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This paper presents a numerical study of the mechanical properties of a binary mixture of similar prolate ellipsoids of various particle shapes and fines' contents using the discrete element method. The ratio of the major axis of a small ellipsoid to that of a large ellipsoid is 0.2. Because all particles have the same shape, the particle shape of the binary mixture is defined unambiguously. The effect of fines content is examined with samples of the same particle shape. The effect of particle shape is examined by comparing the samples of different particle shapes. Very dense samples are generated and sheared under drained and constant volume conditions. The macroscopic and microscopic behaviors of these samples are studied at both peak and critical states. The mechanical behavior can be grouped into two or three regions for our binary mixture depending on the fines content. Although the critical state line varies with fines content, a unified critical state line is identified using the intergranular void ratio, which is a function of fines content and a parameter. The parameter is a constant for the samples of the same particle shape. This constant is different for the samples of different particle shapes. The finding explains why some studies defined the parameter as a function of fines. The intergranular void ratio does not relate well to the peak friction angle. The microscopic behavior is examined through contact normal, particle orientation, and stress partition. Although some microscopic parameters can identify the region where the sample belongs, we do not find any dominant microscopic parameter that relates to the macroscopic behavior of a binary mixture of various fines contents. [ABSTRACT FROM AUTHOR]

Understanding the effect of particle size on the shear strength of granular materials is important for geotechnical design and construction. However, previous studies show contradicting results on the relationship between particle size and shear strength. Additionally, the effect of particle breakage on this relationship has not been fully revealed. In this study, a series of biaxial tests have been simulated with the discrete element method to explore the particle-size effect of sand considering the role of particle breakage. The sand specimens have parallel particle-size distributions. The sequential breakage model has been used to simulate particle breakage, which is a combination of replacement and cluster methods. The main conclusions of this study are: (1) the relationship of peak shear strength and particle size depends on the crushability of particles and relative density of specimens; (2) the particle size and crushability have a very slight effect on the residual shear strength; and (3) at the microscale, the relationship between shear strength and particle size is positively related to the friction utilization ratio. [ABSTRACT FROM AUTHOR]

The recently developed framework for modeling grain breakage in granular materials is shown to be valid for saturated/unsaturated conditions. The model describes the initiation, development, and stabilization of breakage using three parameters, all depending on the strength of a representative particle size. When water is introduced, the strength of the soil's particles is expected to weaken, which leads to increased breakage intensities. The model is able to predict the early onset of crushing and the increase in particle breakage rate, both considered characteristic behaviors of a crushable granular medium in wet conditions. Initially dry and saturated model lines limit the evolution of the model parameters in the intermediate saturation states. A mid-loading change in the saturation of the granular medium results in an increased rate of particle breakage. The experimental data available in the literature is analyzed in light of the proposed model, which allows a better understanding of the development of particle breakage under the influence of water. [ABSTRACT FROM AUTHOR]

Internal erosion is a major cause of dam or dike failures and incidents. In this study, laboratory tests were conducted to investigate the deformation of two gap-graded soils during internal erosion and the stress-strain behavior of the soils that experienced loss of fine particles due to internal erosion. An erosion-controlled experimental method was adopted to achieve a designated loss of fine particles during internal erosion by adding a predefined amount of salt into the soil sample during sample preparation and dissolving the salt in water under a controlled stress condition. Both the radial and axial deformations during erosion were measured using a photographic method. Subsequently, drained triaxial compression tests were performed to study the stress-strain behavior of the soils that had lost different amounts of fine particles. The peak friction angle and critical friction angle of the soil decreased with the loss of fine particles. After a significant loss of fine particles, the stress-strain behavior changed from the initially dilative behavior to a more contractive behavior. [ABSTRACT FROM AUTHOR]

Partially saturated compacted-sand specimens were characterized by using three-dimensional (3D) image registration of dualmodal (neutron and X-ray) tomography data. Neutron and X-ray imaging provide complementary information for precisely identifying the three phases (silica sand, air, and water) of a compacted sand specimen that is partially saturated. Neutron tomography provides high contrast of the water phase, whereas X-ray tomography provides high contrast of the silica sand phase due to different fundamental interaction mechanisms of neutron and X-ray with matters. X-ray interacts with the electron cloud surrounding the nucleus, whereas neutron radiation interacts with the nucleus of an atom. In this paper, a computational technique was developed to unify digital images of dual-modal data obtained at different image resolution and specimen orientations based on the maximization of the normalized mutual information to combine the information from the water phase inferred from a neutron image and information from the sand and pore phases obtained from an X-ray image. The combined microstructure obtained from registered X-ray and neutron images can thus be analyzed in 3D to precisely differentiate silica, air, and water distribution spatially. Microstructure information for granular assembly including pore size distribution and coordination number was determined from X-ray tomography data due to higher precision in identifying solid particle boundaries. Local values of saturation along the height of the specimen are obtained from the pore and water phases segmented from the registered neutron and X-ray images. Image registration of neutron and X-ray images provides the ability to obtain the microstructure of partially saturated sand in terms of the detailed variation of solid particles, air, and water phases spatially. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the concept of effective density for the propagation of small-strain shear waves through saturated granular materials. Bender element tests and resonant column tests were conducted on various granular materials in dry and saturated conditions. Values of small-strain shear modulus for the dry condition are compared with corresponding values for the saturated condition, which are calculated using saturated density and effective density. For bender element tests, the use of saturated density produced errors as high as 28% in the shear modulus, whereas the use of effective density resulted in errors generally less than 5%. For resonant column tests, errors in the shear modulus that were obtained using saturated density were smaller than those for bender element tests because of the lower range of excitation frequency and effect of mass polar moment of inertia of the loading system. A quick chart is provided to help users determine if effective density should be considered for a given application. Effective density is expected to be important for fine and medium sands at high-frequency excitations, such as bender element tests, coarse clean sands at lower frequencies, such as for resonant column tests, and clean gravels at essentially all frequencies of geotechnical interest. [ABSTRACT FROM AUTHOR]

