Fluid flow through fractures in below ground concrete vaults

Abstract Cement and concrete are used extensively to isolate waste materials from the environment and to control groundwater flow rates in mining, waste disposal, and site remediation activities. High quality concrete is a very low permeability material, however it is brittle and subject to cracking. In practice, the permeability of concrete is controlled by the fractures or cracks which form in the structures. Simplified models for flow through and around underground concrete vaults are presented and applied. Results of the applications indicate that flow rates through cracked concrete are a function of size and spacing of cracks. When cracks are small and closely spaced, resistance in passing through the cracks controls flow. However, when cracks are larger and widely spaced, resistance through the adjacent porous medium (i.e., soil) governs flow rates through cracks. If calculations consider both the adjacent porous media and the cracks, maximum predicted flow rates do not occur for the largest or smallest crack sizes and spacings, but somewhere inbetween the extremes. Further calculations illustrate that perched water will frequently be expected on top of vaults justifying use of saturated flow equations. Perched water can occur because the cracked roof of a concrete vault is analogous to capillary barriers used in some engineered cover designs. Although not a critical design concentration, results indicate that keeping the vault roof below saturation to avoid flow into the cracks is difficult and has the best chance of success for smaller, modular vaults, arid locations, and very shallow burial.