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ABSTRACT: Numerical modeling techniques using the combined finite-discrete element method have been utilized to explore the relationship between stress, damage, and crack propagation in concrete. Obtaining post-peak behavior from the Brazilian tension test is difficult because failure occurs abruptly. An experimental test method is described which enables monitoring of the post-peak cracking behavior in the Brazilian tension test by slowing the crack propagation. Experimental testing with this technique proved its capabilities in slowing crack propagation. The combined finite-discrete element method is used to further examine the stress states during this test and to understand the damage evolution present in this test prior to the peak strength. The method also allows for interpretation into the type of damage present throughout the entire test and the zones in which that damage initiates. A comparison is also performed between the numerical results and experimental observations to determine how well the numerical method captures the experimental results. This work enables a good understanding of how fractures form in the Brazilian tension test as well as a comparison between experimental and numerical technique results. 1. INTRODUCTION 1.1. Combined Finite Discrete Element Method In an effort to model inelastic behavior and predict the deformation and failure of materials, the combined finite discrete element method has been employed. The finite-discrete element method (FDEM) is an innovative numerical computation method that combines the efficiency of the finite element method (FEM) with the discontinuity framework of the discrete element method (DEM) to create a highly powerful and efficient system (Munjiza et al., 1999; Rougier et al., 2014). The comparison of DEM and FEM with a schematic visualization of FDEM is shown in Fig. 1. At the contact interfaces of the FDEM framework, there is bonding. The bonding is numerically represented with cohesion points along the boundaries of the deformable particles (Rougier, Knight, Lei, et al., 2014). The cohesion of these bonds is a combination of normal cohesion and tangential cohesion. When the material is exposed to significant stress, these bonds become strained and can become unbonded (Rougier, Knight, Lei, et al., 2014).

Coal combustion byproducts (CCBs) disposed of in unlined landfills can affect the quality of adjacent water resources. In previous studies, CCBs have been found to leach toxic heavy metals such as arsenic, mercury, and lead into groundwater. CCBs include fly ash, bottom ash, and flue-gas desulfurization product (FGD gypsum). Within a landfill, CCBs may be present at different densities associated with depth, compacted primarily from the weight of above materials. This investigation focused on determination of the unsaturated and saturated hydraulic properties of fly ash and bottom ash as a function of density and thus a function of depth within a landfill. Ash samples from a power plant in northern New Mexico were collected for laboratory analysis. Compressibility curves were developed in order to determine what densities may be experienced at a range of pressures. Saturated hydraulic conductivity was determined using falling head tests for multiple densities of each material. Moisture characteristic curves were developed from hanging column tests, pressure plate tests, dew point potentiometer measurements, and relative humidity measurements. The moisture characteristic curves were also measured at a range of densities for each material. Results indicated that the fly ash saturated hydraulic conductivity varied as a function of density for the materials tested and the density could be reasonably predicted using an equation presented. Fly ash unsaturated properties also show trends with variations in density with the variability decreasing as density increases. Fly ash in a landfill can have estimated density, unsaturated and saturated hydraulic properties as a function of depth using the methods in this paper. Bottom ash showed similar trends in compressibility with less variability with respect to the fly ash. The unsaturated and saturated hydraulic properties show some trends, though with high amounts of variability. The density of bottom ash materials in a landfill may be reasonably estimated using methods proposed in this study while unsaturated and saturated hydraulic properties have greater uncertainty.

Long-term performance of a waste-containment facility liner system can be simply and inexpensively monitored in dry climates by relative humidity measurements. An increase in humidity as atmospheric air is circulated through a dry coarse layer within the liner system indicates imminent movement of water into the layer. Continued airflow will evaporate and remove water, thereby restoring the coarse layer to a dry condition. Thus, quantification of relative humidity and water removal rates can be directly used to assess liner system performance. The effectiveness of this approach is increased in dry climates, where the atmospheric air has a low initial relative humidity and leachate production is expected to be small. Simple calculations, numerical simulations, and medium-scale field tests indicate substantial airflow rates can be induced through a coarse layer within a liner system. Air can be circulated actively with blowers, or passively with wind-powered chimneys. System efficiency can be improved by incorporating a coarse layer that has significant primary storage.

Capillary barriers, consisting of fine-over-coarse soil layers, are being considered as an alternative cover component for waste-disposal facilities, especially in dry climates. Infiltrating water is removed from the fine layer by evaporation or transpiration, or percolation into the coarse layer (failure). If the fine-coarse interface is sloped, water in the fine layer can also drain laterally under unsaturated conditions. The effectiveness of two capillary barriers in laterally diverting water was tested. The barriers were 7 m long and 1.2 m thick, built on a 10% slope. One had a homogeneous fine layer, while the fine layer of the other was layered to increase its ability to divert water laterally. The barriers were first subjected to constant infiltration (10 mm/day) followed by exposure to ambient climatic conditions. The layered capillary barrier was successful in laterally diverting water near the interface and did not permit any water to enter the coarse layer. In contrast, the homogeneous capillary barrier failed over its entire length. These results indicate that a significant lateral diversion capacity can be designed into capillary barriers, greatly increasing their effectiveness.

