Special thickness ...

Special thickness polyethylene covering material or special impenetrable preventative linings and then using of specially formulated cementing material, which may include bentonite slurry timed to set at specific time duration perhaps could be an acceptable option. An alternative could be to conduct a mini-research on doing same using various construction available cementation materials or sealants. But put into consideration, the depth, pressure and temperature of the environment in question. These factors are also important when dealing with gas/methane and even contaminants.