When clay dries up it cracks and doesn't heal itself. It can be sealed with dry clay, even sandy clay and add an amendment that allows it to pull back together when rehydrated, but you may not have that specific product available in your area.
For TJ's product to work "best", the pond needs to be full per his recommendations, for max hydrostatic pressure. For any amendment to work, soil has to be 22-28% moisture, it has to be properly compacted and filled before it dries much.
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