Calculation of volume changes by constitutive models for soils and correct transition from contraction to dilation is important for prediction of undrained conditions under which developing pore water pressures may result in instability or in increased stability. Partly drained conditions involving imposed volume changes occur under many field-loading conditions, and prediction of the correct soil response is important for analysis of geotechnical structures. The effects of imposed volume changes on the instability of granular materials are studied in experiments on a fine sand in which water is forced into or out of the sand specimen during shearing. The effects on stress-strain behavior and stability are recorded and discussed. The conditions of imposed volume changes are then simulated with a single hardening constitutive model and compared with the experimental results. It is shown that the model can predict the corresponding loading paths and sand behavior as well as the resulting stable and unstable conditions. [ABSTRACT FROM AUTHOR]

Discrete element method (DEM) simulations are used for measuring the macroscale and microscale changes in stress and fabric for the cyclic loading of a two-dimensional granular assembly. The stress-strain-dilatancy tendencies are similar to those of sands. After repeated loading cycles, the fabric follows a steady-state circuit of evolution within each cycle, but reversals in the loading direction are always accompanied by abrupt changes in the fabric. A consistent and strong inverse relationship is found, however, between the average coordination number and an effective void ratio, defined by excluding particles that do not participate in supporting the applied stress. A strong and consistent correlation is also found between the deviator stress ratio and a trong fabric ratio throughout the cyclic loading. [ABSTRACT FROM AUTHOR]

Granular beds made of activated alumina and magnesium oxide used for recycling/processing water in space life-support systems are usually compressed using a spring mechanism for effective functioning. The spring force is a critical factor in providing appropriate compaction to the entire length of the granular bed, and therefore, it is important to investigate and predict the axial stress and displacement along the bed. This paper presents results from experimental, numerical (finite-element), and analytical models of a full-scale dry granular material bed of activated alumina packed in a cylinder. The finite-element (FE) model, when used with proper contact model and properties, predicted both the axial stress and the displacement along the length of the granular bed well. Janssen's one-dimensional model did not provide as accurate predictions for stress because of simplifications associated with the model. Because the system is long and slender, it is very important to properly model the wall friction in predicting the behavior of the granular bed. All the assessments used in this study indicated that the axial stress decays very steeply along the bed. [ABSTRACT FROM AUTHOR]

This paper presents a bounding surface plasticity model that can be used to simulate complex monotonic and cyclic loading paths. A new mapping rule that only uses the last stress reversal point is introduced to describe the stress-strain behavior of granular soils during loading and unloading. This mapping rule is easy to implement and is suitable for highly erratic cyclic loading conditions, e.g., those induced by earthquake or traffic loading. The application and performance of the model are demonstrated using the results of experimental tests with various stress paths conducted under both monotonic and cyclic loading conditions. The study shows the efficiency of the new mapping rule in capturing the characteristic features of the behavior of granular soils under various loading paths. [ABSTRACT FROM AUTHOR]

The undrained behavior of sand with fines under both monotonic and cyclic loading was investigated experimentally by triaxial testing. The testing program covers fines content ranging from 0 to 30%, and a wide range of initial void ratios and confining stresses. The spectrum of cyclic loading includes one- and two-way cyclic loading, and in the latter case, the cyclic loading may be symmetrical or nonsymmetrical. The influence of fines content can be captured by using the equivalent granular void ratio, , in lieu of the void ratio, . All steady-state data points can be described by a single relationship referred to as the equivalent granular steady-state line. This study proposes the concept of the equivalent granular state parameter, , for capturing the combined effects of the stress state, density state, and fines content. The observed behavior can then be related to and/or . Irrespective of fines contents, the behavior under monotonic loading can be classified into three types, and is a predictor of the behavior type. Cyclic liquefaction, when triggered by adequate cyclic stress and load cycles, can occur in three different forms. Irrespective of the fines content, determines the form of cyclic liquefaction. Furthermore, under certain conditions, the monotonic and cyclic behavior can be correlated. [ABSTRACT FROM AUTHOR]

Anaerobic decomposition of organic materials in landfills is a key source of anthropogenic methane () emissions into the atmosphere. The use of methane biofilter (MB) technology for biological oxidation of into carbon dioxide () and water is an emerging area of interest for researchers and practicing engineers. Studies were undertaken to assess the influence of dissolved organic carbon (DOC), specific oxygen uptake rate (SOUR), and exchangeable nitrogen () on different materials used as filter media in MBs. The obtained results showed that these parameters are strongly correlated with the oxidation potential observed for different porous materials. Low DOC, SOUR, and values [318 parts per million (ppm), 168 ppm, and 228 mg volatile , respectively] were associated with the highest methanotrophic activity rate observed in this study (3.6596 μmol ). [ABSTRACT FROM AUTHOR]

Pressure loss () is a key parameter for estimating biofilter energy consumption. Accurate predictions of as a function of air velocity () are, therefore, essential to assess energy consumption and minimize operation costs. This paper investigates the combined impact of medium particle size and shape on the relationship. The measurements were performed using three commercially available materials with different particle shapes: crushed granite (very angular particles), gravel (particles of intermediate roundness), and lightweight clay aggregrate (almost spherical particles). A total of 21 different particle-size fractions, with particle sizes ranging from 2 to 14 mm, were considered for each material. As expected, decreased with increasing particle size in agreement with earlier findings. The value of , however, also showed a tendency to decrease with increasing particle roundness especially for fractions containing smaller particles. A new model concept for estimating across different particle-size fractions and shapes was proposed. This model yielded improved prediction accuracy in comparison with existing prediction approaches. [ABSTRACT FROM AUTHOR]

