Landfill Caps - changes in hydraulic performance with time

[RockbashinBrit]

I'm trying to put together some estimates of infiltration to a closed landfill with different scenarios of cap design (for a legal case)

I'm after some typical values for how hydraulic conductivity of a clay cap, and of a simple geomembrane would vary as they degrade over time. I need to compare the performance of a simple clay cap which is allowed to degrade, and a multi-layer system that is well maintained.

I've searched high and low but can't find any good references on landfill cap performance with the right information. Surely someone out there has done some research on this?

I do not have access the the HELP3 program, which would be very useful. Any pointers towards useful references/papers/organisations/databases would be gratefully received

Many thanks

Bashin' rocks and drillin' holes!

All models are wrong.

 

[cvg]

The main culprit of clay caps is erosion and settlement. In my experience, the settlement can also exacerbate the erosion. Erosion simply reduces the thickness of the cap thus potentially increasing the amount of water which can flow into the landfill. This all being a function of the permeability, amount of rainfall and the E/T rate. The erosion can occur by both wind and water. Settlement can cause ponding areas which allows additional water to infiltrate. I have seen where this ponding has occured near the top of slopes has cause a "blowout" to occur which potentially erodes through the entire cap layer, exposing the waste. I'm not sure how you could adequately measure or calculate the amount of degradation for a quantitative analysis.

 

[RockbashinBrit]

Thanks cvg. Your observations are well made and I agree that degraqdation is by its very nature difficult to quantify. Nonetheless I still have to find some way of assessing how much water will get into the landfill if it is properly constructed, and if it is poorly constructed and not maintained. Perhaps there are some records of how long landfill caps have lasted before failing/requiring maintainance.

 

[oldestguy]

Another bad thing about clay caps. If you spec a high plasticity, the chances are you will get a high shin-swell clay. When that drys, the cracks go all the way through it and in effect no cap at all. I found that the spec writers don't seem to know about shrink-swell.

 

[howardoark]

If you Google "Alternative Final Cover" you'll find more references than you can shake a stick at.

HELP3 is publically available from epa -

http://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=126967.

HELP is a total piece of crap - the amount of infiltration iscompletely dependent on what the user selects for the evapotranspiration coefficient (so it may be perfect for litigation support work). UNSAT-H is much better and also free

http://hydrology.pnl.gov/resources/unsath/software.asp

.

In general, HDPE is thought to be good for about 300 years and it has a hydraulic conductivity in the 10(-12) cm/sec range, so it can't be compared to clay. The problem is that quality control isn't typically strict enough and after the landfill has settled 8 feet (not uncommon) large pools are probably forming in the liner (or would be except the liner probably has large rips in it by then).

I think monocovers are the way to go. The thickness should be determined based on the local climate.