Mechanical Behavior Of Sand Research Articles

Laboratory investigation of the influence of fines content on the mechanical behavior of sand

The present article seeks to examine and discuss the experimental results that were obtained from a series of straight box shear tests. For this, different soils were prepared with clean sand, which was brought from the Wilaya (Province) of Chlef that is located in northern Algeria, mixed with silt from the same site, clay from the town of M'zilla or clayey loam that is composed half of this same clay and half of silt. The percentages of sand replacement by the selected additions were 0, 10, 20, 30, and 40% in mass ratio. The samples thus prepared were then tested with respect to dry shear, under the normal stresses equal to 100, 200, and 400 kPa. The results obtained showed that the shear strength of the sand from Chlef decreases when the silt content increases from 0 to 40%. With regard to the sand-clay and sand-silt-clay mixtures, it was found that the shear strength decreases until reaching a threshold value that coincides with the addition content of 20%. Then, beyond that value, it begins to increase as the addition fraction increases. Likewise, it turned out that cohesion increases with increasing silt content, while for sand-clay and sand-silt-clay mixtures, this cohesion increases until reaching a maximum value which coincides with the addition of 20% silt content. The same trend was observed for the variation of the direct shear strength, while the friction angle exhibited an inverse behavior.

Effect of cementation on the mechanical response of sands using acoustic emission technique

This study investigates the impact of cementation on the mechanical behavior of sands with various cement content (CSR) in drained triaxial compression, employing both Acoustic Emission (AE) and Environmental Scanning Electron Microscopy (ESEM) measurements. The experimental findings, encompassing quantitative statistics of stress-strain relations, microstructure variations, and AE characteristics, demonstrate that: the addition of CSR from 1% to 20% leads to an exponential rise in peak strength and stiffness, marking a transition from ductile to brittle mechanical failure, which is pinpointed between CSR levels of 5% to 10%. AE characteristics unveil an upward-opening parabola of normalized AE hits with CSR, a clear transition zone identification, and three distinct types of AE rate evolutions corresponding to failure patterns of ductile bulging, shear banding, and brittle fracturing, respectively. It suggests an intimate correlation with the intrinsic differences in micro-mechanical behaviors and AE propagation properties of cemented sands with varying CSRs. Notably, the bulging and shear banding processes are divided by AE into three stages, whereas fracturing is characterized into five stages. Two precursory AE anomalies associated with incipient failure and complex failure modes are observed, emphasizing the advantage of using AE to reflect the internal micro-mechanical behavior of cemented sands over conventional stress-strain manifestations.

A particle-scale analysis of unload-reload hysteresis for normally consolidated kaolin

The mechanical behaviour of sands and clays subject to “normal” (plastic) compression are very similar, albeit at different stress levels, despite the differences in inter-particle interactions (Hertzian normal contact forces or similar for sand) and electro-chemical forces for clays. However the hysteretic behaviour of clay is very different to sand with clays exhibiting “springy” hysteresis loops and ratchetting volumetric compaction. This paper aims to show that whereas the “plastic” compression arises from degradation of macro particles, the “quasi-elastic” unload-reload behaviour is consistent with an increase in the number of platelet edge contacts as clay “stacks” rearrange and compact.

Geotechnical characteristics of siliceous North Sea sands

When deriving drained strength parameters for sandy soils, geotechnical engineers need to be cautious while applying the semi-empirical correlations that are mainly developed from the highly idealised uniform clean silica sands as often used in academia. Many studies have shown that changing fines content changes the mechanical behaviour of sand, and that grading curve and particle angularity affect the strength-dilatancy characteristics. However, when various parameters of sands encountered in nature differ from the commercially sourced silica sands, the counteracting effects are difficult to quantify. In this study, a database of classification tests and Consolidated Isotropic Drained axisymmetric strength tests as well as some accompanying cone penetration tests (CPTs) are collected for siliceous sands from the North Sea. From this extensive database of natural sands, simple expressions are developed to help predicting drained strength parameters with improved reliability. Furthermore, a correction factor, for a commonly applied CPT friction angle qc − φ′p , tx correlation accounting for the grading curve, is developed. These expressions can help in reducing the number of costly advanced strength tests and to gain more information from a soil investigation.

