The most common cause of lifting concrete slabs is tree-root growth, pushing at the soil under the slab and displacing it. There are two approaches to rectifying this problem. The first requires lowering the offending slab, while the second involves raising the surrounding slabs. Choose the best repair method based on your preference.
Lowering the Slab
Dig down immediately next to the sidewalk on the side facing the tree, exposing the tree roots.
Use an ax to chop away the root.
Dig under the sidewalk. Place a large piece of wood in the base of the hole to support the floor jacks. Carefully lift the concrete slab using three floor jacks, spaced evenly on the underside of the slab. Lean the slab on its side, angled away from the hole and stabilize it with cinder block on each side to prevent it from falling.
Remove the tree roots. Place foam board root block in the ground, next to the sidewalk. The tree has a tendency to lose stability after cutting roots and a propensity to die, so base the decision to do this on the size of the tree.
Fill the hole in with dirt and top off the last four inches with washed gravel. Carefully lower the slab on top of the gravel.
Raising Surrounding Sidewalk Slabs
Bore two-inch holes (spaced every two inches) with a masonry drill bit in the ends of the lower slabs.
Contact a ready-mix concrete delivery company to schedule a pour.
Pour concrete into the bored holes with a pressurized nozzle until the slab raises up and is level with the existing slab.
Repeat for each sidewalk slab until all slabs are equal to the height of the raised slab.