Containment Lining
Unconfined bentonite would lift the cap off. Confined, it does the useful thing instead and seals.
— mm
Thickness the liner reaches under its actual confinement
Free swell index is measured in deionised water; calcium-rich or saline groundwater cuts the achievable swell sharply and can halve it over years of cation exchange, so a liner that tests fine on installation may not stay that way. At high overburden the confined thickness can fall below the dry thickness, which the model reports as a negative gain — physically that means the liner is compressing rather than swelling and the hydration assumption no longer holds. Barrier performance is a permeability question; thickness is only a proxy for it.
Geosynthetic Clay Liner Hydration Swell Index & Thickness — free, with the formula and a worked example, at Textile School.