My local county dump is dozens upon dozens of acres square. Perhaps hundreds. It has been closed for most of the time I have lived in the county ( 11 years ).
How did they “cap” it, so that now it is an immense, smooth hill? Grass grows, one cannot see mountains of refuse and junk beneath. How does one go about applying that much dirt, and not go broke? Is it not just dirt? I know that methane is captured and bottled off of the rotting organic materials buried therein. I’ve been told that by an employee at the dump.
Leachate collection system at the very bottom, clay base, trash compacted and then compacted some more, then clean fill dirt added to the top. Seeded and mulched at the top.
They don’t have to worry about going broke at most landfills. There’s very, very big money to be made in trash (tipping fees mostly).
They normally put a layer of clay on top when they “cap” the dump. The clay is dense enough so that trash doesn’t come to the surface. They put dirt on top of the clay, and then plant grass to make it look nice.
The local kids then name the place “Mount Trashmore”
I have worked in 3 landfill sites, and have capped at all of them.
Daily cover is an industry standard 6" of clay. When a whole cell is complete, another 2’ of clay is added. This comes from excavating the next cell to be filled. Finally, 6" of topsoil is added.
To make it look nice, the operator will Hydroseed the area. This is done by spraying a mix of water, seed, straw and a binding agent from a water cannon, usually truck mounted.
Many caps these days are required to be even more impermeable than clay.*
The materials involved are known as geosynthetics, and one common type used for caps is 40-mil HDPE geomembrane. The process of welding the liner panels together is quite intensive, and involves innumerable non-destructive and destructive tests on the welds to ensure they meet the project specifications. Drainage sand and topsoil are placed over the membrane.
BTW, the purpose of a cap is not simply to look nice. A properly designed and installled cap prevents infiltration of water, which results in less contaminated leachate from the landfill. This is especially important with older landfills that were never required to have an impermeable liner beneath the waste.
On a personal note, I’ve spent the last two months as the resident CQA (construction quality assurance) engineer on a new landfill baseliner construction project. A baseliner is even more involved than a cap. Our baseliner consists of two 60-mil HDPE geomembranes with a geocomposite drainage layer between. Most liners also require a clay and/or GCL layer beneath the geomembrane, but as this landfill is designated for ash (from incinerated trash) only, the clay layer is not required by the state.
As far as moving dirt, the general contractor on my project has moved over 100,000 cubic yards of dirt and sand over the last two months. The cost of constructing this one phase of the landfill (one of six planned to date) is several million dollars. The owner of the landfill has very deep pockets, though.
*Actually, even when clay is sufficient, a bentonite-containing geosyntetic clay liner, or GCL, is generally used. A foot-thick layer of clay can be replaced with about a half-inch thick GCL.
In the US, all landfills accepting garbage must comply to Subtitle D standards (40 CFR 258). The only exceptions are areas where there is very little rainfall (no rainfall, nothing rots, no leachate to contaminate the groundwater).
Think of it as a giant tupperware bowl stuffed in a box of clay. Clay base - leachate collection grid - 60 mil plastic - leachate collection - garbage etc (covered with at least 6" good dirt or clay daily) - and, when all is said and done, capped with 60 mil plastic and a methane collection (or venting) system.
The 60 mil plastic is chemically sealed, very neat process.
The geomembrane used in most landfill liners is high density polyethylene (HDPE), which is fused together by heat. The most common method used is a dual hot wedge welder, with extrusion welders used for patches and repairs.
Polyvinyl chloride (PVC) is the next most common material used for liner geomembrane, and I believe it can indeed be chemically bonded (as well as heat bonded).
The landfill liner I spent 12 hours observing the installation of today consists of: subgrade, 60-mil HDPE secondary geomembrane, secondary leachate collection geocomposite (geonet inside of geotextile), 60-mil HDPE primary geomembrane, primary leachate collection slotted pipes, drainage sand, protective sand, with ash to be placed on top for the next decade.
In the landfills that I inspectedl, the methane collection system is a series of pipes under the cap that collect the landfill gasses. Landfill gasses are generated from the breakdown of the garbage. Pipes then extend up out of the landfill and the gasses are then flared (burned off), vented, or processed to remove the methane.
We cap our dumps by lighting cedar incense , because we don’t have a bathroom fan.
Revtim and voltaire, I’m with you. Rest of y’all, I humbly beg your indulgent forgiveness. I’m actually also interested in the legit answers.
I was under the impression that PVC was an environmental problem (I think it was Neal Stephenson’s Zodiac that gave me this idea, though I don’t remember details, and I realize that’s not exactly an impartial source.) Should I be worried about folks using it to Tupperware up huge quantities of landfull debris?
If you’re going to be a wise guy, at least be accurate. :smack:
An acre is not a rectangle. An acre is a square measure ! An acre can be shaped square, round, oval, or any odd shape. As long as it totals 43,560 square feet.