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A series of bender element tests was carried out to investigate the variation of small-strain stiffness (Gmax) of sand–granulated rubber mixtures under long-term loading. Comparisons were made using two different host sands: a non-crushable material called Sydney sand and a crushable material composed of completely decomposed volcanic rock (CDV). The test results suggested that, at the selected stress states in the study (mean effective confining stress ranged from 250 to 333 kPa), the influence of creep on the stiffness of the pure sand specimens was more pronounced for CDV compared with Sydney sand and the presence of rubber amplified creep effects on both types of mixtures. The influence of creep on Gmax was therefore associated with two major mechanisms: the increased compressibility of specimens predominantly because of grain crushing (contribution of sand fraction), and the increased compressibility because of the presence of soft rubber grains (contribution of the rubber fraction). The significant influence of grain breakage on the relationship between stiffness and creep was confirmed by post-test examination of the grain-size distribution curves of pure CDV samples. For rubber–CDV mixtures, the inclusion of soft and viscoelastic rubber grains prevented breakage, in this way reducing the creep mechanism due to grain crushing, but increased the overall influence of creep on stiffness. These conclusions were drawn based on 20% of rubber by mixture weight, and very similar behaviour was observed for 10% rubber. It was therefore understood that the aggravation of creep induced by the increased deformations due to the presence of rubber, for the given contents of rubber included in the study, was more dominant compared with the adverse influence of rubber on samples’ creep by preventing sand grain breakage.

Investigation of dynamic properties of carbonate/calcareous soils is important in earthquake and offshore engineering as these soils are commonly encountered in large-scale projects related with energy geomechanics and land reclamation. In this study, the stiffness and stiffness anisotropy of two types of calcareous sands (CS) from the Western Australia and the Philippines were examined using bender elements configured in different directions in stress path setups. Stiffness measurements were taken on specimens subjected to constant p′ compression/extension and biaxial stress paths and additional tests were performed on three types of silica sands with different geological origins and particle shapes, which were used as benchmark materials in the study. Compared with the three brands of silica sands, the stiffness of the CS was found to be more significantly influenced by anisotropic loading; an important observation of the experimental results was that stress anisotropy had different weighted influences on the stiffness in different directions, thus influencing stiffness anisotropy. Comparisons were made between the specimens subjected to complex loading paths, and respected model parameters as suggested from published expressions in the literature. These comparisons further highlighted that calcareous soils have different responses in terms of stiffness, stiffness anisotropy and loading history, compared with that of silica-based sands.

Granular materials are often subjected to certain degrees of coating in natural conditions due to weathering and soil–environment interactions and the current literature has suggested that the development of surface coatings has an important influence on the macroscopic response of geological materials. In the present study, a new method was developed to artificially coat sand grains with iron oxides and a series of experiments were performed for surface morphological and mechanical characterization as well as quantification of the tribological behavior of iron oxides coated grains. Through the material characterization tests, it was observed that the artificial iron oxides coating notably altered the roughness and hardness of the grains, which in turn resulted in higher inter-particle friction and lower normal contact stiffness. Clear dependency of the coefficient of friction on the magnitude of normal load and the loading history was observed for the coated specimens, which is speculated to be attributed to asperity breakage, plowing damage and debris build-up of the surfaces as also supported by microscopic images. The test results could also provide, qualitatively, some multi-scale insights into the influence of iron oxide coating on the bulk behavior of natural soils as the increase in inter-particle friction angle can be linked with the observed trend of increased critical state strength.

