Medium-strain dynamic behavior of fiber-reinforced sand subjected to stress anisotropy

A comprehensive database is established to investigate the behavior of polypropylene fiber reinforced sands under anisotropic stress state in a wide range of strain amplitudes from about 4 × 10−4% to 1.4 × 10−1%. A fixed-partly fixed Hardin-type resonant column which has a system that allows the specimen to be tested in resonance while maintaining an anisotropic loading path, is utilized. The results show important influence of the fiber content as well as the anisotropic stress state on the normalized modulus reduction and damping increase curves of the reinforced soils. Specifically, the increase of fiber content and stress ratio tend to increase the linearity in the normalized modulus reduction curves. On the other hand, the inclusion of fiber leads to the damping increase curves to shift to greater values, while the stress ratio has an opposite effect. An expression is proposed to predict the normalized shear modulus, as a function of mean effective confining pressure, stress ratio, coefficient of uniformity of the host sand and fiber content. The damping ratio, in a normalized form, is correlated with the normalized shear modulus reduction

The Development of a Micromechanical Apparatus Applying Combined Normal–Shear–Bending Forces to Natural Sand Grains with Artificial Bonds

Natural soils are often cemented, and there has been a need to better understand and properly model their behavior for the safe design and assessment of critical infrastructure. This necessitates the study of cemented soils at the scale of the grain. In this study, a new-generation apparatus is presented that is capable of conducting complex load path tests on two natural sand grains cemented with an artificial bonding component. Thus, the apparatus gives the opportunity to obtain insights into the micromechanics of cemented soils/weak rocks and contribute to the development of more accurate models to be utilized in the discrete element analysis of geomaterials. Apart from the presentation of the major technical features of the new apparatus, a description of the methods which were used for specimen preparation and mounting, as well as a preliminary set of experiments, are presented and discussed in this note

Micromechanical Behaviour in Shearing of Reproduced Flat LBS Grains with Strong and Weak Artificial Bonds

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

The mechanics of a silt-sized gold tailing

Tailing dam failures result in irreversible environmental impacts and cause fatalities. In recent years the mechanical behaviour of tailing geo-materials has received more attention by the geomechanics and engineering geology communities in an attempt to understand better their behaviour in the light of designing safer tailing dams. In this study, the mechanical behaviour of a gold tailing from Brazil is thoroughly investigated by conducting a series of compression and shearing tests as well as dynamic element tests. Fabric effects from the sample preparation method, the susceptibility to liquefaction and the possibility of any transitional behaviour are presented and discussed within a soil mechanics framework. Comparisons are made between the present gold tailing and previously published data on other tailings, giving a general view of the mechanics of tailings and the effects of grading. The results show that for this tailing the rate of convergence for different initial densities to the normal compression line is slow, and so the depositional density would affect the volume to far higher stresses than the material would be expected to experience in-situ. For this tailing any fabric effects from the sample preparation method were found to be very small to negligible with respect to small-strain behaviour and critical state behaviour. For different tailings, even if the particle sizes may cover a wide range, the susceptibility to static liquefaction, as determined by the location of the horizontal asymptote of the critical state line in the specific volume-log stress plane, shows no consistent variation. So it can be concluded that neither the pond nor the upper beach tailings are more susceptible

Investigation of the micro-mechanics of sand–rubber mixtures at very small strains

This study presents a series of discrete element method (DEM) simulations consisting of mixtures of stiff (sand) and soft (rubber) particles, subjected to monotonic triaxial shearing under constant volume at very small strains where pure elasticity governs the behaviour of the samples. The elastic shear moduli of the simulated pure sand and pure rubber samples were first calibrated to values reported in previous experimental works. Sand–rubber mixtures were then simulated with a focus on small-strain stiffness to examine the role of rubber content on the prevailed micro-mechanisms of the samples. The macro-mechanical response of the numerical mixtures showed a decrease in the elastic shear modulus and the deviatoric stress as the soft particle content increased, in line with observations from laboratory tests. Micro-scale information including coordination number, fabric tensor and normal contact force anisotropy was obtained for all tests and the contribution of each type of contact, i.e. sand–sand, rubber–sand or rubber–rubber, in the overall response of the samples, was analysed. The contact force network in the mixtures changed from being sand-dominated to rubber-dominated, with the presence of an intermediate zone in between rubber and sand particles forming a stable contact force network mainly by sand–rubber contacts. Each type of contact was seen to contribute differently to the deviatoric stress in the system as the rubber content increased.published_or_final_versio

