5.6 Landfills, Caps & Containment Systems

Landfill Liner Systems

A modern municipal solid waste (MSW) landfill base liner isolates waste and leachate from the underlying groundwater. Liner components are selected and combined to satisfy a regulatory maximum hydraulic conductivity, typically $k \le 1\times10^{-7}\text{ cm/s}$ under RCRA Subtitle D for MSW facilities (hazardous-waste facilities under RCRA Subtitle C require more conservative, doubly-lined systems).

• Compacted Clay Liner (CCL): A minimum $0.6\text{–}0.9\text{ m}$ ($2$–$3\text{ ft}$) thick layer of fine-grained soil compacted in thin lifts ($150\text{–}200\text{ mm}$) at moisture content on the wet side of optimum to achieve low permeability. Field hydraulic conductivity is verified with sealed double-ring infiltrometer or large-scale field permeameter testing, not laboratory index correlations alone.
• Geomembrane (GM): A continuous polymeric sheet, most commonly $60$-mil ($1.5\text{ mm}$) textured HDPE for base liners, providing an essentially impermeable barrier ($k \approx 10^{-13}\text{ cm/s}$ intrinsic to the polymer) but relying entirely on seam integrity and puncture resistance.
• Geosynthetic Clay Liner (GCL): A thin ($\approx 6\text{–}10\text{ mm}$) manufactured composite of bentonite sandwiched between or stitched/needle-punched to geotextiles, achieving $k \approx 10^{-9}\text{ cm/s}$ once hydrated — used as a CCL substitute where space or borrow-source clay is limited.
• Composite Liner: The current industry standard, pairing a geomembrane directly over a CCL or GCL. The geomembrane provides the primary hydraulic barrier while the underlying low-permeability soil/GCL limits leakage rate through any geomembrane defect (a "backup" barrier) and provides a stable subgrade for geomembrane placement.

Leachate Collection and Removal System (LCRS)

Above the liner, a leachate collection layer — typically $300\text{ mm}$ of drainage sand/gravel ($k \ge 1\times10^{-2}\text{ cm/s}$) or a geocomposite drainage layer — routes leachate to perforated collection pipes. Key design controls:

• Maximum liquid head on the liner: $30\text{ cm}$ ($12\text{ in}$) per RCRA Subtitle D, which limits allowable pipe spacing for a given drainage layer transmissivity and bottom slope.
• Minimum bottom-liner slope: typically $2%$ toward collection piping to promote positive leachate drainage and avoid ponding-induced excess head.
• Chemical/biological clogging: collection gravel and pipe perforations are sized and specified to resist calcium-carbonate and biofilm clogging over the facility's operating life, since post-closure maintenance access is limited once cover is placed.

Landfill Gas Venting and Collection

Anaerobic decomposition of MSW generates landfill gas (roughly 50% methane, 50% CO₂ by volume). Gas management ranges from passive venting (gravel-filled vent trenches or vertical vent wells relying on pressure differential) to active extraction (a network of vertical/horizontal wells connected to a blower manifold under vacuum), often paired with a gas collection layer beneath the final cap to intercept gas before it can migrate laterally or accumulate beneath the low-permeability cover. Active systems are typically required once a landfill's design capacity and gas generation potential exceed regulatory thresholds (e.g., NSPS/EPA Subpart XXX for MSW landfills), and are frequently coupled to flares or energy-recovery facilities.

Interface (Veneer) Stability

Where a soil cover, drainage layer, or protective layer overlies a geomembrane or GCL on a sloped surface, the governing failure mode is sliding along the weakest geosynthetic-to-geosynthetic or soil-to-geosynthetic interface — not a deep-seated slope failure. For the simplified case of an infinite slope with no seepage forces, cohesion, or surcharge, the interface (veneer) factor of safety reduces to:

$FS = \frac{\tan\delta}{\tan\beta}$

where $\delta$ is the critical (lowest) interface friction angle among all interfaces in the cross-section (soil/geomembrane, geomembrane/GCL, GCL/subgrade, etc.), and $\beta$ is the slope angle. Because $\delta$ is frequently smaller than the cover soil's internal friction angle, veneer stability — not the cover soil's own shear strength — usually governs maximum allowable cap or liner side-slope angles. Typical minimum requirements are $FS \ge 1.5$ (static) and $FS \ge 1.1$ (pseudostatic/seismic); when seepage builds up within the cover soil above a geomembrane (a common trigger for veneer failures), an additional seepage force term reduces $FS$ further and is included in more rigorous analyses.

Municipal Solid Waste (MSW) Settlement

MSW settlement occurs in two overlapping mechanisms: immediate/short-term mechanical settlement from self-weight compression and voids collapse under new overburden (additional lifts, cap placement), and long-term, time-dependent settlement driven by biodegradation of organic waste fractions, moisture redistribution, and creep. Long-term settlement is commonly estimated with the Sowers method, which treats post-primary settlement as logarithmic with time:

$\Delta H = C_{\alpha}' \cdot H \cdot \log_{10}\left(\frac{t_2}{t_1}\right)$

where $C_{\alpha}'$ is a modified secondary-compression index for waste (typically $0.03$–$0.10$, higher for fresh, high-organic-content waste), $H$ is waste thickness, and $t_1$, $t_2$ bound the time interval of interest. Because MSW settlement can continue for decades, structures and pavements on or near capped landfills are typically designed with flexible utility connections and settlement-tolerant details rather than relying on settlement termination.

Final Cap/Cover Systems

The regulatory final cover (cap) is a layered system, from top to bottom: a vegetative/topsoil layer for erosion control, a drainage layer to shed infiltrating precipitation off the low-permeability barrier before it can build head, a barrier layer (compacted clay, GCL, or geomembrane — mirroring base liner logic but sized for cover rather than containment loading), and, where active gas systems exist, a gas collection layer immediately above the waste. Cap side slopes are governed by the same veneer-stability check described above, generally limiting cover slopes to $3\text{H}{:}1\text{V}$ or flatter unless textured geomembranes or slope-stabilizing structures (toe berms, buttresses) are used.

Worked Numerical Example: Cover Veneer Stability Check

Problem Statement A landfill final cover system is proposed on a $3\text{H}{:}1\text{V}$ side slope, consisting of a $450\text{ mm}$ vegetative/drainage soil layer directly over a textured $60$-mil HDPE geomembrane barrier. Direct-shear interface testing between the cover soil and the textured geomembrane gives a critical interface friction angle $\delta = 22^\circ$. Determine the static veneer factor of safety and evaluate against the typical $FS \ge 1.5$ criterion.

Solution

Step 1 — Slope angle: $\beta = \arctan\left(\frac{1}{3}\right) = 18.43^\circ$

Step 2 — Infinite-slope veneer FS: $FS = \frac{\tan\delta}{\tan\beta} = \frac{\tan(22^\circ)}{\tan(18.43^\circ)} = \frac{0.4040}{0.3333} = 1.21$

Step 3 — Evaluate against criterion: $FS = 1.21 < 1.5$, so this cover configuration fails the static veneer stability screen. The designer must either flatten the slope, specify a rougher/more aggressively textured geomembrane (raising $\delta$), reduce the cover soil thickness/weight, or introduce a toe buttress/bench to intercept sliding forces — flattening to a $4\text{H}{:}1\text{V}$ slope ($\beta = 14.04^\circ$, $\tan\beta = 0.250$) would raise $FS$ to $0.404/0.250 = 1.62$, satisfying the criterion.