Desiccation cracking shows up on almost every capped cell sooner or later. The cover soil dries out, shrinks, and pulls apart at the surface, sometimes in a tight network of hairline cracks, sometimes in gaps wide enough to lose a boot in. Knowing why it happens is the first step to knowing whether what you're looking at is cosmetic or a problem for the cap underneath.
Why clay caps crack in the first place
Final cover soil specs typically call for a compacted clay layer, often blended with topsoil, because clay's low permeability is what keeps infiltration and gas migration under control. That same clay mineralogy is the reason it cracks. High-plasticity clays shrink as they lose moisture and swell again when wetted. Run enough wet-dry cycles through a cap, especially one that sits through a dry summer after a wet spring, and the soil structure starts to fracture along planes of weakness.
A few conditions make this worse:
- Soil composition. Cover blends heavy on bentonite or other expansive clays shrink more per unit of moisture loss than a sandy loam cap would. Good for the permeability number on the design sheet, rougher on crack resistance.
- Cover thickness and compaction. A thin or poorly compacted lift dries faster through its full depth than a thick, well-compacted one, and it has less soil mass to resist the shrink stress.
- Vegetation cover and root depth. Bare soil or sparse, shallow-rooted turf leaves the cap more exposed to direct sun and wind, which pulls moisture out of the top few inches fast. Deeper-rooted stands can actually pull moisture from lower in the profile too, which is part of why root penetration into a cap is its own separate concern.
- Slope and aspect. South- and west-facing slopes bake harder and longer in most of the country. Steeper grades shed rain faster, so the soil has less time to rewet between dry spells.
- Drought stretches. An extended dry period does more damage than a single hot week. The longer the soil stays below its shrinkage limit, the deeper the cracks propagate.
None of this is exotic. It's the same shrink-swell behavior you'd see in a clay-heavy farm field or a dry pond bottom, just happening on top of waste instead of subsoil.
When cracking turns into a cover failure
A few surface cracks after a dry August aren't automatically a cap problem. Most close back up once the rain returns and the clay swells again. What operators actually watch for is cracking that doesn't fully heal, that deepens season over season, or that lines up with other signs of trouble, like a settlement depression, a seep, or a patch of vegetation that's stressed for no obvious drought reason.
That last one matters more than it sounds like it should. Cracks are preferential pathways. Once a crack opens deep enough to reach landfill gas or to short-circuit the cover's infiltration barrier, the plants rooted right above it respond before the crack is visible as anything more than a hairline on the surface. Gas displacing oxygen in the root zone, or moisture draining straight through instead of being held near the surface, shows up as a patch of vegetation that's off-color or stunted compared to the grass around it, sometimes weeks before anyone walking the cell would notice the ground itself has moved.
That's the gap a quarterly flyover is built to close, reading vegetation stress across the whole cap from above so a stressed patch tied to a developing crack or a gas pathway gets flagged before it's a foot-deep fissure someone finds on a walk-the-cap inspection. You can see how that works at Landfill Cover Monitor.
What to do about active cracking
Standard practice is still the standard practice: fill and compact visible cracks with matching soil, reseed bare patches fast so roots can start holding the surface together again, and keep irrigation or erosion control going during extended dry stretches if your permit and budget allow it. None of that changes. What's worth adding is a routine way to catch the cracks that are forming under the grass before they show up as a problem on the ground.
If you're already walking cells looking for exactly this kind of early signal, it's worth seeing whether a quarterly read of the whole cap would catch it sooner.