The frictional behaviour of geological interfaces has garnered a significant amount of research attention in geophysics and geomechanics with major applications in the characterization of sediments, the analysis of hydrocarbon reservoirs and the study of deep rock mechanics problems. In the present study, we investigated the tribological behaviour of analogue sedimentary rock by performing grain-scale experiments on natural sand particles bonded with calcium carbonate (CaCO3) at their contacts. The cementation was achieved by artificially forming calcium carbonate sediments around the contact area of two particles with chemical solutions based on a newly developed laboratory method by the authors. By differentiating the stage of the chemical reaction when the solution was applied to the specimen, two types of artificial bonds were created, named as (i) the gel type and (ii) the precipitation type. The sediment crystals formed from the two bonding types were distinct based on observations from digital microscope and scanning electron microscope images. Quantitative analysis was performed based on energy-dispersive X-ray spectroscopy, and the results revealed that the percentage of calcium carbonate formed from the precipitation-type cementation was higher than its counterpart. The grain-scale experiments revealed brittle type of breakage of the bonding from both tension and monotonic shearing tests on the analogue rock specimens. A significant influence of the magnitude of normal load was noticed on the shearing strength of both types of bonded specimens, while the gel type of bonding exhibited higher mechanical strength in either tension or shearing tests.

Sand-EPS geofoam is commonly used as a seismic buffer in a variety of geotechnical applications to alleviate damages imposed by seismic vibrations. Evaluation of the properties of the sand-EPS geofoam interface is an essential part of the design and installation of EPS geofoam in direct contact with the parent sand. In this study, a series of laboratory tests are carried out using a large-scale direct shear apparatus to evaluate the monotonic, cyclic and post-cyclic behavior of the sand-EPS geofoam interface. Adopting the relative density of the parent sand, the density of EPS geofoam, the applied normal stress, the cyclic shear strain semi-amplitude and the number of cycles as the variable parameters, their influence on the response of the sand-EPS geofoam interface is investigated. According to the experimental results, with the increase in the number of cycles, sand relative density, EPS geofoam density and the applied normal stress, the equivalent or secant stiffness of the sand-EPS geofoam interface increases while its damping ratio declines. The results of the monotonic (MDS) and post-cyclic monotonic (CMDS) shearing experiments also show that with an increase in the EPS density and normal stress, which offers greater resistance against sliding, the mobilized shear stresses generally increase.

A micromechanical experimental study is presented in this paper investigating the normal contact response of quartz sand grains under cyclic loading. Sand grains with clean surfaces, termed uncoate...

The coefficient of restitution (COR) is an important input parameter in the numerical simulation of granular flows, as it governs the travel distance, the lateral spreading, and the design of barriers. In this study, a new custom-built micromechanical impact loading apparatus is presented along with impact experiments on engineered and natural materials. The COR and energy loss of various grains and base block combinations are studied, including fairly regular-shaped Leighton Buzzard sand (LBS) grains as a natural soil and granite or rubber as base blocks, apart from the use of engineered materials for the grains (chrome steel balls, glass balls) and blocks (stainless steel, brass). The repeatability of the new micromechanical impact loading apparatus was checked by impacting chrome steel balls on stainless steel block. In all the test combinations, the higher and lower values of the COR are found for granite block (ranging between 0.75 and 0.95) and rubber block (ranging between 0.37 and 0.44) combinations, respectively. For the tested grain–block combinations, lower values of COR were observed for impacts between materials of low values of composite Young’s modulus. However, within the narrow range of composite surface roughness of the tested grain–block interfaces no particular trend was observed in the COR values. Compared to glass balls and chrome steel balls, greater scatter in the COR values is observed for natural sand grains. This is due to the variation of the elastic and morphological characteristics among individual LBS grains.

An integrated experimental study is presented which aims at relating the frictional properties at the particle scale to the bulk mechanical behavior for two different types of sands. We performed direct shear tests and inter-particle tests on lunar regolith simulant DNA-1A and Ottawa sand (benchmark material) under both dry and wet conditions. We found higher macroscopic friction angles for the lunar simulant in both dry and wet conditions, a smaller strength decay for Ottawa sand during reversal direct shear tests and similar strength envelopes of both materials under wet and dry conditions. Particle-scale tests evidenced higher inter-particle friction for DNA-1A in wet conditions with respect to the dry case for normal force lower than 2–3 N. For the lunar simulant, the differences between bulk and inter-particle friction appeared to be emphasized in dry condition and an evident effect of water on the friction coefficient was found only at the micro-scale.