Experimental and Numerical Studies on the Direct Shear Behavior of Sand-RCA (Recycled Concrete Aggregates) Mixtures with Different Contents of RCA.

Recycled concrete aggregate (RCA) is a typical construction and demolition (C&D) material generated in civil engineering activities and has been widely used as the coarse-grained filler added to sand for roadbed fillings. The effect of RCA content on the mechanical behavior of sand–RCA mixtures is complicated and still not fully understood. To explore the effect of RCA content on the macroscale and microscopic behavior of the sand–RCA mixtures with various RCA contents, laboratory direct shear tests and numerical simulations using the 3D discrete element method were performed. Experimental direct shear tests on sand–RCA mixtures with different contents of RCA were first carried out. Numerical direct shear models were then established to represent the experimental results. The particle shape effect was also considered using a new realistic shape modeling method to model the RCA particles. Good agreement was observed between the DEM simulation and experimental results, verifying the ability of the numerical direct shear models to represent the direct shear behavior of sand–RCA mixtures. The macroscopic responses of both experimental and numerical tests showed that all samples presented an initial hardening followed by a post-peak strain softening. The peak-state friction angles increased with the RCA content for samples under the same vertical stress. The effect of RCA content on the microscopic behavior based on DEM simulation was also found. The microscopic properties of RCA–sand mixtures, such as coordination numbers, PDFs and contact force transformation features, were analyzed and related to the macroscopic results.

DEM study on effect of particle roundness on biaxial shearing of sand

In this study, discrete element method (DEM) simulations of a biaxial test were used to examine the effect of particle roundness on the mechanical behavior of sands at both the macro and micro scales. First, a stack of microcomputed tomography images were binarized, segmented, and labeled using advanced image processing and analysis techniques. Second, a spherical harmonic (SH) analysis, which involves a complete set of orthogonal functions, was implemented to rebuild the natural particle shape. Then, five templates of virtual particles were built in a DEM simulation, four of which were obtained from SH degrees of 3, 8, 12, and 15, and one template was an elementary sphere. A flexible membrane was numerically generated to allow the material to deform freely under a prescribed confining stress. Finally, the effect of particle roundness on the mechanical properties of granular materials was investigated and discussed. Two shear bands were found to intersect, forming an X shape in both the rotation and displacement fields. Moreover, a lower particle roundness results in higher deviatoric stress and stronger dilation in the volumetric change. A decrease in particle roundness leads to less rotation of particles despite a higher displacement value. In addition, a larger SH degree leads to smaller normalized contact forces of the particles. This implies that decreasing the roundness results in higher anisotropy of the contact forces.

Stress–dilatancy behavior of cemented sand: comparison between bonding provided by cement and biocement

Biocement as an alternative to cement for soil improvement has been studied for the past decade. A comparative study on the cementation effect on the mechanical behavior of sand by biocement and Portland cement is presented. Drained triaxial tests were carried out on both cement- and biocement-treated sand. The yielding of cemented sand is largely associated with the breakage of bonding. Bonding ratio is defined in this paper to quantify the bonding effect. The bonding provided by biocement is stronger than that by cement, and as a result, the stress ratio at yielding for biocement-treated sand is higher than that for cement-treated sand given the other conditions the same. The shear resistance of cemented sand consists of bonding, dilation and friction. The stress–dilatancy relationship of biocement-treated sand is different from that of cement-treated sand or the Rowe’s stress–dilatancy equation. The increase in stress ratio with dilatancy ratio (1 − δev/δe1) is higher for biocement-treated sand.