The propagation of dry granular flows, such as rock and snow avalanches, can be described by depth-averaged models. Different from classical shallow-water equations, these models take into account the anisotropy of normal stresses inside the flowing pile through using an earth pressure coefficient in the pressure term. A new regularization function for calculating the pressure coefficient in the Savage-Hutter-type models at the early stages of dam-break flows and collapses is proposed. In such circumstances the flow lines are significantly curved with respect to the basal surface and a special treatment of the earth-pressure coefficient is required for obtaining a satisfactory agreement with experimental data. The comparison between numerical simulations and laboratory experimental data shows an apparent improvement in describing the early stages of dam-break waves over rough beds. The comparison with experiments over smooth bed surface exhibits minor evidence of improvement. Nonetheless, in this case the proposed formula yields results similar to what obtained by the original Savage-Hutter formula. [ABSTRACT FROM AUTHOR]

For urban drainage, the role of granular media has expanded from particulate matter (PM) filtration to media also engineered for phosphorus (P) mass transfer. In this paper, equilibrium between P as total dissolved P (TDP) and granular substrate-based aluminum oxide (AlOx) media are investigated. Aluminum oxide coated media (AOCM) are clay- (), pumice- (), and concrete-based (), and their native substrates [uncoated media with a clay substrate (UCMs)] serve as control media. Equilibrium is modeled with Freundlich isotherms. Isotherms are evaluated in municipal wet weather (runoff), dry weather (wastewater), and a surrogate deionized (DI) water matrix as a control, each at neutral pH. Compared to UCMs, AlOx-coated (clay, pumice) and AlOx-admixture media (concrete) yield significantly higher equilibrium capacity. For runoff and wastewater, and generate lower capacity compared with the surrogate matrix due to competitive effects, and ligand exchange is hypothesized as a primary mass transfer mechanism. has higher capacity at higher TDP levels with surface precipitation hypothesized as a primary mechanism. For equal TDP levels, capacities of and exceed . The native substrates of AlOx media and also the aqueous matrices have significant impacts on equilibrium. In contrast, Part II compares surface and physical indexes, equilibrium, and leaching for a wide range of media using a surrogate DI matrix. The two parameter Freundlich model is compared to a partitioning parameter to elucidate equilibrium for media with disparate capacities. [ABSTRACT FROM AUTHOR]

Aerobic granulation has been extensively investigated in recent decades but most researchers have focused their studies on granule formation in laboratory-scale reactors using synthetic wastewaters at high organic loading rates (OLR) (). This research studied the aerobic granulation in a sequencing batch reactor (SBR) using real domestic wastewater with an OLR at or lower than . The reactor was operated in three different stages (Stage I, II, and III), according to the applied OLR (1.4, 1.0, and , respectively). After 140 days of operation (Stage II), the reactor biomass consisted mainly of compact mature granules (average diameter of 0.7 mm) with a good settling ability ( approximately ). The OLR increase caused a partial disintegration of the mature granules during the beginning of Stage III. However, the granules were able to quickly recover after the disturbance caused by the higher OLR. The reactor was effective at removing carbon and nitrogen compounds, especially when operated with an OLR of , providing an average removal efficiency of 92% for and 96% for . The results demonstrated that aerobic granules can be formed, using real domestic wastewater, with an effective carbon and nitrogen removal. [ABSTRACT FROM AUTHOR]

Vortex-like flow patterns are often observed in experiments on granular media for which uniform strain is expected based on the loading boundary conditions. These deformations become apparent when the motion associated with uniform strain is subtracted from the total particle motion. Besides presenting an interesting phenomenon that begs explanation, these vortex patterns suggest multiscale structure for nonaffine motion as suggested by modern continuum theories. Further, the authors note that the rotational velocity field added to a uniform strain field produces a planar slip field. Thus, these structures are associated with the slip-band fields that eventually form, which are generally associated with bifurcations in the solution path of the governing partial differential equations. The authors present a procedure to extract these motion fields from discrete-element simulations along with conjugate forces associated with these motions. A key finding from the simulations is that the motions that eventually lead to shear band formation develop throughout the loading history rather than arising as a distinct bifurcation. Further, the pattern of rotational fields, and hence the shear banding pattern, are controlled by the boundary conditions. A question, only partly resolved here, is the origin of forces driving the rotational fields. [ABSTRACT FROM AUTHOR]

To consider the benefit of geosynthetic reinforcement in a mechanistic-empirical pavement design method, the resilient modulus and permanent deformation behavior of a geosynthetic-reinforced unbound granular material (UGM) must be considered. Many researchers conducted repeated-load triaxial (RLT) tests to investigate the resilient and permanent deformation behavior of the geosynthetic-reinforced UGM. However, these tests are difficult to perform, and the results are often interpreted empirically. Thus, implementation of the research results is limited. In this study, an analytical model was developed to predict the resilient modulus and permanent deformation of the geosynthetic-reinforced UGM in RLT tests. The analytical model is compatible with the resilient modulus and permanent deformation models in the current mechanistic-empirical pavement design guide. Both planar and three-dimensional geosynthetics can be analyzed using this model. RLT test results from two published studies were selected to validate the proposed analytical model. In general, the analytical results confirmed and explained the typical test observations from previous studies that geosynthetic reinforcement is more effective in reducing the permanent deformation than increasing the resilient modulus of the UGM sample. A parametric analysis was conducted to investigate the effect of input parameters (i.e., material properties, sample dimensions, and stress level) on the analytical model. The limitations, assumption, and implementation of the model are also discussed. [ABSTRACT FROM AUTHOR]

