One thread had a link to the following site on sealing ponds with clay.
http://tilz.tearfund.org/en/resources/publications/footsteps/footsteps_21-30/footsteps_25/sealing_fish_ponds/
One link a thread referred says to dig the area and compact the soil. Remove any rocks or stone.
A layer of lime that is about 3/4 to 1 1/2 inch thick. I am seeing weights rather than cubic feet. Pulverized limestone weighs about 87 pounds per cubic foot. A 1 inch deep layer would weigh 1/12 of 87 pounds per square foot or about 7 1/4 pounds of limestone per square foot of pond.
Clay, straw and water puddled in with feet sounds a lot like a cob mix, applied by hand rather than feet.
A layer of cob about 5 to 6 cm sounds like roughly 2 inches or a bit more, if my in head conversion isn't too rusty.
The site says keep area wet or cracks can form. Would having a thick layer of straw as an organic layer to retain moisture in the soil between rain falls be enough to keep the bed sealed? Is there a way to keep the bed sealed without using manure such as alternating layers of cob mix and slip covered straw?
Would cob be preferable to earthbags with clay fill for this use? Or would a layer of earthbags provide a stronger "frame" for the structure? It would be only a couple feet deep and the same wide at the top.
Would a French drain be optimal for moving water through the bed to a storage cistern? How deep should the straw layer be to hold enough water to emulate a drainage ditch that does not have any surface water but can support marsh plants?