1.1 Problem Statement
Fractures are prevalent in natural and synthetic structural media, even in the best engineered materials. We find fractures in bedrock, in sandstone aquifers and oil reservoirs, in clay layers and even in unconsolidated materials (Figures 1.1 to 1.4). Fractures are also common in concrete, used either as a structural material or as a liner for storage tanks (Figure 1.5). Clay liners used in landfills, sludge and brine disposal pits or for underground storage tanks can fracture, releasing their liquid contents to the subsurface (Figure 1.6). Even “flexible” materials such as asphalt fracture with time (Figure 1.7). The fact that fractures are inevitable has led to spending billions of research dollars to construct “safe” long-term (10,000 years or more) storage for high-level nuclear waste (Savage, 1995; IAEA, 1995), both to determine which construction techniques are least likely to result in failure and what are the implications of a failure, in terms of release to the environment and potential contamination of ground water sources or exposure of humans to high levels of radioactivity.
Why do materials fail? In most cases, the material is flawed from its genesis. In crystalline materials, it may be the inclusion of one different atom or molecule in the structure of the growing crystal, or simply the juncture of two crystal planes. In depositional materials, different grain types and sizes may be laid down, resulting in layering which then becomes the initiation plane for the fracture. Most materials fail because of mechanical stresses, for example the weight of the overburden, or heaving (Atkinson, 1989; Heard et al., 1972). Some mechanical stresses are applied constantly2 until the material fails, others are delivered in a sudden event. Other causes of failure are thermal stresses, drying and wetting cycles and chemical dissolution. After a material fractures, the two faces of the fracture may be subject to additional stresses which either close or open the fracture, or may subject it to shear. Other materials may temporarily or permanently deposit in the fracture, partially or totally blocking it for subsequent fluid flow. The fracture may be almost shut for millions of years, but if the material becomes exposed to the surface or near surface environment, the resulting loss of overburden or weathering may allow the fractures to open. In some cases, we are actually interested in introducing fractures in the subsurface, via hydraulic (Warpinski, 1991) or pneumatic fracturing (Schuring et al., 1995), or more powerful means, to increase fluid flow in oil reservoirs or at contaminated sites. Our particular focus in this study is the role that fractures play in the movement of contaminants in the subsurface. Water supply from fractured bedrock aquifers is common in the United States (Mutch and Scott, 1994). With increasing frequency contaminated fractured aquifers are detected (NRC, 1990). In many cases, the source of the contamination is a Non-Aqueous Phase Liquid (NAPL) which is either in pools or as residual ganglia in the fractures of the porous matrix. Dissolution of the NAPL may occur over several decades, resulting in a growing plume of dissolved contaminants which is transported through the fractured aquifer due to natural or imposed hydraulic gradients. Fractures in aquitards may allow the seepage of contaminants, either dissolved or in their own phase, into water sources.
Fluid flow in the fractured porous media is of significance not only in the context of contaminant transport, but also in the production of oil from reservoirs, the generation of steam for power from geothermal reservoirs, and the prediction of structural integrity or failure of large geotechnical structures, such as dams or foundations. Thus, the results of this study have a wide range of applications.
The conceptual model of a typical contaminant spill into porous media has been put forward by Abriola...