Vibrations, animal damage and washout can displace and erode supporting materials, creating voids under your concrete slabs that will cause them to sink. Filling these voids with mudjacking or polyurethane material will allow the section to be raise to the correct level.

Even if the concrete is not settled, voids can cause slabs to crack when they are not supported from beneath.  Polyurethane foam can fill the void and prevent further water damage.