The importance of particle rolling resistance to the mechanical behavior of granular materials is well recognized and has been a topic subject to intensive discrete element method (DEM) investigation over the last two decades. However, little effort has been made to explore the energy input and dissipation behavior under the influence of varying degrees of interparticle rolling resistance, especially in relation to the development of shear band. This paper aims to eliminate this deficiency through a comprehensive two-dimensional DEM study on the relationship between the particle-scale energy dissipation and shear band development. Novel insights into the energy allocation at the small- and large-strain stages, and the development of localized bands of sliding and rolling dissipations, as well as the anisotropy of accumulated sliding and rolling dissipations within the shear band are presented for the first time. [ABSTRACT FROM AUTHOR]

Resilient moduli of pavement layers are the basic input parameters for the design of pavements with multiple layers in the current mechanistic empirical pavement design guides. Field measurements are generally believed to provide accurate values to back-calculate pavement layer moduli, but the tests to measure the relevant parameters are expensive, difficult to perform, and cause disturbances to the public. Therefore, the back-calculation of pavement layer moduli from laboratory scale model testing has been a focus of recent pavement research. This paper presents a back-calculation analysis to evaluate pavement layer moduli by using a three-dimensional numerical model developed using the FLAC3D finite difference software. The pertinent measurements that are required for the back-calculation analysis were collected from a cyclic load testing facility under traffic type cyclic loading conditions, with a typical pavement structure consisting of a granular road base lightly stabilized with cement-fly ash over an expansive soft clay subgrade. This study indicates that the stabilized base material had cross-anisotropic resilient properties, with an average vertical resilient modulus of 2,875 MPa and an average horizontal resilient modulus of 1,598 MPa. From this investigation, the resilient moduli of the stabilized granular base layer and subgrade clay were reliably back-calculated from the analysis by using the FLAC3D numerical model. [ABSTRACT FROM AUTHOR]

This paper presents the results from four centrifuge experiments modeling light, nonaqueous phase liquid (LNAPL) migration in a sandy soil. These experiments were performed to evaluate the performance of a soil-cement wall used as a containment barrier and to investigate the effects of groundwater flow on LNAPL migration behavior. Centrifuge modeling experiments were performed at 30 g to simulate 80 days of LNAPL migration through the soil. Pore water pressure measurements and video recordings were used to evaluate the LNAPL migration behavior. Results show that in all tests, the water level was depressed because of the large volume of LNAPL confined between the walls. When groundwater flowed, the LNAPL migrated faster and deeper than when there was no groundwater flow. As a result, the depth of a soil-cement wall should be designed to a greater maximum depth to account for the groundwater-flow effects. In addition, numerical simulations were performed and validated with the centrifuge test results. Both methods showed a good agreement as they provided similar behavior of the LNAPL migration and confirmed the effective performance of the soil-cement wall as a containment barrier. [ABSTRACT FROM AUTHOR]

A model for the water retention behavior of unsaturated granular soil is developed by extending the classic bundled cylindrical capillary representation of pore space to a geometry more closely approximating that of granular porous media. Expressions for pore-scale saturation are derived as functions of matric suction for a three-dimensional unit pore comprising angular pore space bounded by spheres in simple cubic packing order. Water retention curves are modeled by assigning a statistical distribution of pore sizes optimized to best match experimentally determined retention curves. A key model attribute is its capability to capture evolution of fluid partitioning along drainage or wetting paths by differentiating pore water retained as thin films adsorbed to particle surfaces, liquid bridges retained in wedge-shaped pores, and saturated pockets in relatively small pores. Interfacial surface tension, solid-liquid contact angle, wetting direction, and mineralogy (Hamaker constant) are treated as model variables to examine their influences on water retention. Modeled retention curves compare well with measured curves obtained for glass beads, natural sands, and soils from the literature. The model effectively estimates hysteretic wetting-drying response, surfactant-induced surface tension lowering, and air-water interface area as a function of saturation. Capability to model evolution of fluid distribution has potentially important implications toward more robust understanding of macroscopic hydraulic, electrical, thermal, and mechanical behavior of unsaturated soil. [ABSTRACT FROM AUTHOR]

High spatial resolution () neutron tomography was performed on partially water-saturated compacted silica sand specimens with two different grain morphologies (round and angular) at Helmholtz Zentrum Berlin using cold neutrons at the cold neutron radiography and tomography beam line. A specimen mixed with heavy water was imaged for contrast comparison purposes. Microfocus X-ray imaging was also performed on these specimens with slightly higher resolution () using geometric magnification to locate the solid phase (silica particle boundaries) more precisely. Image processing was performed to remove unwanted gammas detected because of the gadox scintillator used for the high-resolution neutron imaging system. The visualization of solid, gas, and liquid phases for different grain morphologies is presented at the grain level. Using dual-modal contrast possible from simultaneous use of neutrons and X-rays, the authors introduce, for the first time, an improved ability to distinguish solid silica, liquid water, and gas phases. Quantitative analysis using three-dimensional tomography data is demonstrated for obtaining void ratio, void percentage variation over the height, and particle size distribution. [ABSTRACT FROM AUTHOR]

One of the most critical components in the oxygen-generation and water-processing assemblies for human habitation in space is the deionizing (DI) bed-a packed bed of ion-exchange resin beads-that purifies water. The DI bed shrinks during the course of its operation, and therefore, for the bed to work satisfactorily, it must be kept properly compressed. To understand the force-transferring mechanism along the bed, sets of experimental programs were developed and conducted on the individual particles and bed samples of a DI granular material, Amberlite IRN-78. The presence of water reduced the load-bearing capacity of the individual particles, because most of the wet (water submerged) particles tested failed under relatively small crushing force, whereas the dry particles withstood a much higher load. The particle crushing force was found to be closely represented by normal and Weibull distributions. The DI material bed has a relatively small internal friction angle (shear strength). The lateral pressure coefficient was found to be relatively high. The material exhibited time-dependent behavior, creep, and stress relaxation, possibly through particle deformation and rearrangement. The friction force between the DI medium and the wall of the housing cylinder was significant, and increased rapidly in a nonlinear fashion with increasing sample length. The effect of friction was more significant for more densely packed beds. The loading and unloading tests showed that the compacted material bed exhibited an anelastic type of hysteresis behavior. The uncompacted bed was seen to have nonlinear load-deformation behavior, with a rapid increase in displacement with load, whereas the compacted bed showed linear or near-linear, load-displacement behavior. [ABSTRACT FROM AUTHOR]

