic Boyce model was used to describe the resilient strains. Total stress and effective stress calibrations were performed with a measurement of the initial suction. Eventually, these effective stress calibrations were performed with one set of parameters.

Shearing response of geotextile-reinforced calcareous soils using monotonic triaxial tests

Different types of geosynthetic reinforcements have been widely used for improving shear strength of soil materials in recent decades. In this study, two different calcareous soils reinforced by geotextile layers were evaluated by monotonic compression triaxial tests. The calcareous soils were obtained from Hormuz Island and Boushehr Port located in the north coast of the Persian Gulf. Different reconstituted samples were prepared at dense conditions with and without geotextile layers and consolidated under different effective confining pressures ranging from 100 to 600 kPa. Different layers of reinforcements including one, two and three geotextile layers were placed in the reinforced samples and sheared under drained condition. The results show that confining pressure has a significant effect on the efficiency of reinforcements. Evidently, increasing the reinforcement layers decreased the particle breakage of the calcareous soils used in this study.

A simple anisotropic bounding surface model for saturated clay considering the cyclic degradation

A simple elastoplastic bounding surface plasticity model accounting for anisotropy and cyclic degradation of saturated clay is developed within the framework of critical state soil mechanics and the bounding surface plasticity. The associated flow rule is adopted and merely three constants more than those of the Modified Cam-Clay model are required for simplicity. A preferable rotational hardening rule is generalised into the domain of dynamic model after comparing different kinds of rotational hardening rules to describe the development of anisotropy. The radial mapping rule and the mapping origin moving with stress reversal are incorporated into the bounding surface plasticity concept to capture the behaviours of clay under cyclic loads. A new interpolation function of plastic modulus associated with the length of the accumulated deviatoric plastic strain is introduced into the model to describe the cyclic degradation. The developed model is validated by undrained monotonic and cyclic triaxial tests on isotropically and anisotropically consolidated clay. The proposed model is capable of reproducing the stress–strain behaviours of saturated clay, and capturing the anisotropy and cyclic degradation.

Study of the mechanical behaviour of reclaimed asphalt aggregates without binder addition

The objective of this paper is to investigate a source of reclaimed asphalt aggregates without binder addition. To this end, a series of triaxial tests was performed. First, monotonic triaxial tests allowed to determine failure resistance of the material at 20°C, 50°C and at 20°C with a prior preheating at 50°C. Then, the resilient and permanent behaviours at 20°C were studied thanks to Repeated Load Triaxial (RLT) tests with different levels of loading. The results show that increasing the temperature test implies a drop of failure resistance but a higher cohesion. RLT tests reveal a stress dependent resilient behaviour. Finally, the experimental results were compared with calibrations of the non-linear elastic Boyce model.

Experimental study on fibre reinforced sandy soils behaviour under static loadings - drained and undrained conditions

The effect of the addition of 0.5% of randomly distributed polypropylene fibres in a round grained sand, composed mainly of silica, is presented on laboratory scale. The samples were compacted with two void ratios (0.75 and 0.63), corresponding to values of relative density of 50% and 90% respectively. 24 monotonic triaxial tests were performed, with initial effective mean pressures of 20, 100 and 200 kPa. Half of samples were tested in drained conditions, and the other half in undrained conditions. For the samples subjected to drained conditions, the effect of the fibres was identified at the post-failure stage, by showing sustained increase of strength. The positive effect of fibres could also be observed through the increase of material friction angle. In the samples subjected to undrained tests, the addition of fibres reduced the contractive behaviour, mainly for those more compacted.