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11 11 (1.,2/13/2011),Vino6Jain RE: FW contact~in11 fo landfills when you need 7 feet of cover soil? ( *" Comment: Please remember that the test is a quality control test not a performance. The effective stress is 5 psi, but the hydraulic stress is 2 psi which is about 4.6 ft of water. This is not the water pressure that you would see in a landfill cap.) I am not sure if I did my calculations right but I did a simple calculation to compare the use of a GCL with permeability properties of 5x10-9 cm/sec against the Title 27 requirement of minimum of 1 ft of compacted soil with hydraulic conductivity of 1x10-6 cm/sec. Using s=vt where s=thickness of barrier material (cm) v=hydraulic conductivity (cm/sec) t=time to travel through barrier material (years) for Title 27, solving for time (t) I got: t(barrier soil)= s/v= [1 year/3.1536e7sec]x[1ft(30.48cm/ft)]/1 x10-6 cm/sec= .9665 years for the GCL assuming the confined thickness is 1 cm, under a confining pressure of 5 psi with 2 psi head (effective confining pressure on GCL=7 psi) I got: t(gcl)=s/v=[1 year/3.1536e7sec]x[(1cm)]/5e-9 cm/sec=6.342 years The problem is I do not have the confining pressure conditions specified in ASTM 5084. For GCL used in a landfill cover application I have much less confining pressure. Typically, design engineers are requiring only 2 feet of cover soil. The article that Miguel sent me clearly states that with less confining pressure hydraulic conductivity in a GCL increases. I believe that is why manufacturers of GCL during installation of their product do not want the GCL to get wet under unconfined conditions (no cover material placed on GCL prior to a rainfall event). Once the bentonite expands under unconfined conditions its hydraulic properties are irreversible. Simply drying the material out does not restore its hydraulic conductivity properties to its initial state. (***""*"**The biggest issue with gcl getting prehydrated before putting a load on it is the bentonite squeeze out during loading. Example, when the gcl is prehydrated and you step on it, the gcl will leave a footprint where the bentonite was displaced. However, you are correct that at lower loads the hydraulic conductivity will increase because there is more pore space for the water to leak through. As the pressure increases, the pore space decreases and it is harder for water to flow. You're comment above on drying out and rehydrating is incorrect. There have been studies on wet/dry cycles on gcl, and it does not affect gcls. Dr. Rowe and Brachman at Queen's University would be good to discuss this issue with as they have done research)