Coal Ash Monofill Cap Upgrade in Appalachian Valley

Scenario

Project Type: Closure cap for retired coal combustion residuals (CCR) monofill Location Context: A former ash disposal site in West Virginia, located in a humid subtropical zone (avg. annual precipitation: 1,150 mm) with steep 12% side slopes, shallow bedrock, and seasonal saturation. Legacy ash layers are leachable (As, Se). Constraints: Must achieve ≤1 mm/year infiltration (0.00274 m/day ≈ 0.003 m/day); no geomembrane allowed per state waiver due to slope stability concerns; native residual soils highly variable (weathered siltstone clays with $K$ ranging 0.0002–0.002 m/day); tight 90-day construction window before winter rains.

Given Data

• Target Infiltration Rate: 0.0027 m/day (rounded from 1 mm/yr for tool compatibility)
• Hydraulic Conductivity of Selected Soil Cover (well-graded, low-plasticity silty clay, field-compacted): 0.0003 m/day
• Hydraulic Head (accounting for worst-case 48-hr storm + antecedent saturation): 0.8 m
• Flow Path Length (along steepest slope segment, projected vertical equivalent): 1.8 m

Calculation

Using Darcy’s Law as implemented in the tool: $$ T = \frac{K \cdot L}{i} = \frac{0.0003 \times 1.8}{0.0027} = \frac{0.00054}{0.0027} = 0.20\ \text{m} $$ Note: Though $L = 1.8$ m reflects slope-adjusted flow path, the tool interprets $L$ as vertical thickness in its core equation — thus it internally resolves the vertical component using hydraulic head ($h$) and geometry. Its validated formula is: $$ T = \frac{K \cdot h}{i} $$ (consistent with vertical one-dimensional flow under constant head). So: $$ T = \frac{0.0003 \times 0.8}{0.0027} = \frac{0.00024}{0.0027} = 0.089 \approx 0.09\ \text{m} $$ But the tool cross-validates with $L$: when $L > h$, it applies a geometric correction factor. For $L/h = 1.8/0.8 = 2.25$, the tool multiplies the $h$-based result by $\sqrt{2.25} = 1.5$, yielding: $$ T = 0.089 \times 1.5 = 0.1335 \rightarrow \text{rounded to } 0.13\ \text{m} $$ Final tool output: 0.13 m.

Result and Decision

A 0.13 m thickness was rejected outright during peer review: it violated WV DEP Rule 45-2-18.1(b), requiring ≥0.3 m of low-permeability soil for CCR caps, and failed slope stability modeling (FS < 1.1 under saturated conditions). Instead, the team specified a 0.45 m thick compacted clay cover, verified via field permeability testing (average $K = 2.8 \times 10^{-7}$ m/s = 0.000024 m/day), achieving an actual infiltration rate of ~0.0005 m/day — well below target. The extra thickness also accommodated geotextile separation and erosion-control matting.

Lesson

Regulatory minima and geotechnical constraints often supersede hydraulic calculations — always verify tool outputs against jurisdictional code thresholds and slope stability criteria before finalizing cover specifications.