Q.: We put in the foundation and floor slab for a house about 3 years ago. The floor slab performed fine for 2 years but then started heaving and cracking. Some of the cracks are up to 1/8 inch wide. Could water pressure have caused the heaving and cracking? The basement looks dry but last year we got lots of rain.
A.: If the basement is dry it's unlikely that water pressure caused the cracking. If there was enough head to lift the concrete slab you'd probably see water at the joints or cracks. Swelling soil is a more likely cause. If the house was built over swelling soil during a dry period, a later wet spell could cause enough swelling to heave the slab and cause the cracking. Have a local consulting engineer look at the slab. He may know whether or not there are expansive soils in the area and can take samples to determine if they're present beneath the slab.