Compaction grouting repairs a sinkhole by pumping a very stiff soil-cement grout into the ground under pressure through steel pipes. The grout stays in a mass at the pipe tip and pushes the loose soil around it tighter, which closes voids and firms up the ground over the limestone. An engineer designs it; a pump places it.
That last sentence is where we come in, and it’s worth being clear about from the start. We pump. Your remediation contractor or engineer designs the repair, specifies the fill, and supervises placement. Our sinkhole grouting service is the placement part of that job, run to someone else’s plan.
What Makes Compaction Grout Different?
Compaction grouting is a ground improvement method that uses a low mobility grout, sometimes called LMG. The Federal Highway Administration’s ground modification manual describes it as a grout injected at 1 inch of slump or less, usually a soil-cement mix with enough silt to make it plastic and enough sand to give it internal friction. Slump is the measure of how much a sample sags when the test cone is lifted, so 1 inch is very stiff. Ordinary concrete flows far more than that.
The stiffness is the whole point. According to the FHWA manual, the grout “does not enter soil pores but remains in a homogeneous mass.” It forms a bulb at the tip of the injection pipe. As more grout goes in, the bulb grows and squeezes the surrounding soil denser.
Compare that to the grout most people picture: a runny mix that soaks into gaps. That kind of grout (permeation grouting) fills pores without moving the soil. Compaction grout moves the soil on purpose.
Why Do Florida Sinkholes Need Grouting?
North Central Florida sits on karst limestone. The Florida Department of Environmental Protection explains that slightly acidic groundwater dissolves cavities in the limestone over many years. Sand and clay above those cavities can ravel down into them. When the cavity can no longer carry the weight above it, the ground drops.
The state recognizes three types:
- Solution sinkholes, where little sediment covers the limestone and the rock dissolves near the surface.
- Cover-subsidence sinkholes, where thick, permeable sand slowly ravels down and forms a gentle circular depression.
- Cover-collapse sinkholes, where a thicker clay layer bridges the void until it gives way suddenly.
A settling slab or a dip in the yard isn’t automatically a sinkhole. DEP notes that broken pipes, decomposing debris, and poorly compacted fill can all look like one, and a licensed professional geologist or engineer has to confirm it.
When it is a sinkhole, grouting goes after the mechanism. The FHWA manual’s Hillsborough County case history describes the typical Florida approach: grout pipes installed within and around the depression, and a sand, cement, and fly ash grout pumped into the soil above the sinkhole. The purpose is to stop water carrying soil down into the rock, compact the soil the sinkhole disturbed, and fill any open voids left behind.
How Does Compaction Grouting Work, Step by Step?
Every job is designed for its own site, but the sequence follows the same logic.
- Investigation and design. The engineer drills borings and may use ground penetrating radar or electrical resistivity to map loose zones and voids. From that data they set the hole layout, depths, grout mix, and the refusal criteria that tell the crew when to stop.
- Drilling and casing. A drill rig installs steel grout casing down to the planned depth, often into or just above the limestone.
- Grout supply. Grout is either batched on site or delivered by ready-mix trucks. Because the spec is so tight, the FHWA manual says each truckload should be slump tested and rejected if it’s out of spec.
- Staged injection, bottom up. Grout goes in at the bottom of the casing first. The pipe is then pulled up in short stages, typically 1 to 3 feet, with another round of grout at each stage. Perimeter holes are usually grouted before interior holes, working inward.
- Refusal. Each stage stops when it hits a limit the engineer set: a maximum pressure at the pipe header, a set volume, or measurable movement of the ground surface or a nearby structure.
- Verification. Additional holes are drilled between the first ones in a split-spaced pattern. If those take much less grout, the ground between them has tightened up.
Pumping rate gets more attention than people expect. The FHWA manual warns that pumping too fast, especially in fine-grained soils, can build up water pressure in the ground and damage adjacent structures. Slow and controlled is how it’s supposed to look.
Where Does the Concrete Pump Fit In?
Stiff grout has to be pushed, and the FHWA manual names concrete pumps (modified as needed) as the pump type used for low mobility grouting. That’s the piece of the job we handle.
Here’s what our side of a grouting job looks like:
- Access review. You or your remediation contractor walks us through the site. We look at pump position, the hose run to the casings, fill volume, and timing, alongside the project engineer.
- Fill supply. Justin coordinates directly with the concrete plant before each pour, unless your team is managing grout or fill supply.
