I found a site or 3 that talked about them but the better of them was this:
DRY WELLS should be considered when drainage problems are severe. The best solu- tion to roof-water drainage problems is to connect all downspouts to either a storm sewer (depending on municipal laws), or dry well. This requires the installation of drainage pipes or tiles in an underground trench about 30 cm (1 ft) beneath the surface near the downspout, sloping at 4 cm per metre (1/2" per foot) from the house to the storm sewer or dry well.
A dry well is just a large hole in the ground filled with rocks and rubble and covered with wood boards or a concrete slab to keep out topsoil. It is used where storm sewers are not available for drainage. One method of making a dry well is to take a 45 gallon (200 L) oil drum, remove the ends, cut a hole in one side for the drainpipe, and punch a few dozen random holes in the drum for water seepage. Bury the drum at least 46 cm (18" deep), filling it with rocks and rubble. Cap it with wood planks or a concrete slab to keep it from filling up with dirt.
You can also build a dry well with concrete blocks laid on their sides and separated at the joints by a few centimetres to form a hollow in the centre. A 1 metre (3 ft) square hole is mini- mum; a 1 x 2 metre to a 1 x 3 metre hole is best. Dry wells should always be located at least 3 metres (10 ft) from foundation walls.
If you have any particular ?'s, post back or e-mail me directly. My best to ya and hope this helps.