I think it WOULD be an issue, because swelling clays generally swell when wetted, then shrink when dried. There would be potential for cracking, loosening, etc. as a result. In a liner, you might be able to keep the clay approximately at the compaction moisture fairly well if there is a membrane liner over it, but you can't expect to keep the cap at constant moisture long term.
Bentonite is widely used as a liner and is full-blown swelling clay. What's at issue is the amount of confinement. If properly confined (i.e., once under a measured amount of fill soil, lechate collection materials or just refuse) I wouldn't expect damaging swell. On the matter of wetting and drying cycles, once under a few feet of cover I would anticipate that the range of seasonal moisture change would be too slight to worry about.
Here's where the body is buried: To properly place clay liners, you MUST place them wet of optimum. There is greater than a 2 fold decrease in hydraulic conductivity between the same clay compacted below the optimum moisture content and above the optimum moisture content. For the case that you place the clay in the 90 percent saturation range and then let it lie - it'll crack (a bad thing). Therefore, you MUST immediately cover a clay liner after placed and compacted to spec.
Nice question, but what is needed is expressions of experience with these materials, not theory.
I had the unique opportunity some years ago to visit some covered landfills that had been built to state specifications for cover.
The state enforcement guys were bragging up the imperviousness of this form of cover. They wanted to show me one of the better landfills. I had along a folding ruler. These "experts" from the state agency were mortified when I showed them the deep, wide, cracks in this cover material, running from surface into the trash below.
After that I had no difficulty convincing them that their spec was not the best and that lower plasticity material was a better choice.
Fine, low-plasticity material is highly susceptible to rill and head cut erosion unless good ground cover can be established. Settlement (differential)of the landfill can create drainage flow areas that were not anticipated. Erosion in these low areas, especially down the side of the landfill can create large gullies very quickly. Soil cover material had better be thick and non-erosive to prevent exposure of the cover in these gullies.
Using a fine-grained soil as barrier in the final cover system can be an issue, especially if leachate will be collected during post-closure. The effectiveness of a soil cover will degrade over time due to waste settlement, freeze/thaw cycles, etc. According to the literature, post-closure permeability may increase by a couple orders of magnitude (or more).
Here in my neck of the woods (upstate New York) I've been working with two landfills that were capped with soil covers over a decade ago (18" 1E-7 cm/s glacial till + 24" barrier protection layer + 6" topsoil). Hauling records indicate that leachate flows have been increasing significantly at both of these sites over the years. I think most of the landfills in this area nowadays are capped using geomembranes.
