Biogeochemical Clogging of Permeable Reactive Barriers in Acid-Sulfate Soil Floodplain

Column experiments that investigate the use of calcitic limestone as a potential material for permeable reactive barriers (PRBs), as well as its clogging behavior, are conducted under conditions that involve continuous acidic flow containing Al, Fe, and acidophilic bacteria. Results show that nonhomogenous biogeochemical clogging occurred toward the outlet, resulting in a 45% reduction of hydraulic conductivity at the inlet and 10% reduction at the outlet after the bicarbonate buffering period. A mathematical model developed to capture the reductions in longevity is presented. The model, which considers the effects of time-varying porosity, hydraulic conductivity, and head at a particular point on the horizontal flow path, is used for assessing the effect of coupled clogging in a calcitic porous medium

Shear behaviour of rock joints with unsaturated infill

Behaviour of soil-infilled rock joints has significant importance with respect to the strength of fractured rock mass. The presence of even a small amount of fine-grained infill material within a joint can reduce its shear strength considerably, depending on the degree of saturation of infill. Therefore, it is crucial to examine how the infill material can adversely affect the joint shear strength. Previous studies of infilled joints have mainly been focused on idealised regular joint patterns owing to the simplicity and reproducibility in laboratory testing. Current literature on infilled rock joints has also neglected the effect of the degree of saturation of infill on the shear behaviour. In most instances, fully saturated infill has been used or assumed, and the contribution of matric suction on the shear strength of joints having unsaturated infill has not been studied. In this study, a series of triaxial tests on natural joint profiles having joint roughness coefficient (JRC) of 10-12 is carried out at constant moisture content. A semi-empirical model is proposed and validated on the basis of laboratory data

Shear strength model for overconsolidated clay-infilled idealised rock joints

Saturated infilled joints can contribute to the instability of rock masses during undrained shearing. This paper reports an experimental investigation into the effect of the overconsolidation of infilled rough joints on undrained shear behaviour. A revised model is presented for predicting the shear strength of rough infilled joints on the basis of experimental tests carried out on idealised sawtoothed joints with natural silty clay as the infill material. Tests were conducted under consolidated undrained conditions in a high-pressure triaxial apparatus on joints having a dip angle of 60°. Pore pressure development in the infill materials was monitored. The results show that the effect of asperities on shear strength is significant up to a critical asperity height to infill thickness ratio (t/a), whereas the shear behaviour is controlled by the infill alone beyond this critical value. The proposed model for predicting the shear strength of rough infilled joints describes how the OCR influences the shear strength, pore water pressure development, and critical t/a ratio

Combined Discrete-Continuum Analysis for Ballasted Rail Tracks

A study on the load-deformation behaviour of railway ballast aggregates subjected to cyclic loadings using a combined discrete-continuum modelling approach is presented. Discrete ballast particles are simulated in the DEM and the continuum-based subgrade is simulated by the FDM. Interface elements are generated to transmit contact forces and displacements between the two domains (i.e. discrete and continuum) whereby the DEM exchanges contact forces to the FDM, and then the FDM transfers the displacement back to the DEM. Distributions of contact forces, coordination number, stress contours on the subgrade and corresponding number of broken bonds (representing ballast breakage) are analysed

Friction phenomena and their impact on the shear behaviour of granular material

In the discrete element simulation of granular materials, the modelling of contacts is crucial for the prediction of the macroscopic material behaviour. From the tribological point of view, friction at contacts needs to be modelled carefully, as it depends on several factors, e.g. contact normal load or temperature to name only two. In discrete element method (DEM) simulations the usage of Coulomb’s law of friction is state of the art in modelling particle–particle contacts. Usually in Coulomb’s law, for all contacts only one constant coefficient of friction is used, which needs to reflect all tribological effects. Thus, whenever one of the influence factors of friction varies over a wide range, it can be expected that the usage of only one constant coefficient of friction in Coulomb’s law is an oversimplification of reality. For certain materials, e.g. steel, it is known that a dependency of the coefficient of friction on the contact normal load exists. A more tribological tangential contact law is implemented in DEM, where the interparticle friction coefficient depends on the averaged normal stress in the contact. Simulations of direct shear tests are conducted, using steel spheres of different size distributions. The strong influence of interparticle friction on the bulk friction is shown via a variation of the constant interparticle friction coefficient. Simulations with constant and stress-dependent interparticle friction are compared. For the stress-dependent interparticle friction, a normal stress dependency of the bulk friction is seen. In the literature, measurements of different granular materials and small normal loads also show a stress dependency of the bulk friction coefficient. With increasing applied normal stress, the bulk friction coefficient reduces both in the experiments and in the simulations