- Pumping to spec. We set up and pump the fill per the engineer’s spec, at the pace and in the stages the grouting crew calls. We work to the placement plan, not in place of it.
- Washout. We wash out on site according to the contractor’s rules, never into storm drains or unprotected ground.
The equipment is our Schwing line pump fleet, which suits grout and cementitious fill. A line pump is trailer-mounted and runs hose along the ground, so it fits yards and side lots where the drill rig and casings already take up space. Stiff material is harder on any pump and any hose, and a clog in the line stops the injection mid-stage. So the mix, the slump, and the hose route get settled with us before the first truck shows up.
We’ve pumped concrete across North Central Florida since 2003, from Lake Butler out to karst country around Gainesville and Ocala. If your grouting contractor is lining up a pump, have them call us early so access and grout supply get sorted before drilling wraps up.
What Should You Ask Before Grouting Starts?
Homeowners usually meet the grouting crew after the engineering is done. It’s still your property, and these questions come straight from what the job depends on:
- Who is the engineer of record, and who is on site supervising placement?
- What are the refusal criteria? Pressure, volume, and allowable movement should all be written down.
- How will movement be monitored? Ask how the crew will know if the slab, walls, or ground start to lift.
- Who supplies the grout, and who tests it? Batched on site or ready-mix, someone should be checking slump.
- Where do the pump, trucks, and washout go? A loaded mixer truck can weigh 60,000 pounds, which matters on a lawn, a septic field, or an old driveway.
Clear answers to those tell you the job has been planned properly.
When Is Compaction Grouting the Wrong Fix?
Compaction grouting is common for Florida sinkholes, but it isn’t right for every site.
Soil type matters. The FHWA manual says densification works best in free-draining soils like sands, gravels, and coarser silts. In fine-grained, saturated soils the water can’t get out of the way fast enough, so the grout may just shove soil aside without tightening it.
Some sites need deeper support. In a paper presented at the Thirteenth Multidisciplinary Conference on Sinkholes, engineers Edward Zisman and Daniel Clarey note that compaction grouting is widely used and usually less costly and less intrusive than other methods. They also document cases where improper use made marginal ground worse, and argue that some subsurface conditions call for deep foundation support instead.
Other methods exist. DEP lists grout, foam, and engineered systems of plugs, pins, and porous concrete among the repairs licensed engineers use. Picking between them is the engineer’s call, based on what the borings show.
Frequently Asked Questions
What is compaction grouting?
Compaction grouting is a ground improvement method that injects a very stiff, low mobility grout (1 inch of slump or less) into loose soil through steel pipes. The grout stays in a mass at the pipe tip and displaces the soil around it, densifying loose ground, filling voids, and in some cases lifting settled structures.
What is the difference between jet grouting and compaction grouting?
Compaction grouting pushes stiff grout into the ground and squeezes the soil around it tighter without mixing with it. Jet grouting uses high-velocity jets of grout, sometimes with air and water, to cut and mix the soil in place into columns of soil and cement. One compacts the existing soil; the other partly replaces it.
Is sinkhole grouting the same as pouring concrete into a sinkhole?
No. Sinkhole grouting places cementitious grout or compaction grout under pressure, through pipes, in stages set by an engineer. Pouring concrete into an open hole doesn’t compact the loose soil below or seal the path into the limestone, so the ground can keep moving.
How much does sinkhole grouting cost in Florida?
It depends on the engineering, the number and depth of grout holes, and how much grout the ground takes, which nobody knows exactly until injection starts. The engineer or remediation contractor prices the repair. Our part, the pumping, is billed hourly with a minimum, with no fuel surcharges and no primer fees.
Is my property in a Florida sinkhole zone?
Much of North Central Florida sits on karst limestone, but that alone doesn’t tell you about your lot. The Florida Geological Survey keeps records of reported subsidence incidents, and a licensed professional geologist or engineer can evaluate your specific site.
Does Croft Concrete Pumping design sinkhole repairs?
No. We pump. Your remediation contractor or engineer designs the repair, specifies the fill, and supervises placement. We deliver the grout or cementitious fill on site per their plan.
Booking a Pump for a Grouting Job
Owner Justin Croft runs the company and the crew, so he’s the one you’ll talk to about access, and the one responsible for the truck showing up. We’re fully insured, with general liability and workers’ compensation on every project and certificates of insurance available on request.
Call (386) 623-1782, email office@croftconcrete.com, or request an estimate with your contractor’s placement plan in hand.