Concrete slab-on-grade foundations often develop sag or settlement, out near the interior portions of the slab foundation, due to consolidation of the underlying fill materials. Typically, these fill materials are gravel, but sometimes they are soil. If there is a deep amount of fill beneath the slab, regardless of the type of fill material, settlement or consolidation of the fill can occur and this can lead to slab settlement over time. Plumbing leaks can also lead to severe slab settlement as the leaks erode the underlying soil fill and washout cavities beneath the slab. Shallower-bearing slab foundations on grade can also develop settlement due to these same plumbing leaks or improperly compacted fill materials. Many insurance companies argue that the plumbing leaks are due to plumbing line settlement that occurred along with the ground consolidation/settlement, and this is due to improper construction practices-e.g., poor compaction of the fill materials beneath the slab. Hence, homeowners insurance may not pay for the repair of a settled slab or repair of plumbing leaks beneath a slab.
If the slab settlement is due entirely to settlement or consolidation of the fill materials beneath the slab, and there are no plumbing drain line breaks/cracks/problems, then the slab can be re-leveled by mud-jacking, which is a process of injecting grout under pressure, beneath the slab, and slowly lifting the slab back to level.
If the slab settles for whatever reasons, and fractures the plumbing drain lines, and they are leaking, then the slab cannot be grouted because the grout would enter the drain lines and stop them up. Hence, the only solution in this case is to map out the drain lines beneath the slab, via video camera and sensors, then bust up the slab, repair/replace the fractured drain lines, then replace the overlying concrete. In this case, it may be best to simply pour a self-leveling concrete mixture over the repaired area to restore level to the settled concrete slab surface. This may also entail having to reset/resquare interior doorways, which have wracked from the long-term slab settlement. Or, the replaced concrete above the plumbing repairs can be structurally keyed into the original, surrounding slab, via closely-spaced rebar dowels, making the patch very strong. Then the repaired slab can be pressure-grouted, after plumbing repairs, to bring the slab back to level.
How to get started
The first thing to do in any situation is to perform a hydraulic plumbing drain test to see if there are any breaks/separations. An inflatable stopper is placed in the outside sewer drainpipe and a toilet is removed inside the home. The faucets are all run until the drain lines beneath the slab fill with water and rise to the surface of the concrete floor slab at the removed toilet. The water level in the sewer pipe is then monitored to see if it drops over time-thus indicating a break in the under-slab sewer piping. The cost for this test is around $ 375.00-up. If the water level does not drop, then there are no plumbing leaks and the slab can be pressure grouted.
Cost for Repair
If no leaks exist, the cost for mud-jacking the slab back to level costs about $ 6.00/sq.ft. with a minimum fee of around $2,000.00. Additional costs include labor for moving furniture and floor coverings in the area of grout repair, possible replacement of damaged floor coverings in the area of grout repair, repair of wracked doors, repair of drywall cracks and repainting in the area of drywall repairs.
If leaks exist, then you must get an estimate for repair from a licensed plumber specializing in this type of repair. The slab must be busted up and removed in order to repair the underlying leaking pipes. Again, in this case, after plumbing repairs, it may be best to use a self-leveling concrete compound to relevel the settled floors inside the home.