To “MudJack” or Not
(after slab foundation repairs)
If after a slab foundation is repaired by installing piers AND the slab is lifted upwards as part of the repairs, a void space will be created between the bottom of the slab and the dirt. The question to be discussed in this blog is: should the void space be filled (mudjacked) or not. The answer is: it depends.
Several years ago, most foundation repair companies (in the North Texas area) automatically included in their proposals the cost to inject a grout slurry mix into the void space (usually referred to as mudjacking). The traditional slurry mix consists of cement, soil and water (which is a very low grade of concrete) that can be pumped into the void spaces. The slurry then hardens and will provide support to the slab (as the dirt used to before the lifting of the foundation).
However, now days, mudjacking after foundation repair does not seem to be done as frequently as it once was. I suspect that there are several reasons for this but one of them could be that if the slab was built on expansive clay, some believe that it is better to leave a void space between the bottom of the slab and the dirt. This allows the clay to expand some without impacting the slab (depending on how big the separation is and how expansive the clay is).
I know of a foundation repair contractor who during the drought of 2006, lifted several slab foundations and then completely filled the void space with slurry. Unfortunately, when the rains returned and the expansive clay soils expanded, the foundations heaved up off the piers. Depending on the severity of the upheaval, this is very difficult to remedy.
Because of this potential for upheaval when a slab is built on expansive clay, many engineers do not require that the void space be filled with slurry.
However, there are other engineers who insist that all voids be filled. Their position is based on the true premise that a slab-on-grade foundation (slab that is supported by the dirt) was designed by the original engineer to be fully supported, so all void spaces should be filled.
Unless circumstances dictate otherwise, I typically do not recommend filling the void left between the soil and the slab after a foundation is lifted. This decision is based on the assumption that expansive clay soils may eventually expand and heave the slab upwards. Yes, there is a risk that the slab may not be strong enough to span above a void space without deflecting downward. IF this deflection occurs, then the void will need to be filled (with a slurry or polyurethane foam).
To restate the issue: there is a risk in NOT mudjacking (not filling the void) AND there is a risk in mudjacking (filling the void). To me, the more reasonable risk is to not mudjack unless a problem develops and the unsupported portion of the slab deflects downward. If this deflection occurs, then mudjacking (or filling the void with polyurethane foam) will need to be done. It should be said that the downward deflection of the slab may not occur for months (years) after the foundation work has been completed. Even though I have never experienced this on my projects (which includes thousands of repairs to slabs), it can potentially occur.
However, if the void space is filled soon after lifting the slab and the clay expands and pushes the slab upwards, there usually is not much that can be done that is financially reasonable (except wait and hope the soils will eventually shrink back down). This latter course of action is technically the correct thing to do but it also has the higher risk of upheaval of the slab if that slab is on expansive clay soil.
I recommend that the homeowner discuss this issue with his/her engineer and with the foundation repair company.