Cap Design Calculator

An engineered cap seals contaminated soil in place rather than removing it, and this calculator builds one up layer by layer using standard EPA/RCRA minimum thicknesses sized to meet the closure performance standards in 40 CFR 264.310 — the regulation that requires a RCRA Subtitle C final cover to minimize long-term liquid migration, promote drainage, minimize erosion, and have a permeability at or below that of any bottom liner system, without itself dictating a single prescriptive thickness. From the bottom: an optional 300mm gas vent layer for sites with volatile organics or landfill gas; a low-permeability barrier that's either 600mm of compacted clay alone or 300mm of clay backed by a 1.5mm (60-mil) HDPE geomembrane (sized with a 10% area allowance for seams and anchor trenches); a 300mm sand/gravel drainage layer; a protection layer sized to whichever is larger, 450mm or the site's actual frost penetration depth (so cold-climate sites automatically get a thicker cap to prevent freeze-thaw cracking of the barrier below); and a 150mm topsoil layer to support vegetation. All layer volumes are simply thickness times the cap's footprint area.

The drainage adequacy check applies Darcy's law (flow equals hydraulic conductivity times hydraulic gradient times cross-sectional area) using a typical sand conductivity of 1×10⁻³ m/s and the entered surface slope as the gradient, then compares that annual drainage capacity against peak infiltration estimated from annual precipitation — a result under 100% signals the drainage layer as designed cannot shed water fast enough to prevent it from ponding on or infiltrating through the cap. This is a preliminary sizing tool, not a stamped design: it doesn't check settlement, slope stability, or the specific geotechnical properties of imported fill, and real designs need a licensed engineer's review plus site-specific soil testing (particularly for the clay barrier's actual permeability) before construction.