Slab Jacking and Underpinning
Slab jacking and underpinning are powerful techniques for rectifying settlement and consolidation problems. They are the absolute worst thing you can do to try and restore a slab heave problem.
Slab jacking is the pressure injection of a grout under a slab to lift a ‘sunken slab’. Take care not to confuse a sunken slab with a edge heave slab because they look identical. There is quite possibly a void between a edge heave slab and the ground just like you would expect on a slab spanning over ground consolidation. Filling this void with a grout and using the grout to bring the slab back to level without consideration of the soil moisture profile under the slab is a recipe for heave reversal and a whole set of new cracks in the opposite corners and directions. How can you tell the difference? If you are considering using slab jacking, first you must rule out edge heave!
Do these things:
- Do not engage a slab jacking expert to determine if your house should be slab jacked. That is akin to asking a dentist if you need to visit the dentist five times a year or asking an optometrist whether you need glasses. The answer will just about always be yes – whether you do or you don’t. Do not engage a salesperson to diagnose slab heave or consolidation problems.
- Consider slab jacking only if you are trying to overcome consolidation of loose soils or uneven settlement of fill. The only way to prove loose fill or soft soil is with a soil test. If the ground around the outside of the building is firm and you still think you have consolidation of soil, core the slab and have the soil under the slab tested for bearing capacity and density.
Gaps Under a Slab
This is why gaps under a slab do probably do not constitute consolidation of loose soils:
- Soil consolidates under its own weight and under applied loads. We can assume that vertical consolidation is greater under a greater load in a homogenous loose or soft soil.
- Houses apply more load to the ground under the perimeter footings.
- Therefore more vertical consolidation should occur under the perimeter walls.
- Where gaps exist between the slab and the ground, the house is not applying any force to the ground, Therefore the soil in this area is consolidating under its own weight.
- Therefore the slab surface profile for consolidation of loose soils should be a doming slab, not a dishing slab.
Gaps between a slab and the ground under the middle of a slab therefore indicate that the edges of the slab are lifting and the concrete is spanning over the void. This will occur even in an unstiffened slab as the slab goes to catenary action. You may not even identify cracks in the bottom of the slab. The catenary forces in the slab will be resisted by the slab reinforcement which, by the time you account for top cover and thickness of the mesh, is about central in a 100mm thick slab. A slab in catenary action will span over the ground and you may not even observe cracking in the concrete. Do not fill the gap by slabjacking!
Slab Doming and Underpinning
Conversely, before establishing the need for underpinning of edge footings, rule out centre heave and edge shrinkage.
- Establish consistent moisture contents around the perimeter of the slab and under the slab by coring the slab and testing soil moisture contents.
- Establish that the soil under the edge of the slab is looser or the uncontrolled fill is deeper) than under the middle of the slab.
- Establish a correlation between locations of higher load and greater vertical settlement.
- Look for slab surface cracking adjacent to the perimeter footings. Even with slabs reinforced with edge beams, catenary action of the slab is unlikely due to the holding up effect of the soil under the slab and the lower pressures needed to support the slab over a large distributed area. If edge settlement is occurring, a line of cracks running parallel to the slab edge is possible as the slab forms a hinge to accommodate the uneven vertical movement.