I haven't either. But, I suppose if you're wanting to "dig your heels in" or "get a good toe-hold," birkies would be the answer. Pliable dirt or wet sand would be your best venue for "birkiedoing." Now, sand ... that is a whole 'nother thread ...

moonjen (whose last venture into "sand" was in 1967 and is still finding grit in her underclothes)