The way a labor contractor backfills the area of work, whether new construction, rehab or service work, is extremely important. Many municipalities will inspect the foundation or open slab before the foundation is poured (or repaired) is evidence as to the importance of why this is so important.
You may have seen a new home being built and the foundation walls being backfilled with the site spoils. You run the very real risk of the ground settling if the material under the floor or against the side of the foundation is not backfilled properly. All ground will settle, but if the proper steps are taken, this will rarely have adverse affects under reasonable conditions.
If the foundation is not properly backfilled (or unreasonable conditions occur), is where mud jacking becomes necessary. The best way to illustrate this situation, is to provide a real life scenario. A service tech was dispatched to a customer whose waste piping had come apart right under his floor in his laundry room in front of his garage.
Upon arrival the plumbing technician verified that the piping had, in fact, come apart under the slab in the laundry room. The next step is to find out why. The technician made a small hole in the wall to insert a Ridgid Mini Sea Snake (small color LCD fiber optic camera) to see inside the wall to get a diagnosis. What the technician discovered is that the floor had pulled away from the underside of the slab in the laundry room and under the garage floor.
Upon further inspection we saw that the ground had fallen in a significant portion of the laundry room and part of the garage. The waste piping servicing the laundry room was undersized and completely exposed. Because a permit had to be taken, the plumbing inspector wanted the piping to be changed to the proper size upon replacement.
The challenge was to determine how to sawcut or break-up the concrete without running the risk of the concrete falling. There were portions where the ground had fallen 3ft from the underside of the concrete slab. This, clearly, can be extremely dangerous. Another issue that can arise is that when making the plumbing repairs and backfilling the void is to make sure the backfill is evenly distributed to avoid the same issues in the future.
In order to support the underside of the slab a slurry was injected through small bore holes in the concrete to bring the ground up to the bottom of the slab. Once the slab was supported, the concrete could be safely cut.
But, of course, there was another problem to consider. The slurry used for the mud jacking could enter the open sewer lines. To remedy this, the broken line was isolated from the active sewer by inserting an inflatable test ball through a clean out which prevented the slurry from entering the sewer.
Then, the mud jacking would be completed under the laundry room and garage. The slurry would be allowed to cure, the floor sawcut where the pipe was to be repaired/replaced. Once that was complete, the plumbing technician would dig down to the defective piping and repair and backfill using the code approved materials and method.
Obviously, if no plumbing was being repaired or replaced the mud jacking would be done with a concrete slurry so it would harden into a stable base while filling all of the voids under the slab.