Straining depicts the geometrical retention of fine particles within materials' pore network. Whether or not fines can be retained depends on (1) the structure of materials' pore network as well as (2) fines' size distribution, manner of deposition, and retained amount. In this paper, the main results of straining tests are presented for randomly packed beads filters. The experimental program included a series of tests performed in both dry and water-saturated filters. The tests provided information on the dynamics of fines in the filters' pore network and changes of filters' efficiency. For instance, data supported the existence of four distinct stages: the filling of accessible cavities, the formation then clogging of the preferential pathways, the transfer of fines toward the lateral unobstructed capillaries, and eventual formation of a surface cake. The postulated mechanisms inherent to every single stage are discussed in terms of retention rate, straining length, lateral transfer, and progressive obstruction of constrictions. Additionally, the straining mechanisms were also probed in hydrodynamic conditions at increasing water flows. [ABSTRACT FROM AUTHOR]

This paper presents the material response of a granular material under different gravity fields using the discrete element method (DEM). A bidispersive porous medium is prepared and sheared under various gravity conditions. The medium is composed of two kinds (shape and size) of ellipsoids. Each type of particle occupies 50% of the whole mass. The ellipsoids are randomly generated and deposited in lunar gravity (1/6 g), Earth's gravity (1 g), and elevated gravity (10 g). Various friction coefficients are used during the deposition stage, which yields samples of different initial densities. Afterward, samples are isotropically consolidated to different initial confining pressures, and the friction coefficient between particles is 0.5. A total of 25 numerical samples have been created. Very large strain is applied to the sample under drained or undrained conditions. The granular material comes into the critical state. Overall, the gravity effect on peak friction angle (shear strength of the material) is negligible. The critical states of all samples are very similar. There is one observed gravity effect; a greater level of gravity produces samples of higher coordination number. [ABSTRACT FROM AUTHOR]

This research study was performed to examine the shakedown behavior of geogrid-reinforced unbound granular materials and evaluate the factors that affect the shakedown stress limits. To achieve this objective, the study was performed through conducting repeated load triaxial (RLT) tests on both unreinforced and geogrid-reinforced unbound granular specimens. A multistage RLT test, in which only one sample is needed to determine the shakedown stress limits, was selected in preference to a single-stage RLT test, in which several tests on multiple samples are required. Five geogrids of different tensile modulus and different aperture geometries (three rectangular or biaxial and two triangular or triaxial) were used. The test results clearly demonstrated the potential benefits of placing the geogrid within the unbound granular base specimen in terms of permanent deformation reduction. The benefits of the geogrid are more prevalent at higher stress levels and for higher tensile modulus geogrids. The inclusion of the geogrid has more of an effect on delaying the occurrence of range B shakedown behavior (i.e., increasing the plastic shakedown limit) than that of range C shakedown behavior (i.e., increasing the plastic creep limit) for specimens prepared at the optimum and the dry side of optimum. For specimens prepared at the wet side of optimum, geogrids have a negligible effect on delaying the occurrence of range B shakedown behavior (i.e., increasing the plastic shakedown limit) but have a significant effect on delaying the occurrence of range C shakedown behavior (i.e., increasing the plastic creep limit). Of the five geogrids used, the triaxial geogrid TX2, with triangular geometry and the highest tensile modulus, performed consistently better than the other four geogrids. [ABSTRACT FROM AUTHOR]

This work deals with the evaluation of the most suitable expression for the motion resistance of a debris flow. In particular, it focuses on inertial debris flows, i.e., granular-fluid mixtures in which the particle inertia dominates both the fluid viscous force and turbulence; it provides, through an order-of-magnitude analysis, the criterion to be satisfied for a debris flow to be considered inertial and shows that most of real-scale debris flows match this description. The analytical relation between flow depth, depth-averaged velocity, and tangent of the angle of inclination of the free surface is then used in steady, uniform flow conditions to approximate the flow resistance in depth-averaged mathematical models of debris flows. That resistance formula is tested against experimental results on the longitudinal profile of steady, fully saturated waves of water and gravel over both rigid and erodible beds, and against field measurements of real events. The notable agreement, especially in comparison with the results obtained using other resistance formulas for debris flows proposed in the literature, assesses the validity of the theory. [ABSTRACT FROM AUTHOR]

In this study, a simple idealization of heat transfer by conduction at the interparticle contacts is utilized for the evaluation of thermal conductivity of granular soils when subjected to external loading conditions. Discrete element method simulations employing this idealization were performed to examine the impact of loading the soil in a consolidated drained triaxial test environment on soil thermal conductivity. Results of conducted simulations show that shear-induced anisotropy results in an anisotropic thermal conductivity tensor. The results also indicate that contact density affects the average thermal conductivity of granular materials. The larger the average number of contacts per particle, or the coordination number, the larger the thermal conductivity. During shearing, the coordination number tends to decrease, resulting in a reduction in soil thermal conductivity with dense soils, showing a larger decrease in thermal conductivity upon shearing. [ABSTRACT FROM AUTHOR]

