Expert: Dan Griffin - 3/27/2008

Question
I have to put a 2000lb generator on a 10 ft by 5 foot slab. The ground has a 1 foot grade along the 10 ft length. Assuming I have an 8 inch thick slab with 4" of the slab sticking out at the top before the 1 foot grade on the top end, what would be the best way to support the other end of the slab which at this point is not in contact with the ground. I was thinking possible excavating the dirt and putting a gravel base in, then backfilling the dirt to the same grade level and then pouring the slab. Any ideas? If this plan sounds ok how deep should the gravel base be? We are in the North East. Thanks

Answer
I appreciate your concern for bearing capacity and for drainage.  Too often people can ignore these issues.

2000#, a ton, sounds heavy.  Evenly disributed over 50 square feet is actually 40#per SF, not quite so bad.

YOu need to remove any and all sod over the entire area.  Raise and level the entire pad area to the level of the grass at the high end, use sand or whatever is used for compactable fill in your area.  Form and pour a thickened edged slab.  Dig a trench around the edge of the slab and use 2 or even 3 2x4's to form the concrete or use a 2x10 all the way around.  Set the form for the top finish grade first, making 4" above the grass at the high end your point of beginning.  I hope this is clear - the basic slab is 4" thick with a thick edge that makes it look like it might be 12" thick.  This adds great stiffness and strength to the concrete and allows your finish grade to slope gracefully past the slab.