DEAR TIM: The concrete patio on my new home is sinking. It used to tilt away from the house. Now it tilts back towards the house. A similar thing is happening to the concrete slab inside my neighbor's garage. What do you think caused this to happen? Could it have been prevented? Is there a way to fix the problem, other than installing new concrete? S.W.

DEAR S. W.: There is a very good possibility that both concrete slabs were installed on poorly compacted fill dirt. Sub-surface erosion and shrinking soils are also possibilities. Fill dirt is almost always placed along side of house and garage foundations after the foundation work is completed.

Rarely does a builder take the time to compact this dirt. Mother Nature will do the job, but she sometimes takes years to complete the task.

Soils consist of solid particles and the spaces (voids) between these particles. The void spaces can be filled with either water and/or air. Void spaces are great for grass, bushes and trees. However, void spaces in soil can cause big problems for buildings and concrete slabs. Concentrated loads, such as buildings or slabs can literally squeeze air and water from soils. When this happens, the soil sinks and the buildings or slabs follow closely behind.

Your problem could have been avoided. Instead of installing fill dirt, your builder could have installed granular fill such as sand or a sand and gravel mix. These materials can be compacted quite easily with a hand held vibratory compactor. This material should also be used to fill trenches that cross sidewalks and driveways.

Your builder also could have pinned your slab to your house foundation. This process involves drilling holes into your foundation and inserting steel bars (pins). A network of inexpensive steel reinforcing bars are then fastened to these pins. The concrete patio or slab is then poured with this steel roughly in the center of the concrete. Should the soil beneath the slab settle or sink, the slab stays in place, supported by the pins. Do not use this method for structural slabs (those that support the weight of a structure or machine).

If you are fortunate enough to have a slabjacker in your area, you should not have to replace the concrete. These individuals can float a slab back to its original position by pumping a mixture of sand, cement, fly ash, and other additives beneath your slab.  Not all companies use the same product to lift the concrete. You may find a company or two that pump two fast-reacting chemicals that combine and expand under the concrete slab. These chemicals form a structural polyurethane foam that is unaffected by water that may seep under the slab.

To install the mixtures or chemicals under a tilted or fallen slab, they simply drill strategically placed holes into the slab. Using a portable pump and flexible hoses, they fill these holes with the special mixture. Lifting a slab using this method can often be accomplished in a few hours. Often the cost to perform this service is less than half that of replacing a new slab.

There are numerous benefits to slabjacking. It can be done in virtually any weather. The material injected beneath the slab provides a strong base. There is little or no disruption to landscaping. Nothing needs to be moved off the slab, as the pump can lift the weight of the slab and anything you have placed on it. This should be great news for your neighbor!