Directional strength and stiffness characteristics of inherently anisotropic sand: The influence of deposition inclination

Evolution of soil fabric during deposition, sedimentation and subsequent shearing has been the focus of innumerous experimental and numerical studies in the past. However, the effect of inclined bedding plane on the mechanical behavior of granular materials has not been well recognized mostly due to the difficulties associated with the relevant simulations in the laboratory. In this study, a comprehensive experimental program is carried out to rigorously examine the significant influence of inclined bedding plane and deposition angle on the mechanical behavior of sands over a wide range of strains, from very small to large levels. To this end, specimens at different bedding plane angles are first prepared, frozen and then subjected to several consolidated drained (CD), consolidated undrained (CU) and bender element tests. Results show that the small strain shear stiffness and tangent axial stiffness are heavily influenced by the major principal stress direction and the deposition inclination. In terms of tangent axial stiffness, this effect is highlighted particularly before peak shear strength where the mechanical behavior of the granular material is dominated by particle interlocking. Besides, the anisotropic response of the soil is more pronounced at higher isotropic confining pressures. The volumetric behavior, internal friction angle and shear band direction are also observed to be remarkably dependent on the angle of bedding plane.

An anisotropic plasticity model incorporating fabric evolution for monotonic and cyclic behavior of sand

Fabric anisotropy has a significant influence on the mechanical behavior of sand. An anisotropic plasticity model incorporating fabric evolution is formulated in this study. Information on the overall stress–strain relationship and micromechanical fabric states from DEM numerical tests is used in the development of the constitutive model, overcoming the difficulties of fabric measurement in physical tests. The framework of the model and its formulations for fabric evolution, plasticity, and dilatancy enables it to capture the strength, shear modulus, and dilatancy of sand under both monotonic and cyclic loading. The model is validated against DEM numerical tests and physical laboratory tests on samples with different initial fabric, showing good agreement between the simulation and test results for the anisotropic stress–strain behavior of sand. The use of DEM test data also allows for the validation of the model on the micromechanical fabric level, showing that the model can reproduce the fabric evolution and its influence on key constitutive features reasonably well. The model is further applied to analyze the liquefaction behavior of sand, exhibiting the significant influence of fabric anisotropy on both liquefaction resistance and postliquefaction shear deformation.

Bearing Capacity and Failure Mechanism of Strip Footings on Anisotropic Sand

AbstractSand typically exhibits anisotropic internal structure (or fabric), and the fabric anisotropy has a dramatic influence on the mechanical behavior of sand. Meanwhile, the fabric evolves when...

Study on the mechanical behavior of sands using 3D discrete element method with realistic particle models

Presented is a study on the geometrical characteristics of sand particles and the mechanical behavior of sand material under external loading. Based on computed tomography technique, a reconstruction method of granular particles was developed and used to build a database of 3D geometrical models for sand particles. The studied sand particles showed good regularities in morphological characteristics and thus were suitable to be used for the random generation of numerical samples. DEM tests using realistically shaped particles were proven to better simulate the mechanical behavior of the sample during elastoplastic loading stage, which was an issue for the simplified spherical particles. The generation, extension, and breakage of the force chains controlled the strain softening behavior of sands. Anisotropy analysis using the spherical harmonic series showed that the evolution of anisotropy directions and parameters corresponded well with the macroscopic mechanical behavior of the material. Pore volume computation based on Voronoi diagram was performed to illustrate the formation and evolution of localized shear zone. The mesoscopic analysis showed that particle shape significantly influences the mechanical behavior of sands and thus should be properly modeled in numerical simulations.

