what do need to provide a dust bath in my fixed run?

We have just taken delivery of 3 hens (18 weeks old) and initially made a dust bath up with some sandy soil in a shallow dish approx 1.5ft in diameter. They have not used it yet and it is now wet!!
Any advice?

my girls chose their own dust bath! i have a big sort of half barrel type planter, that did have flowers in. The girls liked the idea of that, scratched out a shallow sort of dip in it, and dust in there. useful, cos you can put the diatom straight in there and they can dust themselves for mite etc.  The secret is you MUST keep it dry, i suggest you put it under a covered area of the run.