Place a carpenter's level onto the foundation floor and follow the slope of the floor to its lowest point to find the most likely location of the void beneath the foundation. Ignore the built-in slope of the foundation leading to any drains contained in the concrete floor.
Drill a line of 1-1/2-inch diameter holes from the lowest point of the foundation toward the foundation center. Stop at the point where the floor no longer slopes toward the washed out void beneath the house. Use a hammer drill to drill through the concrete foundation along the chosen line with the holes spaced 6-inches apart.
Attach the outlet hose to the mudjacker. Run the hose down the hole at the highest point of the foundation. Lower the hose until it clears the foundation base. You will feel a change in resistance as you move the hose through the hole, indicating that it’s cleared the slab. Stop moving the hose once it reaches the bottom of the slab. Place the carpenter’s level next to the hole with the level positioned along the slope of the floor.
Mix one part Portland cement with three parts fly ash to create the concrete slurry for the mudjacking process. Add some expander additive to the mix, following the manufacturer’s directions on the amount. This will prevent the slurry from shrinking as it cures. Use a hoe to fold enough water into the mix until it reaches a batter-like consistency. Use the spade to move the slurry from the wheelbarrow to the mudjacking machine's hopper.
Open any doors or windows to increase the room’s ventilation. Adjust the mudjacker controls to 10 PSI. Turn on the machine to begin pumping the material. Pump the slurry through the hose and into the space beneath the foundation. As you pump the slurry, the foundation will rise slightly. Keep pumping until the carpenter’s level displays the floor as level again.
Move the level and the hose to the next hole down the line. Repeat the pumping process, filling the void under the foundation until the second hole is level. Continue down the line of holes until the void beneath the house is filled and the foundation no longer sags. Turn off the machine after finishing with the final hole.
Mix three parts sand with one part Portland cement in a small bucket. Add water to the mixture until it is the same consistency as peanut butter. Use a trowel to push this patching mix into the holes in the foundation. Insert the patching mix until the holes are filled and the compound is level with the surrounding foundation floor.