The shearing behaviour of reproduced flat LBS grains artificially bonded with ordinary Portland cement (OPC) and plaster of Paris (PP) was examined using micromechanical experiments. Monotonic shearing tests showed a distinct variation in the load–displacement relationship at low, medium and high normal loads, and a nonlinear shear strength envelope was proposed. For OPC-bonded sand grains, a brittle–ductile transition at 20–30 N normal load was observed and three breakage mechanisms in shearing (chipping, shear cracks and crushing) were distinguished in accordance with the changes in the load–displacement curves. OPC-bonded sands showed a predominant dilation at lower normal loads, whereas PP-bonded sands were highly compressive. Based on previously published works using element-scale tests, a new mechanism for dilation under micromechanical testing was proposed in the study. Cyclic shearing tests were conducted on OPC-bonded sands, and the effects of increased displacement amplitude and normal load were highlighted.

Drained triaxial shearing tests were performed on a well-graded compressive sand (completely decomposed granite, CDG) and its mixtures with granulated rubber tires to investigate the effects of rubber size and content on their mechanical behaviour. Three sizes of rubber particles, GR1, GR2, and GR3, were used with size ratios to CDG (D50,rubber : D50,CDG) of 0.9, 3.5, and 7.2, respectively, and the rubber content ranged from 0% to 30%. The results show that for CDG–GR1 mixtures, the strength decreases with increasing rubber content, while for CDG–GR2 and CDG–GR3 mixtures, the strength decreases only at 10% rubber content and then increases markedly with increasing rubber content. The increase of strength is mainly because the inclusion of large rubber particles widens the particle size distributions of the mixtures, resulting in denser packings. The denser packings also lead to a decrease in compressibility. At larger size ratio and higher rubber content, the CDG–rubber mixtures show higher shear strength and lower compressibility than pure CDG, which indicates the CDG–rubber mixtures are very suitable to be used as filling materials.

We examined the normal and tangential contact behaviour of gouge materials between nominally flat quartz grains using micromechanical experiments. Primary mixtures of plastic clay (montmorillonite) and non-plastic silt (crushed aggregates) were used as gouges to focus the investigation on the tribological behaviour of gouge materials at dry, partial saturation and submersion conditions. The presence of gouges resulted in a decreased normal stiffness and frictional strength compared to pure quartz surfaces, and complex crushing behaviour. From cyclic shearing tests, montmorillonite showed the lowest secant stiffness and maximum energy losses, which were more pronounced at lower normal loads. Montmorillonite in its plastic and semi-solid states indicated that variation of post-peak softening and friction coefficient depend on the normal load at a given partial saturation level. In the sudden submersion simulation tests, both before and after the start of shearing, montmorillonite showed a significant decrease in the coefficient of friction (compared to dry gouge), unlike non-plastic silt.

Granular materials are of interest in various fields of engineering and science and they commonly display rough morphologies which govern their constitutive behavior. One of the emerging applications of granular materials is their use as proppants in hydraulic fracturing and the present study provides an investigation into the normal contact behavior of three types of proppant simulants using micromechanical-based experiments. As the complex processes in hydraulic fracturing necessitate accurate modeling of the order of nano-to-micrometers of displacements, a micromechanical investigation of grain contact response accounting for extremely small ranges of deformations is necessary to provide insights with potential applications in contact mechanics modeling. The proppant simulants used in the present study are composed of quartz sand grains from different origins with variations of their local surface morphologies and nano-to-micro scale roughness. Based on their local radii, which govern the contact behavior of non-conforming surfaces, the proppant simulants would approximately correspond to mesh 16/30–20/40. In view of their global radii, these grains would be considered on the upper bound of potential sizes used in hydraulic fracturing. In terms of normal contact response, the data were fitted using two contact mechanics models and a discussion is elaborated on their applicability in simulating the normal load–displacement behavior of grains at their contacts. It is shown that surface asperities play an important role in the constitutive behavior of the grain contacts and control the different zones of response as observed from both experiments and analytical expressions. Natural grains used as proppants may display smoother surfaces compared with other geological materials, such as weathered rocks or crushed rocks. However, their roughness is still an important influencing factor that should be considered in the simulation of multi-scale problems, especially when micrometer-level displacements are important to be encountered. Based on differentiation of the normal force for a given range of displacements, tangent stiffness was derived and the data showed that tangent normal stiffness (KN) increases with normal displacement. Thus, the load (or stress) state of a given contact is important to provide appropriate values of the ratio (KT/KN), where the tangential stiffness (KT) decreases rapidly with the increase of the sliding displacement.

