Mudjacking is a process by which a ‘mud’ is pumped, with pressure behind it, under a concrete slab, with enough pressure to raise, or jack up, the concrete slab.
Pressure grouting is the same pumping under a concrete slab, but with just enough pressure to fill the void under the slab, but not enough pressure to raise the concrete.
If the concrete slab is raised, especially over 2 inches, it leaves a large void under the slab. The slab was not designed to act as a bridge member, supported at each end, and there is a danger the slab will sag into that void, in the future, if not supported in the middle. The slab was not designed to act as a bridge member, or it would have had a lot more steel reinforcement and would have been poured a lot thicker. Therefore it is important to pump a concrete/soil mixture under the slab, with pressure to fill the void sufficiently.
But that is not the main reason pressure grouting is so crucially important after raising the foundation slab. The pressure grouting mud will SEAL OFF the void from moisture penetration under the slab. Ponding water, that will pond in the large void created by lifting the slab, will accumulate and create foundation problems in the middle of the slab. Usually this will occur in the first 2 years after the slab is raised.
Many foundation contractors can offer a cheaper price in foundation repair, because they have omitted the pressure grouting to fill the void, but the homeowner is subsequently left with a defective home. As water ponds in the void, the middle of the home will settle, and the walls will soon be cracking.
The average cost of repairing the middle of the slab is about $10,000, much more expensive than perimeter foundation repairs. And still the homeowner must then do the pressure grouting that was omitted in the first contracted foundation repair, and then repair the sheetrock cracks, and re-stretch the carpet.
The average cost of filling the void after foundation repairs is about $3,000.
Voids less than 2 inches are probably not necessary to pressure grout, because the 2 inch void is mostly on the edges of the foundation, which is strongly supported by the underpinning piers or pilings, and the void quickly decreases to about 1/2 inch in the first 3 feet. The moisture penetration must reach 8 feet inside the home or more to affect the center load bearing areas, which would need about a 4 inch drop or more.
Only non load bearing areas can be lifted, such as taking a sag out of the middle of a room. Mudjacking is popular to lift parking lots, sidewalks, highways and streets, and flatwork areas, where all the area to lift is relatively the same weight. It is dangerous and ill advised to attempt to lift a load bearing wall with mudjacking.