Capping a Landfill: Gel Rates on 150 mm of Stony Cover Soil

Closing a 4 hectare cell means putting a cap on it, and a cap is a hostile place to grow anything. The specification called for 150 mm of cover soil over the geomembrane, a stony, compacted material with 30 percent gravel and a pH of 8.4, seeded with a mix of fescue and legume. The first attempt in 2021 produced 20 percent cover after a season. The second attempt, with gel and a different placement method, produced 68 percent.

Landfill capping is neither agriculture nor landscaping, and the standard gel advice transfers badly. There is no root zone to speak of, the soil is thin and skeletal, and the surface temperature runs 10 degrees above a normal field. This article covers what a cap actually asks of a polymer and how to dose for 150 mm of stony cover.

Why a Cap Is Not a Field

The cover layer is 150 mm thick, sitting on a drainage geocomposite over a membrane. There is no subsoil reservoir, so any water that leaves the cover layer is gone from the system. Roots cannot go deeper than 150 mm, and in practice most of the root mass is in the top 80 mm. Surface temperature on a black membrane under a thin cover can hit 55 degrees in summer, which is above the point where many grass species stop photosynthesising.

Those conditions change the calculation. A polymer in a normal field is a reservoir below the root zone. In a cap it is the only reservoir, and it has to work within 150 mm of the surface, where drying is fastest. That means a higher dose than a field, a shallower placement than a field, and a granule that re-wets reliably after repeated drying.

Gel Rate for 150 mm of Cover

For a 150 mm cover, dose by volume rather than by area: 2.5 to 4 kg per cubic metre of cover soil, which for 150 mm depth works out at 375 to 600 g per square metre, or 3.75 to 6 tonnes per hectare. That sounds enormous against the 6 to 10 kg per hectare of field agriculture, and it is, because the treated volume is 150 mm deep rather than 600 mm and because the material has no natural water holding capacity to fall back on.

In practice most projects cannot afford the top of that range. A workable compromise is 2 kg per cubic metre in the top 80 mm only, applied as a pre-swollen slurry during placement of the cover, which lands at 160 g per square metre and roughly 1.6 tonnes per hectare. On the 4 hectare cell, that was 6.4 tonnes, which at 3,800 dollars per tonne was 24,000 dollars, about 6,000 dollars per hectare. Against the cost of the cap works, it was 2 percent of the cell closure budget.

Stony Soil and the Granule That Never Gets Wet

Thirty percent gravel means 30 percent of the surface area is stone in contact with nothing, and the actual soil volume available to a granule is 30 percent smaller than the depth suggests. It also means the cover holds very little water: a skeletal material like this has a plant-available water capacity of maybe 6 to 8 percent by volume, against 18 percent in a loam.

Placement in a stony material has to be by slurry. Dry granules broadcast onto a gravelly surface sit in the gaps, and the first watering washes them into the stone layer where they are inaccessible. On the second attempt at the cell, the contractor mixed the gel into a slurry in a 5,000 litre tanker and sprayed it onto the cover in two passes, incorporating the top 60 mm with a rotavator between passes. The granules ended up distributed through the fines rather than in the voids.

Monitoring Settlement and Vegetation Cover

A capped cell settles, and differential settlement cracks the cover. Monitor three things quarterly: vegetation cover by quadrat, surface cracking along the cell perimeter, and moisture at 75 mm with a hand probe. Cover below 50 percent after the first season means the seed mix or the establishment water was inadequate, not necessarily that the gel failed.

One important limitation: hydrogel does not fix a cap that is losing water through the membrane, and it should never be used to justify a thinner cover layer. The polymer is there to keep a shallow root zone alive long enough for the sward to close, and once closed, the sward itself becomes the water management system. For grades with the salt tolerance a cap leachate environment demands, check the potassium agricultural options in the SAP powder specifications before you price the cell.