In materials science and engineering, a significant amount of research has been carried out using indentation techniques in order to characterize the mechanical properties and microstructure of a broad range of natural and engineered materials. However, there are many unresearched or partly researched areas, such as, for example, the investigation of the shape of the indentation load–displacement curve, the associated mechanism in porous materials with clastic texture, and the influence of the texture on the constitutive behavior of the materials. In the present study, nanoindentation is employed in the analysis of the mechanical behavior of a benchmark material composed of plaster of Paris, which represents a brand of highly porous-clastic materials with a complex structure; such materials may find many applications in medicine, production industry, and energy sectors. The focus of the study is directed at the examination of the influence of the porous structure on the load–displacement response in loading and unloading phases based on nanoindentation experiments, as well as the variation with repeating the indentation in already indented locations. Events such as pop-in in the loading phase and bowing out and elbowing in the unloading phase of a given nanoindentation test are studied. Modulus, hardness, and the elastic stiffness values were additionally examined. The repeated indentation tests provided validations of various mechanisms in the loading and unloading phases of the indentation tests. The results from this study provide some fundamental insights into the interpretation of the nanoindentation behavior and the viscoelastic nature of porous-clastic materials. Some insights on the influence of indentation spacing to depth ratio were also obtained, providing scope for further studies.

In the present study, the micromechanical behaviour at the interfaces of sand grains coated with kaolinite powder is examined. Two different classes of coated grains were produced, one with lighter coating and one with heavier coating. The interface micromechanical experiments were conducted on pairs of grains and the emphasis was placed on the normal and tangential load-displacement relationships and the inter-particle friction of the different pairs highlighting the differences between uncoated and coated grains and also between the different classes of coating. All the experiments were conducted using a micromechanical custom-built loading apparatus and for the analysis of the data, analytical models from the literature were used, where adequate modifications were applied in specific in the tangential direction to match with the experimental results. Iteration processes, image analyses and microscopic image observations were adopted in the study of the frictional behaviour of the grains as well as the study of the damage of the surfaces. Work done and rate of work done concepts were introduced and linked to the damage of the surfaces of the grains and the role of repeating the shearing tests following the same shearing paths were particularly emphasized. The results from the study can provide some useful insights into the grain-scale behaviour of soil grains as well as input parameters in DEM studies.

Pumice has a large potential to be used in structural and geotechnical engineering projects as a lightweight aggregate, and it is a material of unique characteristics. Gravel-sized pumice from Yunnan, China, was examined in the present study with a custom-built micromechanical apparatus investigating the properties of pairs of grains at their interfaces subjected to monotonic and cyclic tests under different saturation states. Based on interferometer measurements, the vesicular surface of the pumice was characterized to be very rough. From the interface micromechanical experiments, it was shown that the virgin surfaces appeared to be very soft under normal loading tests, during which micro-asperity breakage was caused. After preloading and preshearing, the normal load–displacement curves became much smoother and the Young’s moduli, which remained relatively low in magnitude compared with other geo-materials previously studied, could be quantified applying the Hertzian model. Large hysteretic loops were observed from cyclic normal load tests. Plastic displacements dominated the first normal loading cycle, while elastic displacements prevailed in the subsequent cycles. High values of the apparent coefficient of friction were observed from the monotonic shearing tests. A modification of the power value of the Mindlin and Deresiewicz model was necessary to be applied during the fitting of the experimental data so that theoretical and laboratory curves could match. It was shown that this power had a direct correlation with the coefficient of friction.

