Floating Slab Foundation Is Settling

Foundations

• Floating slabs are one of three basic categories of concrete foundations. As their name implies, they consist of a large slab of concrete that "floats" unsupported on the soil. Usually the slab and the soil are separated by a thin layer of crushed gravel that improves drainage. Sometimes the foundation features a so-called waffle raft slab with reinforced concrete beams running across the underside of the slab. Besides being economical, floating slab foundations may be an effective approach for soils that shift substantially in response to changes in moisture content.

Settling

• Settling occurs when the ground beneath the foundation moves and transmits this motion to the foundation above it. If the soil beneath the foundation becomes compacted or compressed, for example, the foundation may slowly sink. Alternatively, if the foundation is built on clay that expands significantly when it absorbs moisture, the upward pressure can move or rearrange the foundation. The worst problems arise when you have differential settling, where some parts of the underlying soil settle or move to a greater extent than others. This kind of stress can cause the foundation to crack and fail.

Warning Signs

• When clay beneath a foundation expands, it tends to uplift the corners, imposing stress on the walls and the center of the slab. If your foundation slab is settling, you may observe a number of peculiar changes around the house. Floors that bulge, walls with vertical cracks, displaced moldings and doors that no longer close are all telltale warnings that point to differential settling. Also look out for cracked bricks, window frames that no longer align with the wall, cracked sheetrock and mysterious new cracks in the floor.

Solutions

• The two most common solutions are slabjacking and piering. In the slabjacking approach, builders pump grout underneath a part of the slab and thereby lift it to make it level with the rest. The piering approach relies on steel posts or piers; builders drive these down into the soil with hydraulic rams and use them to stabilize the distressed slab. If you think your foundation is damaged, you should consult with a builder or engineer in your area to figure out which solution is most appropriate and how best to implement it.