Experimental study on the coefficient of restitution of grain against block interfaces for natural and engineered materials

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 micro-mechanical 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/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 micro-mechanical 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 0.75-0.95) and rubber block (ranging between 0.37-0.44) 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

Multiple contact compression tests on sand particles

Particle crushing has been recognised to be of key importance for many engineering applications. In soil mechanics, this phenomenon has become crucial in defining a complete framework able to describe the mechanical behaviour of sands. In this study, the effect of multiple discrete contacts on the breakage of a grain was investigated, crushing coarse grains of a quartz sand and a crushed limestone sand between a number of support particles, thereby varying the number of contacts, i.e. the coordination number. The stress at failure was calculated when the particle broke, which was through a number of distinct modes, by chipping, splitting or fragmenting which were observed with the use of high speed microscope camera. The Weibull criterion was applied to calculate the probability of surviving grain crushing and the fracture modes were observed for each configuration of the supporting particles. The data showed that in addition to the number of the contacts the nature of those contacts, controlled by the particle morphology and mineralogy, play a significant role in determining the strength of a particle. The sphericity affected the strength for the softer limestone while the local roundness at the contacts was important for the harder quartz sand. Catastrophic explosive failure was more often observed in particles with harder contacts while softer contacts tended to mould relative to their neighbouring particles inducing a more frequent ductile mode of crushing

Experimental study of sand grains behavior at their contacts with force- and displacement-controlled sliding tests

Discrete element modeling requires the proper quantification of the behavior of grains at their contacts including the normal force – displacement and tangential force – displacement relationships to be used as input for contact modeling purposes. This paper reports on recent advances in soil mechanics experimentation which allowed measuring the grain contact behavior of small sand particles quantifying friction and stiffness with sliding tests of a force-controlled or displacement-controlled type. The particular focus of this work is on the micromechanical behavior of quartz type grains of size between about 1 and 5 mm. A description of the developed micromechanical apparatus at City University of Hong Kong is first discussed and its important different capabilities with previously developed apparatus is briefly reviewed. Subsequently, a limited set of new data is reported and discussed along with a review of recently acquired results published in the literature associated with the contact behavior of quartz sand grains. These sliding tests have covered a wider range of normal contact forces from about 0.5 to 8 N, and the results indicated that, for this range of confining forces, there is not any notable change of the inter-particle coefficient of friction. Keywords: Micro-mechanics, Inter-particle friction, Inter-particle stiffness, LBS grains, Experimental mechanic

Dynamics of potential fill–backfill material at very small strains

The paper presents a synthesis of past and recently acquired laboratory test results on granular soils using wave propagation techniques at very small strain amplitudes. Resonant column tests on uniform to well-graded coarse sands and gravels of angular and low sphericity grains were analyzed. Empirical-type equations were developed for the prediction of the elastic modulus and material damping at small strains considering the effects of the grading characteristics, the isotropic effective stress and the void ratio. The G0–p' relationship, expressed through exponent nG, was affected by the sample preparation method. For the narrow range in relatively low pressures adopted in the study, it was observed that nG decreased slightly with an increase in relative density. Due to the limited initial void ratios of those tests, the effect of the preparation method was not incorporated into the proposed formulae for the nG prediction. In this direction, additional experiments from the literature, which adopted the resonant column and bender element methods, were further analyzed, including soils of variable types tested with a wider range in relative densities. By employing typical formulae from the theory of elasticity, the bulk modulus and the changes in void ratio were estimated based on the change in isotropic effective stress in the literature data. Considering the recent micromechanical experimental findings associated with the nature of the contact response of soil particles, the importance of soil type and particle-contact behavior in the constant-state response of soils was demonstrated and quantified. Material damping values ranged from about 1.10% to about 0.45% for p' from 25 to 200 kPa with a slight decrease in Ds0 with an increase in pressure