Pressure drop () as a function of air velocity () is a key parameter controlling air cleaning biofilter cost efficiency. At present the relationship in biofilter materials must generally be determined experimentally, as no universal link between the relationship and material physical properties have been established. The objective of this study was, therefore, to investigate the link between the relationship, material particle size distribution, and wetting conditions for a common biofilter medium, Leca. Measurements of was carried out for 36 different Leca particle size fractions with different mean particle diameter () and particle size range () under both dry and wet conditions. Measurements showed that increasing and decreasing together with wetted conditions in the filter generally resulted in decreasing for the same . A set of expressions allowing for accurate estimation of the relationship based on , . and wetting conditions were established, enabling reliable estimation of filter pressure drop (and operation costs) in Leca and similar granular materials. [ABSTRACT FROM AUTHOR]

As human activities expand to the Moon, Mars, and other extraterrestrial bodies, it will be necessary to use local resources rather than bringing everything from Earth. In situ resource utilization (ISRU), or planetary surface engineering, starts with excavation and dirt-moving. The current study focuses on excavation of lunar regolith simulant by blading with and without preripping (mechanical raking) and points out the need for considering the relative proportion of coarse grains in regolith when dealing with excavation force and energy. The coarse-grain content of the lunar regolith, estimated from 11 Apollo cores, can reach 30% by mass. Prior ripping of vibrationally compacted beds of a standard fine regolith simulant can decrease total excavation resistance (when subsequent blading is included) by up to 20% for relative regolith densities greater than 60%. The effect of coarse grains on the response of compacted regolith to excavation was more significant than would be expected in most terrestrial practice. In conclusion, it is suggested that careful matching of excavator design to local coarse-grain content of the lunar regolith needs to be considered in designing a planetary surface engineering architecture. [ABSTRACT FROM AUTHOR]

Conventional pavements rely on stiff upper layers to spread traffic loads onto less rigid lower layers. In contrast, an inverted pavement system consists of an unbound aggregate base compacted on top of a stiff cement-treated base and covered by a relatively thin asphalt concrete layer. The unbound aggregate interlayer in an inverted pavement experiences high cyclic stresses that incite its inherently nonlinear granular media behavior. A physically sound, nonlinear elastoplastic material model is selected to capture the unbound granular base in a finite-element simulator developed to analyze the performance of inverted pavement structures. The simulation results show that an inverted pavement can deliver superior rutting resistance, as compared with a conventional flexible pavement structure with similar fatigue life. [ABSTRACT FROM AUTHOR]

This paper deals with progressive substitution of hydrated lime by granulated blast furnace slag to produce new calcium silicate materials obtained by autoclaving, representing an alternative economic raw material. The slag grain-size grinding and the heat treatment in saturated vapor pressure autoclave conditions were investigated to study the compressive strength behavior of the new material. The results showed a decrease in compressive strength from the substitution. The microstructure analysis showed that reaction products consist mainly of 11 Å tobermorite and xonotlite. When increasing the autoclave temperature, it results in an increase in xonotlite relative to tobermorite. It is also observed that the amorphous composition of granulated blast furnace slag does not release new distinct phases of hydrates. The autoclaved products were characterized using X-ray diffraction (XRD) technique and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDX). [ABSTRACT FROM AUTHOR]

Several tests are used for the characterization of unbound materials for pavement applications. The resilient modulus has been one of the most common tests for design specification of unbound materials. The California bearing ratio (CBR) is another laboratory test that is frequently used. The dynamic cone penetrometer (DCP) test is a more common test for in situ quality assessment/quality control of unbound materials. For better connection between design and quality assurance (QA)/quality control (QC), it would be helpful to have a reliable, mechanistic method for correlating test results. This is particularly true for the use of new materials, for which there is not an extensive body of data to empirically draw such connections. This paper presents a framework for a unified approach for modeling these tests. A discrete-element method (DEM) is used to simulate the CBR test, the DCP test, and the resilient modulus test. An initial evaluation demonstrated that the simulations can account for the effect of aggregate shape, size, gradation, friction, and stiffness. As such, this methodology shows promise for the development of mechanistic-based correlation between test results. These results are presented, along with some limitations of the current model and challenges for the future. [ABSTRACT FROM AUTHOR]

This paper examines the nonlinear stress-strain behavior of lightly stabilized granular base materials and presents a method to predict them based on modified and extended Ramberg-Osgood expression from unconfined compression (UC) testing. A typical granular material was lightly stabilized with 0.5-3.0% cement-flyash (CF) as well as with 1.5-3.0% slag-lime (SL) and tested in UC with internal deformation measurement setup. This study indicates that the proposed mathematical model can accurately predict the nonlinear stress-strain relationships of the lightly stabilized materials obtained from the experiments. The parameters involved with the proposed model were initial elastic modulus , 0.2% proof stress , ultimate strain , and the exponents for quantifying the nonlinearity of the curves and . Values of all the parameters were determined from the experimental stress-strain responses and reliable regression relationships were developed between these parameters and unconfined compressive strength (UCS). [ABSTRACT FROM AUTHOR]

Using a high-strength geosynthetic for encasement of granular columns increases the strength of a given column and improves its stress-displacement response. This paper describes the results from a series of three-dimensional finite-element analyses that were performed to simulate the behavior of a single geosynthetic-encased column in soft clay. To examine the sensitivity of the results to the constitutive model that was used to simulate the behavior of the encased granular soil, analyses were performed using models for the encased soil possessing various levels of sophistication. For each model that was studied, comparative analyses were performed to simulate the behavior of a dense and a loose granular soil within the encasement. The results demonstrate the importance of selecting a constitutive model that accurately captures the shear-induced volume-change behavior of the encased granular soil. Additional findings provide guidance for other researchers seeking to model the behavior of geosynthetic-encased columns and are useful for practitioners looking to enhance their understanding of the performance of different constitutive models for simulating the behavior of similar foundation elements. [ABSTRACT FROM AUTHOR]