Effective skeleton stress and void ratio for constitutive modeling of fiber-reinforced sand

Inclusion of flexible fibers such as polypropylene and polyester is an effective method for soil improvement, as it significantly enhances the soil strength and ductility. A proper constitutive model is essential for assessing the stability and serviceability of fiber-reinforced slopes/foundations. A new method for constitutive modeling of fiber-reinforced sand (FRS) is proposed. It assumes that the strain of FRS is dependent on the deformation of the sand skeleton only, while the effective skeleton stress and effective skeleton void ratio, which should be used in describing the dilatancy, plastic hardening and elastic stiffness of FRS, are affected by fiber inclusion. The effective skeleton stress is dependent on the shear strain level, and the effective skeleton void ratio is affected by the fiber content and sample preparation method. A critical state FRS model in the triaxial stress space is proposed using the concept of effective skeleton stress and void ratio. Four parameters are introduced to characterize the effect of fiber inclusion on the mechanical behavior of sand, all of which can be easily determined based on triaxial test data on FRS, without measuring the stress–strain relationship of individual fibers. The model is validated by triaxial compression test results on four fiber-reinforced sands under loading conditions with various confining pressures, densities and stress paths. Potential improvement in the model for incorporating fiber orientation anisotropy is discussed.

Study on pore fabric, dilatancy, dissipation function and yield function for sand

The mechanical behavior of sand is affected by the anisotropic fabric. Integated with the micromechanics theory, and linked with void fabric tensor, the relations among macroscopic volumetric strain, shear strain and void fabric are established, and it is further shown that dilatancy and anisotropy under applied shear force of sand are necessarily induced. By means of thermomechanics theory, the inclined elliptical yield function in dissipation space is built, and considering the effects of anisotropic fabric, there exists three important lines, just as the normal consolidation line, phase transformation line and peak value line, and corresponding with three characteristic lines passing through the origin in true stress space which relatively truly reflect the mechanical characteristics for sand, especially dense sand.

Exploring the effect of 3D grain shape on the packing and mechanical behaviour of sands

This paper investigated the dependence of three-dimensional grain shape characteristics on particle size by applying X-ray micro-computerized tomography. A series of packing and triaxial shearing experiments are performed, which suggest that the macroscale behaviour of sands at different scales of stress states progressively correspond to the characteristic scales of the microscale morphological descriptors of particles. That is, the packing behaviour are more reliant on grain aspect ratio (AR), whereas the critical state mechanical behaviour are more reliant on grain sphericity. As the AR of particulates increases, the extreme void ratios and packing index decrease. The shearing friction angles at the critical state decrease as the sphericity of grains increases and the intercept void ratios become smaller. The drained peak friction angles and maximum friction angles of undrained stable and strain-hardening specimens of angular sands are larger than those of rounded sands at the same state parameter level because the interlocking of angular sands with many projecting corners is stronger than that of rounded sands during shearing. At the same initial state level, rounded sands are more susceptible to softening and liquefaction than angular sands under undrained conditions, because smooth corners permit easier grain rotations in undrained unstable specimens.

Effect of fines content and plasticity on undrained shear strength of quartz-clay mixtures

Data obtained from previous soil borings revealed that, natural soils free of fines are rarely encountered in the field. On this basis, the aim of this study is to investigate the effects of plastic fines on mechanical behavior of sand (quartz)-clay mixtures. Two types (bentonite and kaolinite) of clay were mixed with quartz at rates ranging among 0% to 100% by dry weight. Evaluations were made based on changes in threshold fines content (FCt) with clay type and mechanical properties, consistency limits, and compaction characteristics. The results indicate that undrained shear strength (su) decreased up to 30% kaolinite content while increased with bentonite content from 0 to 100% which is an evidence of the effect of clay type on FCt. Furthermore, quartz-kaolinite mixtures have greater maximum dry unit weight that quartz-bentonite mixtures. On the contrary, the undrained shear strength of quartz-bentonite mixtures was greater than quartz-kaolinite mixtures.

Effect of Sisal Fiber and Polyurethane Admixture on the Strength and Mechanical Behavior of Sand.

