Q: We are working at a site with a permitted landfill cell with a leachate collection system that includes a drainage geocomposite with 12 inches (305 mm) of leachate collection sand overlying it. My question has to do with the percentage of fines in sand on a geocomposite. Our leachate collection pipe spacing is such that the leachate should not exceed the thickness of the geocomposite during landfill operations. (We calculated a max impingement rate and did calculations that accounted for reduction factors and factors of safety.)
We have historically specified leachate collection sand with a maximum of 5% passing the No. 200 sieve (as a fraction of the No. 4 sieve) and a minimum hydraulic conductivity of 5 × 10-3 cm/sec. This material was easy to come by. However, the borrow pits around the site are becoming depleted, and clean sand is no longer available, as it has a higher percentage passing the No. 200 than specified, but the hydraulic conductivity still meets the specification. My research indicates that many states require a maximum of 5% fines (based on the whole sample not the sand fraction).
I have used the soil retention criteria found in Robert M. Koerner’s Designing with Geosynthetics (2012, vols. 1 and 2, 6th ed., Xlibris, Bloomington, Ind.) to check that the sand is appropriate for use with the geocomposite, and it is.
Are you aware of any guidance on the percentage of fines in sand on a geocomposite?
A: This is a fantastic question and one that is timely. With the scarcity of quarried gravel, drainage geocomposites are regularly being used as a component of leachate collection and removal systems (LCRS). We typically see 12 inches (305 mm) of 5 × 10-3 cm/sec sand specified as drainage geocomposite cover soils. Several states require a maximum of 5% fines. However, I think it is a great addition to the soil specification. From the Hazen equation to predict sand permeability, d10 is squared. Hence, it has significant impact on the permeability. With that written, please do not specify 0% fines for this soil. Note that mining and transporting such sand from any borrow site will lead to 2%–3% fines.