Recycled concrete aggregate (RCA) is an excellent substitute of natural aggregates and its use in civil engineering projects has received great attention in recent years. One of the important factors that control the behavior of RCA when used as granular construction material or ballast in geotechnical and infrastructure engineering projects is the mechanical behavior at the contacts of the grains. In the present study, an experimental work is presented investigating the micromechanical behavior of RCA with a focus on the normal and shearing contact response of pairs of grains. Based on these tests, normal and tangential contact stiffness as well as the interface friction coefficient are quantified and thoroughly compared with studies presented in the literature on a broad range of geological and engineered materials. Microscopic image and white light interferometry were applied to inspect changes of grain surface characteristics before and after shearing, while, the basic characterization of the grains was conducted performing SEM analysis, roughness and micro-hardness measurements. The results from the study provide some basic understanding of the contact behavior of RCA interfaces and can be useful in DEM simulations of aggregate assemblies.

The study reports on the small-strain dynamic behavior of two types of carbonate sands from Western Australia and the Philippines. Basic characterization of the soils was performed in terms of specific gravity, grading information, angle of shear strength at critical state, particle shape characterization and composition analysis. Piezo-element inserts were utilized to carry out the dynamic tests. For the Western Australia (WA) carbonate sand, both bender and extender element tests were performed, thus the shear modulus, the Young’s modulus and the Poisson’s ratio were examined. Both vertical and lateral bender element tests were performed on a set of specimens from the Philippines (PH) carbonate sand to study the shear modulus and, from which, no fabric anisotropy of the reconstituted specimens was found. It was observed that the overconsolidated specimens had higher stiffness than those during the first loading stages for both carbonate sands. In the pressure range of the study, grain breakage was small and its effect on the behavior of the samples was almost negligible. Empirical equations in the literature proposed from quartz sands could not predict the stiffness of the carbonate soils satisfactorily. In this regard, a preliminary study was carried out adopting the assumed void ratio that only considers the inter-particle voids instead of the summation of inter- and intra-particle voids; based on this concept, the predicted and measured stiffness (including both small-strain shear modulus, Gmax and small-strain Young’s modulus, Emax) were found to be satisfactorily close.

The study of rate and shearing effects is of major interest in geomechanics and petroleum engineering research. In this paper, a new micromechanical apparatus is presented along with calibration and reliability tests, which is designed in a way that the interface behavior of granular materials can be examined for a broad range of shearing velocities of five-orders of magnitude utilizing a dynamic data-logger and high-resolution sensors. The results showed that the shearing velocity between 0.4 and 1,340 mm/h did not influence the response of dry grain-grain interfaces including engineered and natural grains, but it significantly affected the frictional behavior of composite grain-rubber interfaces.

Polymers, such as polydimethylsiloxane, can be utilised as functional coatings with water repellent properties in granular materials. These new surface-modified granular materials have potential applications in barriers to control wetting. However, limited knowledge exists on the mechanical behaviour of particles with synthetic coatings. Here, a fundamental insight is provided by investigating the micromechanical behaviour of a coated sand with extreme water repellency by means of a custom-built inter-particle loading apparatus. To synthesize the water repellent coatings, an industrial sand was subjected to 3% mass ratio of dimethyldichlorosilane. In a comparative investigation between coated and uncoated (natural) sand particles, the results revealed (1) a decrease of the normal stiffness and (2) a dependency of the inter-particle friction coefficient on the normal force for the coated sand particles. In particular, the inter-particle friction coefficient significantly increased at low normal forces (lower than 2 N) with this effect decreasing as the coatings were damaged under a high normal force (over 5 N), and with the micromechanical behaviour of the coated sand converging towards that of the uncoated (natural) sand. The results are compared to theoretical models and mechanisms are proposed to explain the findings. By changing the nature of the coatings, this paper provides a basis for future research on granular materials with functional coatings for other applications.

We investigate the inter-particle tangential force–displacement behavior of a broad range of granular materials. The major experiments are conducted using a custom-built micromechanical loading apparatus and the emphasis of the work is placed on the microslip displacement. We show that for all the materials tested, the microslip displacement increases with the increase in normal force and that extended threshold displacements are observed for rougher and softer grains. An analytical expression proposed in the literature is modified, incorporating material micro-hardness in a normalized form, to establish an expression which can be used in micromechanical-based analysis of problems involving geological materials.