One major problem related to sandy soil is its low shear strength and cohesion in engineering. Although much effort has been made to strengthen sand mass with satisfactory performances, most undertakings lack environmental considerations. Thus, a combination of natural fiber and macromolecule polymer material attempts to achieve both strength and eco-friendliness. In the present investigation, sisal fiber (SF) and water-based polyurethane (PU) were used to reinforce sand. A series of unconfined compression tests were carried out on sand specimens at different percentages of fiber contents (0.2%, 0.4%, 0.6%, and 0.8% by weight of dry sand) and polymer contents (1%, 2%, 3%, and 4% by weight of dry sand). The results showed within our test range that the unconfined compressive strength (UCS) as well as post-peak strength of specimens increase with fiber and polymer contents. The inclusion of fiber and polymer significantly improve the ductility of specimens. The effect of dry densities on UCS were studied with three proportions. It is found that a high dry density led to an increase of UCS due to an effective contact area increase. The interactions were studied by observation through scanning electron microscopy (SEM) images. The presence of water-based polyurethane has the potential to improve the interparticle cohesion of sand due to its unique network membrane structure. The fiber reinforcement benefit depends strongly on the friction, interlocking force, and bond strength at the interface.

An Experimental Investigation in Effect of Confining Stress and Matric Suction on the Mechanical Behavior of Sand with Different Fine Content

An Experimental Investigation in Effect of Confining Stress and Matric Suction on the Mechanical Behavior of Sand with Different Fine Content

Discrete element simulation of mechanical characteristics of sands under principal stress axes rotation

The mechanical behavior of sands and other granular materials subjected to rotation of principal stress axes is significantly dependent on the micro fabric properties and their evolution. The discrete element simulation for sands under cyclic rotation of principal stress axes was validated by comparing the numerical simulation and laboratory test results on the development of two normal strains,the relationship between shear stress and shear strain,the evolution of the volumetric strain,and the effects of initial void ratio and stress ratio. Four variables characterized by the spatial distribution features of particle major axis orientation and interparticle contact normal directions were used to describe the fabric properties of granular materials. The evolution of the four fabric variables in a single cycle was studied for granular materials subjected to cyclic rotation of principal stress axes. The effects of initial void ratio and stress ratio on the evolution of fabric variables were also investigated in detail.

Experimental investigation on the deformation characteristics of granular materials under drained rotational shear

Rotational shear is the type of loading path where samples are subjected to cyclic rotation of principal stress directions while the magnitudes of principal stresses are maintained constant. This paper presents results from an experimental investigation on the drained deformation behaviour of saturated sand in rotational shear conducted in a hollow cylinder apparatus. Two types of granular materials, Leighton Buzzard sand and glass beads are tested. A range of influential factors are investigated including the material density, the deviatoric stress level, and the intermediate principal stress. It is observed that the volumetric strain during rotational shear is mainly contractive and most of strains are generated during the first 20 cycles. The mechanical behaviour of sand under rotational shear is generally non-coaxial, i.e., there is no coincidence between the principal axes of stress and incremental strain, and the variation of the non-coaxiality shows a periodic trend during the tests. The stress ratio has a significant effect on soil response in rotational shear. The larger the stress ratio, the more contractive behaviour and the lower degree of non-coaxiality are induced. The test also demonstrates that the effect of the intermediate principal stress, material density and particle shape on the results is pronounced.

Experimental studies on the influence of intermediate principal stress and inclination on the mechanical behaviour of angular sands

A comprehensive experimental study has been made on angular sand to investigate various aspects of mechanical behavior. A hollow cylinder torsion testing apparatus is used in this program to apply a range of stress conditions on this angular quartzitic fine sand under monotonic drained shear. The effect of the magnitude and inclination of the principal stresses on an element of sand is studied through these experiments. This magnitude and inclination of the principal stresses are presented as an ``ensemble measure of fabric in sands''. This ensemble measure of fabric in the sands evolves through the shearing process, and reaches the final state, which indeed has a unique fabric. The sand shows significant variation in strength with changing inclination of the principal stresses. The locus of the final stress state in principal stress space is also mapped from these series of experiments. Additional aspects of non-coaxiality, a benchmarking exercise with a few constitutive models is presented here. This experimental approach albeit indirect shows that a unique state which is dependent on the fabric, density and confining stress exists. This suite of experiments provides a well-controlled data set for a clear understanding on the mechanical behavior of sands.