Rolling friction plays an important role in the heap formation of granular materials. In this paper, three rolling friction models, which are incorporated into the discrete element method, are compared with simulation of sandpile formation in detail. The experiments were first implemented for cement particles to verify the reliability of simulated results. An approximate approach, which can be used to determine the friction coefficient between particles and between particles and a boundary, is also proposed. Then, numerical simulations parallel to the experiments were carried out by using the three rolling friction models. The simulated results are detailed and discussed compared with the experimental results. In addition, the rolling friction between the particles and boundaries was also studied through an example of a sphere moving on a horizontal plate with an initial translational velocity. The numerical results suggest that the rolling friction model described by the relative angular velocity can well reflect the rolling friction behavior and that the rolling friction tangential force has little effect on the simulated result. [ABSTRACT FROM AUTHOR]

The dynamic compaction (DC) method is a versatile ground treatment technique with growing popularity. It is applicable to a wide variety of soil types and conditions, particularly sandy materials and granular fills. This study presents a case history of the dynamic compaction with a high energy level of 6,000 kN·m on granular deposits at a site in China. The reclaimed site featured loose backfill with heterogeneity and saturated silt. In order to properly deal with such soil conditions and to optimize the DC design, field tests were conducted to determine the influencing factors in DC. Deformation tests were performed to ascertain the rational spacing of impacts and the optimal number of drops and to provide proofs to the adjustment of the original DC procedure. Monitoring of the pore water pressure helped obtain the time delay between passes. The approach to assess the depth of improvement was discussed based on interpretations of the spectral analysis of surface waves (SASW) test. Analysis of the SASW and plate-load tests demonstrated significant improvement in the soils at the site, with no obvious weak layers. Following dynamic compaction, the allowable ground-bearing capacity and the depth of improvement at the site were no less than 270 kPa and 7.4 m, respectively. [ABSTRACT FROM AUTHOR]

This paper concerns the problem of designing standard artificial tunnels for rock boulder protection. The proposed approach consists in uncoupling the problem of the dynamic response of the granular dissipative soil cushion placed on the top of artificial tunnels from the dynamic response of the reinforced concrete structure underneath. An already conceived elasto-viscoplastic constitutive model capable of simulating the penetration process of the boulder within the soil stratum and reproducing the force acting on the boulder is briefly described. Its simplified one-dimensional formulation is outlined. A modified version of the model, taking into account large displacements occurring when either impacts on loose granular soils or high energetic content impacts on dense sand strata are considered, is also introduced. To validate the approach, some in situ test results are numerically simulated. A simplified numerical approach is proposed for obtaining the evolution with time of both impact force and boulder penetration, bypassing the use of the rheological model. This goal is achieved by introducing some abaci obtained numerically with reference to an ideal dense sand soil stratum of reference. [ABSTRACT FROM AUTHOR]

In order to investigate the influence of water on compression characteristics of decomposed granite soils, single-particle crushing and one-dimensional compression tests were carried out on three types of decomposed granite soils as well as on quartz-rich silica sand under both dry and wet conditions. Results showed that the initial crushing strength of a single particle was reduced and strength variability increased due to the weakening effects induced by the presence of water. Moreover, it was observed that the one-dimensional compression behavior of decomposed granite soil was related to the initial crushing strength. Finally, the magnitude of initial crushing strength was also affected by the degree of weathering of the soil. [ABSTRACT FROM AUTHOR]

The present paper discusses the application of a high-cycle accumulation (HCA) model originally developed for sand for the prediction of permanent deformations in an unbound granular material (UGM) used for base and subbase layers in pavements. Cyclic triaxial tests on precompacted samples of an UGM have been performed in order to validate and calibrate the model. The stress amplitude, the initial density, and the average stress were varied. The test results are compared to those of air-pluviated samples of sand (subgrade material). Some significant differences in the behavior of both materials under cyclic loading are outlined. It is demonstrated that the functions describing the intensity of accumulation can be maintained for an UGM with different material constants, but that the flow rule must be generalized in order to describe the anisotropy. Recalculations of the laboratory tests show a good prediction of the modified HCA model. [ABSTRACT FROM AUTHOR]

This paper presents the findings of a laboratory investigation on the characterization of a freshly quarried granular base material lightly stabilized with slag-lime cementitious binder involving unconfined compression (UC) testing and indirect diametrical tensile (IDT) testing, both with internal displacement measurements. The UC test investigation involved the determination of the unconfined compressive strength (UCS) and four different types of stiffness moduli from both internal and external displacement measurements. The IDT testing included the determination of IDT strength as well as the static and dynamic stiffness moduli (i.e., SSM and DSM) of the lightly stabilized granular base material from monotonic and cyclic load IDT testing. This study indicates that the stiffness moduli of a lightly cementitiously stabilized granular base material can be determined consistently from UC testing by measuring the deformations internally, and the modulus is more reliably defined as either the tangent modulus at half the ultimate stress or secant modulus at 0.02% strain. The UC stiffness modulus could be estimated reliably from the corresponding UCS value using the regression relationships established from this study. The IDT strength was determined to be equal to 0.1143 times the UCS value and this conforms to the recommendation given in design guides such as AUSTROADS. Moreover, the IDT, DSM, and SSM of a lightly cementitiously stabilized granular base material could be estimated from the UCS of the same material using the correlations developed in this paper which could be used in pavement design. [ABSTRACT FROM AUTHOR]

Linear regression techniques were used to develop practical models to predict total organic carbon (TOC) breakthrough in bituminous granular activated carbon (GAC) adsorbers. Models were developed for two field-scale GAC sizes (8×30 and 12×40 mesh) and two empty bed contact times (EBCTs) (10 and 20 min). Model input parameters include two water quality variables, influent TOC concentration (TOC0) and pH, that impact performance. The dependent variables for the models were normalized breakthrough time, throughput in bed volumes, to six fractional (TOC/TOC0=0.2, 0.3, 0.4, 0.5, 0.6, and 0.7) and three mass (TOC=1.0, 1.5, and 2.0 mg/L) effluent concentrations. Model development was performed using small-scale breakthrough data from 35 different source waters; external model validation was performed with small-scale breakthrough data from 14 source waters; a sensitivity analysis was performed to ensure that the models effectively capture expected breakthrough trend; and a scalability test was performed to verify the models’ ability to predict breakthrough for field-scale GAC adsorbers. [ABSTRACT FROM AUTHOR]

