If, as you indicate, the soil below the topsoil is clay, then digging a hole and filling it with rock...thus creating what is referred to as a drywell...will only serve to create a hole in the ground filled with rock and water. Clayey soils do not, as a rule, drain very well. They act more like a sponge, and the water goes nowhere fast, if ever. If, however, you can dig that hole and install a pipe to drain the water away to some other destination lower than the drywell, then the drywell might work, but more as a catch basin, like you'd see in the street. Otherwise, you'd be better served by seeking to regrade your back yard in order to drain the water away to the sides, and hopefully off your lot.
[This message has been edited by treebeard (edited March 05, 2003).]