A concrete slab almost never cracks because the concrete failed — it cracks because the soil underneath moved and left a void with nothing supporting the weight above it. Grout injection is the repair trade’s answer to that problem: pumping a flowable cement-based slurry through small drilled holes to fill empty space, re-establish bearing, and in many cases hydraulically lift a sunken slab back to grade. Contractors have been doing versions of this since the 1930s, and it remains one of the most cost-effective structural repairs in residential construction. Here is how the process actually works, what it costs, and where it beats — or loses to — polyurethane foam.
What Grout Injection Actually Is
The grout in question has nothing to do with tile grout. Injection grout is a pumpable mixture of Portland cement, water, and fillers such as sand, fly ash, or fine limestone, mixed to a consistency somewhere between pancake batter and thick milkshake depending on the job. A crew drills holes through the slab — typically 1 to 2 inches in diameter on a grid of 3 to 8 feet — then pumps the slurry through a hose and nozzle at pressures usually under 100 psi for residential work. The material flows into voids, displaces water and loose soil, and hardens into a solid mass that can exceed 1,000 psi compressive strength within days.
When the goal is lifting rather than just filling, the operator keeps pumping after the void is full. Because the slurry cannot compress, continued pressure pushes upward against the slab, raising it a fraction of an inch at a time while the crew monitors elevations with a laser or zip level. This is the technique most people know as mudjacking or slabjacking.
Where It Gets Used
Around a house, grout injection shows up in more places than most homeowners realize. Sunken driveway, sidewalk, and patio slabs are the classic candidates — a settled approach slab at the garage that traps water against the foundation is the single most common call. Interior floor slabs in basements and garages get lifted the same way. Beyond flat work, contractors inject grout to fill abandoned pipes and old septic tanks, pack the washed-out zone behind retaining walls and pool shells, and stabilize the soil under footings. On larger jobs, a stiffer variant called compaction grouting pushes a low-slump grout into loose soil to densify it, but that is engineered work with a drill rig, not a Saturday service call.
One important boundary: this article covers cement grout under and behind concrete. Repairing an actively leaking crack in a poured foundation wall is a different job that uses injected resins under high pressure, chosen for their ability to penetrate hairline openings and bond the crack faces. Grout injection is a volume-and-bearing tool, not a crack-sealing tool.
The Process, Step by Step
A competent slab-lifting crew follows the same basic sequence on nearly every job. First they map the settlement, checking elevations across the slab to find the low corner and estimate void depth — sounding the concrete with a hammer reveals hollow zones by their drum-like ring. Next they drill the injection holes, placing them near the lowest points and spacing additional holes so the grout can travel between them. Pumping starts at the deepest settlement. The operator fills until refusal or lift begins, then moves to the next hole, working the slab up evenly to avoid cracking it with a one-sided push. A slab should come up level as a plate, not hinge upward at one corner.
Once elevations check out, the crew patches the holes with non-shrink cement. The patches stay visible — that is the honest cosmetic cost of the method — though they fade with weathering. Most residential jobs finish in two to four hours, and the slab can take foot traffic immediately. Keep vehicles off a lifted driveway for 24 to 48 hours while the grout cures to working strength.
What Grout Injection Costs
Mudjacking is priced by the job, but the drivers are square footage, lift height, and access. National averages run $3 to $8 per square foot of slab, with typical single-slab jobs landing between $500 and $1,500. A two-car driveway with several settled panels commonly runs $1,000 to $2,500. Compare that with tear-out and replacement: new concrete flatwork runs $8 to $15 per square foot plus demolition and hauling, so injection often costs 50 to 70 percent less and is done in hours instead of a week of forming and curing.
Most contractors carry a minimum service charge of $300 to $600 regardless of job size, which makes bundling worthwhile — if the crew is already on site for the driveway, adding a settled sidewalk panel might cost $150 instead of a standalone $500. Warranties of 1 to 5 years against re-settlement are standard; treat anything longer with skepticism unless the contractor also addressed the drainage problem that caused the void.
Cement Grout vs. Polyurethane Foam
The main competitor to traditional grout injection is high-density polyurethane foam lifting, marketed under names like PolyLevel and often called polyjacking. Foam wins on hole size (5/8 inch versus 1 to 2 inches), weight (2 to 4 pounds per cubic foot versus roughly 100 for cement slurry), and cure time — foam reaches 90 percent strength in about 15 minutes. The weight difference matters on marginal soils: a heavy cement fill adds load to ground that already proved it can settle.
Cement grout wins on price and mass. Foam jobs typically cost 30 to 50 percent more for the same lift, and cement’s density makes it the better choice for filling very large voids economically — filling an abandoned 1,000-gallon tank with foam would be absurdly expensive, while a flowable cement fill handles it for a few hundred dollars in material. Grout is also inert, mineral-based, and unaffected by groundwater once cured. For a typical settled driveway, either method works; get quotes for both and weigh the price gap against foam’s smaller holes and lighter load.
When Injection Is the Wrong Answer
Grout injection restores support; it does not repair broken concrete. A slab shattered into small pieces, one with severe spalling or surface scaling, or one that has settled more than about 2 inches with wide cracks is usually a replacement candidate — lifting badly fractured concrete tends to open the cracks further. Injection also cannot fix the cause of settlement. If a downspout is dumping 500 gallons of roof water beside the slab every storm, the soil will wash out again and take your repair with it. Fix gutters, extend downspouts at least 4 feet from the slab, and correct grading before or alongside any lifting work.
Expansive clay soils deserve special caution. Slabs on clay heave upward in wet seasons and drop in droughts; pumping a heaved slab full of grout at the wrong time of year locks in the wrong elevation. In clay-heavy regions like Texas and Colorado, ask the contractor how they account for seasonal movement before signing anything.
Hiring It Out vs. DIY
This is professional work. The pump, mixer, and drilling equipment rent poorly, the material math is unforgiving, and over-pressurizing can crack the slab you are trying to save — or lift it past grade, which is far harder to undo. Bagged “slab lift” products sold to homeowners exist, but without elevation monitoring and pump control they amount to guesswork under pressure. What a homeowner can do is verify the diagnosis: probe along the slab edge with a rod to confirm voids, photograph elevations against a string line, and get two or three bids that specify hole count, material type, and lift targets in writing. A good bid reads like a plan. A vague one reads like a change order waiting to happen. Handled by the right crew, grout injection turns a sinking slab into a stable one for a third of the cost of replacement, usually before lunch.