The paper addresses the question of whether the number of particles in a noncemented granular assembly will affect the mechanical characteristics of the assembly: its strength and stiffness. The question is answered by applying the discrete element method to assemblies of different sizes. To isolate the effect of assembly size, apart from the scatter that usually accompanies such simulations, multiple assemblies were tested. The two-dimensional assemblies had nearly identical initial porosities and fabrics, and they were all loaded in biaxial compression. Two different boundaries were tested: periodic and wall boundaries. We find that the peak compressive strength decreases with assembly size for both types of boundaries and over a range of assembly sizes that contain 256 particles to over 66,000 particles. Stiffness is only slightly reduced and only with wall boundaries. Deformation is less uniform in the larger assemblies, with deformation concentrated in a smaller fraction of the assembly area. An analysis of deformation patterning shows that at least a few thousand particles are required for realistic microband patterning. [ABSTRACT FROM AUTHOR]

Micromechanics of rapid granular flows is studied in a two-dimensional planar granular Couette flow apparatus. The device is capable of generating particulate flows at different shearing rates and solid fractions. Monosize plastic disks are sheared across an annular test section for several shear rates. The motion of particles is recorded through a high speed digital camera and analyzed by image processing techniques. The average and fluctuation velocity profiles are obtained and granular temperature relations with shear rate are investigated. Average streaming velocity across the shear cell decays slightly faster than exponential, and is rather Gaussian when not too close to the wall. Fluctuation velocities and granular temperature across the shear cell are related to effective shear rate. Interparticle collisions are estimated from the particle trajectories and probability distribution of collision angles obtained from particle collision data. In dense flows, three peaks of collision angles are observed, which signal the onset of triangular structure formulation and cause crystallization. It is found that the distribution of collision angles is anisotropic. [ABSTRACT FROM AUTHOR]

This study examined a common form of filtration, a passive radial cartridge filter (RCF) system, to physically separate hetero-disperse particulate matter in rainfall-runoff. The RCF tested utilizes aluminum-oxide coated media with a uniform pumice substrate (d50m=3.56 mm) gradation. To examine the RCF, this study applied laser diffraction and real-time pressure sensor measurements to validate a computational fluid dynamics (CFD) model to predict particle separation and filter head loss for hetero-disperse particulate matter (PM). Filter hydrodynamics are resolved using a macroscopic approach for the porous media with a k-[variant_greek_epsilon] turbulence model coupled with the Ergun equation. PM fate was resolved using a discrete phase model. CFD results closely followed measured data for filtration and head-loss response for all flow rates. With influent PM at 200 mg/L(d50m=16.3 μm), effluent PM ranged from 32 to 57 mg/L for surface loading rates of 24 to 189 L/m2 min, respectively. There was agreement between measured and modeled data for effluent PM and head loss. CFD postprocessing provided added insight into the mechanistic behavior of the RCF by means of three-dimensional hydraulic profiles, particle trajectories, and pressure distributions, illustrating that a RCF is nonuniformly loaded. As part of design and regulation, such physical testing coupled with modeling is a required precursor to uncontrolled field testing, regular maintenance and certification of a BMP. [ABSTRACT FROM AUTHOR]

The time-dependent mechanical response of an artificial granular medium consisting of glass beads is experimentally examined by performing one-dimensional creep tests on both saturated and dry specimens. The time-dependent compression of the oedometric sample, which is related to the internal rearrangement of the granular assembly, is modeled using an elastic-viscoplastic strain hardening constitutive model. The procedures used to identify the parameters defining the model are discussed; the incremental strain of the material is decomposed into reversible elastic and irreversible creep components. Finally, the capabilities of the proposed model in predicting the time-dependent response of the material are investigated through a comparison between experimental results and analogous results derived from computational modeling. [ABSTRACT FROM AUTHOR]

We probe the origins of nonaffine deformation in discrete element simulations of densely packed granular systems under biaxial compression, using a new local measure of nonaffine micropolar deformation. This measure represents the deviation of particle motion from that dictated by a measure of micropolar strain and curvature, devised on the scale of a particle and its first ring of neighbors. Highly correlated mesoscopic structures emerge and contribute significantly to nonaffine deformation, an aspect of material behavior that is largely neglected in existing constitutive models. Distinct regimes of deformation were observed: a globally affine regime in which particles move in uncorrelated Brownian motion with some “rattlers” present followed by a globally nonaffine regime involving rattlers, microbands, vortices, confined buckling of force chains, and persistent shear band(s). Structural development leading to and during persistent shear banding is elucidated. The highest levels of nonaffine deformation were observed during drops in the macroscopic stress ratio: here, nonaffine deformation is essentially confined to the shear band, where confined buckling of force chains is the intrinsic mechanism. Degrees of correlation between the local measures of nonaffine strain and curvature confirm the key role of particle rotations in shear bands. Probability density functions of local nonaffine deformation exhibit power law behavior cut off by an exponential tail, consistent with experiments. Self-diffusion anisotropy was observed inside the shear band. The degree of diffusion is greater tangential rather than normal to the shear band; particles in buckling force chains exhibit the highest degree of diffusion along the band. Self-diffusion of particles after the peak stress ratio was found to be superdiffusive during each drop in stress ratio, and diffusive during each rise in stress ratio. The implications of these results for constitutive modeling are discussed, with particular attention paid to the significance of confined buckling of force chains. [ABSTRACT FROM AUTHOR]