Any good flatwork contractor knows that a concrete slab is not self-supporting. That’s why he’ll prepare the ground first by placing 6-inch lifts of well-compacted gravel; otherwise the slab could settle and crack.


No Visible Means of Support
Typical residential slabs aren’t designed to span over empty space. They need solid ground under them, including a layer of firmly compacted gravel. When there’s soft soil under a slab instead of the proper sub-base, the likely result is settling and cracking of the slab.
Illustrations: Harry Whitver

But sometimes life is complicated. For example, the soil under the imported gravel sub-base may not be suitable for bearing the load. Some soils just don’t compact well. Even worse—sloping bedrock underneath the problem soil. Now you have to place and compact your imported gravel over a hard base that’s not level. (Good luck with that.)


Drill, Lift, and Grout
Slabjacking is a two-part operation. First, the contractor drills holes into the slab. Then, a cement grout is pumped into the voids to lift the slab and provide a level base of support. Sometimes, steel pins driven down to bedrock or firm soil are used to help with the lifting.

Flowable Fill
Sometimes it’s hard to compact the soil or gravel you would ordinarily use under
a slab. In those cases, one option is to use a special low-strength concrete mix called “lean fill” or “flowable fill.” You can place this material the way you would concrete, and it will harden to provide firm support.

Slab settlement is common enough that there’s a whole industry of specialists who know how to repair it. A typical fix involves “slabjacking,” or “mudjacking.” A contractor drills holes in the affected slab, inserts driven piers down to rock or competent soil, lifts the slab up with support brackets, and then injects the voids under the newly lifted slab full of flowable grout that will harden to provide firm support. There’s nothing new about this process, which is outlined by John Meyers in “Slabjacking Basics” from the September 1990 issue of JLC, a sister publication of BUILDER.

Prevention requires thorough compacting of the underlying soil and the gravel sub-base. But in problem situations, consider using “flowable fill,” a lean mix of gravel and cement that you can pump or pour like concrete. It’s not as strong as structural concrete, but it’s stronger than compacted soil or gravel—and requires no compaction. For more on this technique, see “A Quick Cure for Problem Soils” from the January 2000
issue of JLC.