A numerical procedure has been developed for predicting dilation (porosity) and gas permeability changes in rock salt. The hierarchical single-surface constitutive model of Desai and co-workers is used a finite element program to calculate the state of stress and strain surrounding excavations in rock salt. The elastoplastic constitutive model accounts for strain hardening, a non-associative volumetric response and stress-path-dependent behaviour. The calculated stress and strain fields are used in a flow model based on the equivalent channel concept to predict permeability. Parameters for both the mechanical and permeability models are developed from laboratory test results. Two field experiments adjacent to underground excavations are modelled. The extent of the dilated rock zone around the excavation is predicted well, but the magnitude of the porosity and gas permeability is underpredicted very near the excavations. This discrepancy is attributed to model parameters derived from loading-only laboratory tests, whereas significant unloading occurs in the field. The shape of the yield surface was found to be an important factor in dilation and permeability predictions. Similar stress, strain and permeability fields were obtained with different model types (plane strain or axisymmetric) and initial stress states, and with instantaneous and progressive excavation.

The permeability of sandstone was measured in triaxial tests. Within the range of confining pressures used here, permeability as a function of differential stress was found to decrease until a minimum value was reached at about 60–70% of the fracture strength, i.e., at a stress that lies between the crack-initiation stress and the stress corresponding to the volumetric strain reversal. Thereafter, permeability started to rise, and at about 85–90% of the fracture strength its rate of increase was greater. The permeability method was not sensitive enough to mark the point of crack initiation and the onset of crack coalescence. However, differential stress versus volumetric strain curves were, in general, similar to differential stress versus permeability curves. Thus, the permeability method can be successfully used to study the fracture mechanism of rocks.

The Geocomposite Capillary Barrier Drain (GCBD) has been developed and tested to prevent positive pore pressures from developing by laterally draining water while it is still in tension. The GCBD consists of two key layers that function as long as the water pressures in the system remain negative: (1) a transport layer that laterally drains water and (2) a capillary barrier layer that prevents water from moving downward. prototype GCBD systems have been tested in a 3 m long lateral drainage test apparatus. For most test conditions, the GCBD systems drained water under negative pressures at a rate sufficient to prevent any positive water pressures from developing in the overlying soil. Further, the drain system served as a barrier as it prevented downward flowing water from moving into the underlying soil.

Ross` solution for the divergence capacity of a capillary barrier is modified to incorporate constant hydraulic conductivity anisotropy of the fine layer. The analysis assumes steady infiltration and that the principal directions of the hydraulic conductivity tensor are parallel and normal to the fine-coarse contact. The solutions for divergence capacity and length have two terms. The first term, similar to the solution of Ross, represents the diversion due to the presence of the capillary barrier and is proportional to the hydraulic conductivity parallel to the contact. The second term, which arises from infiltration into an anisotropic medium independent of the capillary barrier, is a function of the fine layer thickness and will be small for engineered barriers. Thus an anisotropic fine layer with a greater hydraulic conductivity in the direction parallel to the fine-coarse contact will extend the performance of a capillary barrier as measured by its divergence capacity and length. 6 refs., 2 figs.

Effective sealing of penetrations (boreholes) in geological materials is becoming increasingly important as it is recognized that groundwater resources need to be protected from potential contamination. In the undisturbed geological system, potential contaminants are separated from freshwater aquifers by low-permeability aquitards including evaporite deposits such as bedded and domal halites. Improper plugging, sealing or abandonment of boreholes which penetrate aquitards can allow the bore- hole to act as a conduit between the contaminated and freshwater regions. Borehole seal performance has been recognized as a potential environmental concern by numerous researchers (e.g. USEPA, 1977, 1987; Freeze and Cherry, 1979; National Research Council, 1985; Smith and Browning; 1993; Smith et al., 1993).

This report presents a preliminary evaluation of design concepts for the eventual sealing of the shafts, drifts, and boreholes at the Waste Isolation Pilot Plant Facility. The purpose of the seal systems is to limit the flow of water into, through, and out of the repository. The principal design strategy involves the consolidation of crushed or granular salt in response to the closure of the excavations in salt. Other candidate seal materials are bentonite, cementitious mixtures, and possibly asphalt. Results from in situ experiments and modeling studies, as well as laboratory materials testing and related industrial experience, are used to develop seal designs for shafts, waste storage panel entryways, non-waste containing drifts, and boreholes. Key elements of the ongoing experimental program are identified. 112 refs., 25 figs., 1 tab.