Concrete Slab Sinking: Uneven Heave, Voids, and Polyjacking Decisions

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   From a safe walking position, photograph the slab and mark the visible low edge, high or heaved area, cracks, open joints, trip edges, and any place that rocks or deflects under ordinary foot traffic. Use a straightedge and a level only on stable, accessible areas; do not climb onto unstable sections or place a vehicle on the slab to test it.

   Expected result: Record whether the depression is localized at an edge or corner, whether a neighboring section is raised, and whether cracks are tight, widening, offset, or accompanied by rocking.

   Next step: Save the photos and measurements for a qualified concrete or foundation assessment. Keep heavy loads off the affected area when it is visibly unsupported or unstable.

   Stop if: Stop immediately if the slab shifts, breaks, exposes a deep void, or cannot be crossed safely; keep people and vehicles away and arrange professional assessment.

   Why this matters

   Why: A localized low area with an adjacent high area suggests differential movement but does not identify whether the cause is an empty subbase, soil movement, drainage, or structural slab damage. Widening cracks, a rocking panel, or a sharp offset means the slab should not be treated as a simple cosmetic level problem.

2. Check 2

   Walk the perimeter without entering a gap or excavation and inspect visible ground, joints, edges, downspout discharge, grading, hose or irrigation areas, and signs of soil loss such as muddy paths, washout channels, damp seams, or an exposed underside.

   Expected result: Note whether water is directed toward the slab, whether soil is missing at the low edge, and whether the gap changes after rain or irrigation.

   Next step: Document the suspected water path and correct only obvious, low-risk surface drainage such as redirecting a loose downspout extension without digging near utilities. Ask the contractor to investigate concealed drainage or plumbing before recommending injection.

   Stop if: Do not enter a void, undermine the edge, dig, probe beneath the slab, or handle a leaking utility; stop and call a qualified professional if water is actively removing soil or a support edge is exposed.

   Why this matters

   Why: A washout or water path supports a loss-of-support hypothesis and means lifting alone could leave the cause active. No visible drainage clue does not rule out a buried leak or a subsurface void.

3. Check 3

   From the surface, inspect for intersecting cracks, shattered or spalled concrete, displaced joints, exposed reinforcement, broken corners, heaved sections, and any nearby wall, step, door, or utility feature that moved with the slab.

   Expected result: Separate a mostly intact slab with a localized low spot from a slab with extensive cracking, broken edges, differential movement, or movement connected to a building element.

   Next step: Give the contractor the complete photo set and ask whether a structural or geotechnical assessment is needed before any lift. Do not select a method based only on the advertised material.

   Stop if: Keep off the slab and escalate promptly if there is a large open crack, a broken or rocking panel, exposed undermined edge, sudden change, or movement of an adjoining wall or footing.

   Why this matters

   Why: Localized loss of support can be a candidate for professional stabilization or lifting, but extensive structural distress or movement tied to a foundation may require a different repair or replacement. Injection can fill a void without restoring damaged concrete or fixing a moving foundation.

4. Check 4

   Without intentionally loading the damaged area, note whether ordinary walking, a door or gate crossing, rainfall, or normal non-heavy use causes visible flexing, rocking, new sounds, or water movement at the edge. Do not use a car, jack, pry bar, or stacked weight as a test.

   Expected result: Record whether movement is repeatable with normal low-risk activity, only appears after water, or is absent while the surface remains uneven.

   Next step: Include the circumstances and video or photographs in the service request. Ask for a support evaluation rather than requesting a predetermined polyjacking dose or lift.

   Stop if: Stop all loading if movement increases, a crack opens, material falls away, or the slab rocks; keep the area clear until assessed.

   Why this matters

   Why: Repeatable deflection is consistent with poor support but cannot determine the void size or safe injection pressure. Movement after water points toward an active drainage or washout issue; a static but badly cracked slab may be damaged even without visible flex.

5. Check 5

   Before authorizing work, ask the qualified contractor to identify the suspected cause, explain how support or voids will be evaluated, and state why stabilization, controlled lifting, repair, or replacement fits the observed condition. Ask how drainage, soil movement, cracks, adjoining structures, and post-repair movement will be handled.

   Expected result: A sound proposal connects the observed distress to a defined investigation and includes a method-specific safety and verification plan. A proposal that promises to fill or lift based only on surface height, with no cause or damage assessment, leaves the main failure mode unresolved.

   Next step: Proceed only after the contractor explains the limits of the proposed repair and any need for drainage, structural, geotechnical, or replacement work. Obtain a second qualified assessment when the slab is connected to a building or has severe damage.

   Stop if: Do not drill, inject, mix grout or foam, operate a pump, jack the slab, or authorize work that lacks a cause assessment and a controlled stop plan.

   Why this matters

   Why: Polyjacking and mudjacking are professional construction processes involving drilled access points, pressure-controlled injection, and monitoring; the homeowner cannot safely choose the hole pattern, material quantity, or lift rate from a visible gap. FHWA also distinguishes filling a void from raising a slab.