In this study, the effect of fibre reinforcement on the small-strain shear modulus (Gmax) of sandy soils is investigated. A comprehensive set of 47 specimens composed of different types of sand reinforced with variable percentages of polypropylene fibres were tested using a Hardin-type resonant column apparatus embedded with bender element inserts to study the effect of fibre inclusion and content on the Gmax of sands with different particle shapes and gradations. Accordingly, an expression of Gmax for sands, previously proposed by the last two authors and their colleagues, was systematically modified in this study to include the effect of fibre content to estimate the small-strain shear modulus of fibre reinforced sands. An additional set of dynamic tests was performed to verify the newly proposed expression. It was shown that the newly developed expression can predict the Gmax of sand-fibre mixtures with an accuracy of around ±20%, with the error being limited to ±10% in the majority of the cases.

The mechanical behaviour of rubber mixed with different types of host sands is investigated from micro- to macro-scales. The change of the mobilized friction angle because of rubber inclusion is associated to the change of inter-particle friction angle and the friction angle due to particle rearrangement. The mobilized friction angles at the peak and critical states were obtained from triaxial testing data, and the inter-particle friction angles at different sand and sand-rubber interfaces were measured by means of grain-scale experiments. It is found that: (i) The peak friction angle decreases after adding rubber particles for all host sands, which is more significant for dilative sands. (ii) For smooth and non-breakable sands, adding rubber increases the friction angle at the critical state, due to the increasing inter-particle friction from sand-rubber interfaces, but the reduction of sand-sand contacts weakens the interlocking, leading to easier particle rearrangement. For rough and breakable sands, adding rubber decreases the friction angle at the critical state, since the inter-particle friction of sand-rubber interfaces is lower than that of pure sand interfaces, but leads to less particle breakage and thus harder particle rearrangement. (iii) State parameter works well in assessing the effect of rubber inclusion on the peak strength.

Recycled rubber in granulated form is a promising geosynthetic material to be used in geotechnical/geo-environmental engineering and infrastructure projects, and it is typically mixed with natural soils/aggregates. However, the complex interactions of grains between geological materials (considered as rigid bodies) and granulated rubber (considered as soft bodies) have not been investigated systematically. These interactions are expected to have a significant influence on the bulk strength, deformation characteristics, and stiffness of binary materials. In the present study, micromechanical-based experiments are performed applying cyclic loading tests investigating the normal contact behavior of rigid–soft interfaces. Three different geological materials were used as “rigid” grains, which have different origins and surface textures. Granulated rubber was used as a “soft” grain simulant, this material has viscoelastic behavior and consists of waste automobile tires. Ten cycles of loading–unloading were applied without and with preloading (i.e., applying a greater normal load in the first cycle compared with the consecutive cycles). The data analysis showed that the composite sand–rubber interfaces had significantly reduced plastic displacements, and their behavior was more homogenized compared with that of the pure sand grain contacts. For pure sand grain contacts, their behavior was heavily dependent on the surface roughness and the presence of natural coating, leading, especially for weathered grains, to very high plastic energy fractions and significant plastic displacements. The behavior of the rigid–soft interfaces was dominated by the rubber grain, and the results showed significant differences in terms of elastic and plastic fractions of displacement and dissipated energy compared with those of rigid interfaces. Additional analysis was performed quantifying the normal contact stiffness, and the Hertz model was implemented in some of the rigid and rigid–soft interfaces.

We investigated experimentally the tribological behavior of tailing grain contacts using a micromechanical apparatus which allows high precision of force and displacement measurements to derive contact stiffness. A technique was developed to apply biopolymer coating to the grain surfaces and the emphasis of the study was placed on the investigation of the influences of saturation conditions, the presence of polymer-based coating, and the abrasion on the frictional behavior of the grains. Material characterization was based on interferometry, micro-indentation and elemental composition analyses. Elastoplastic displacements dominated the first cycles of normal loading and the Young’s modulus was interpreted based on different contact models. The tailing grains showed significantly higher inter-particle friction compared with that of quartz grains. Three major characteristics which influenced the frictional behavior of the grain contacts were the abrasion, which was more dominant in the first loading cycles, the high roughness of the grains and the presence of the biopolymer coating which increased significantly the friction.