We've use raised beds both in the garden & on the allotment & on both occasions they were on compacted clay soil. We just lightly forked the soil before placing the bed timber down. In the garden we had no 'leakage' as it was fairly level, but at the allotment we had slight leakage due to the badly uneven ground. We solved the problem by adding slivers of marine ply inside the main frame where it was bad.
We also put membrane down between the beds and topped this off with bark chippings as paths.
Hope this helps
MW
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