This project explains the cement pressure grouting process to stabilize the soil and fill voids after jacking up the porch foundation. My front porch foundation had settled several inches due to insufficient backfill compaction when the house was built. Raising the foundation left a large void under the foundation, porch slab and sidewalk that needed to be filled. This project is continued from Porch Foundation Repair – Jacking with Helical Piers.
Foundation Gap after Jacking
The gap under the sidewalk is easy to see in the following photo. There are similar voids under the porch slab and foundation. The helical foundation piers alone will support the porch foundation but I preferred to have all voids filled for load bearing contact between the foundation and the subsoil:
Soil Stabilization and Void Filling
There are two primary methods for soil stabilization and void filling:
Each method has it’s pros & cons and both are expensive.
- Foam is a more recent innovation, uses small injection holes, adds very little weight and is less likely to further compact lose soils. If foam is pressure injected through pipes it will permeate and stabilize the soil.
- Beware of foam applied with a spray gun because it will only fill voids under the foundation where the nozzle can reach.
- Cement-based pressure grouting is a long-standing technique. A cement slurry about the consistency of pancake batter is injected through pipes. The cement cures and hardens the soil. It’s possible the added weight could cause loose soil to compact and settle further if the grout is not injected deeply enough.
My foundation repair company, Atlas Piers, recommend cement-based pressure grouting. I liked the idea of cement grout because it hardens and discourages chipmunks from burrowing under the foundation and porch slab. I know from experience the little critters will burrow under a concrete slab creating a maze of tunnels.
Porch Foundation Repair – Cement Pressure Grouting
I contracted with Southeastern Pressure Grouting, Inc. based on a referral by the foundation repair company. I called Southeastern and explained what was needed. Given the straightforward nature of the job, I e-mailed several photos of the porch along with the dimensions (9 ft by 18 ft). Southeastern estimated 50 feet cubic feet but it may vary depending on the actual size of the voids and soil conditions.
I was also concerned that pressure grouting might damage the 3/4 inch polyethylene water line that runs under the porch. Southeastern explained the grout wouldn’t damage it because it flows around and encases the line.
Pressure Grouting Cost and Warranty
The cost for pressure grouting consists of a fixed setup fee plus a material charge per cubic foot of grout. One cubic foot of grout equals one 70 lb bag of cement. My job used 60 cubic feet grout, which is 2.2 cubic yards… that’s the size of 3 ft x 4 ft x 5 ft box! The total charge was $2,322.
Southeastern provides a transferable “Lifetime of the Structure” warranty and will re-support any area at no cost for labor, material and equipment.
Cement-based Pressure Grouting
Pressure grouting was scheduled the day after the foundation repair. A large panel truck arrived towing a grout mixer/pump. The three man crew from Southeastern were really nice, explained the grouting process, examined the porch to decide where to place the grout injection pipes and did a superior job. The fellow manning the injection pipe has been on the job for 9 years and worked on large jobs at Atlanta Hartsfield International Airport.
Drill Holes Injection Pipes
A total of seven holes were made using a hammer drill using an 18 inch long by 3/4 inch diameter drill bit: 6 holes in the porch and 1 hole in the sidewalk. The holes were drilled between flagstones which I’ll fill with buff colored cement to match the porch.
I had no idea how thick the concrete slab is under the flagstones. The porch slab thickness ranged from about 4 inches to almost 10 inches thick. The home builder didn’t cut corners on concrete! Notice the 18 inch long drill bit still hasn’t broken through in this photo; it punched through moments after the photo was taken:
Four holes drilled at this end of the porch. The foundation settled the most at the corner where the workman is standing, therefore the largest voids will be at this end:
The holes were checked with a fiberglass probe rod to verify soil is at the bottom. Fiberglass is important because it doesn’t conduct electricity on the odd chance an electrical cable is touched. (The gas, electric, TV and phone utilities were previously marked.)
Void under Porch Slab
The gap between porch slab and subsoil is quite large, note the shadow from the flashlight indicating the size of the void. The porch slab might crack and collapse if the void isn’t filled to support the slab:
An injection hole is also drilled in the sidewalk because it was raised with the porch foundation:
Install Pressure Grout Injection Pipes
The workman used a creative way to drive the injection pipes into the subsoil:
- Place a short round-head bolt into the end of the pipe.
The bolt prevents clogging the pipe with dirt.
- Insert the pipe into the hole.
- Hammer the pipe into the subsoil with a heavy steel ram.
- Raise the pipe to let the bolt fall out.
Four of the six injection pipes in the porch are shown. The concrete slab cracked in front of the patio furniture stacked at the far end:
Rags are stuffed in the holes to seal the grout injection pipes:
Cement Injection Pressure Grouting
A cement slurry with chemical additives is pumped pressure into the subsoil. The workman monitors the pressure gauge to determine when the grout has filled the soil and voids, then raises the pipe a foot or so and continues grouting until the pressure builds. This process repeats until the grout completely filled the voids.
I placed my hand on the grout hose and felt the grout pumping. When the workman carefully withdrew the pipe not a drop spilled on the porch. I remarked that’s kinda amazing; the workman replied “Not how I do it!”.
I was concerned the pressure might be so great that it would blow out the OSB plywood panel against the backfill dirt above the basement wall. Pressure grouting can involve a 100 psi or more but only 5 psi is needed for this job and the plywood panel was fine.
Eventually the grout started flowing between the cracked joint between the porch and house wall. This is a good sign!
Sidewalk Pressure Grouting
This section of the sidewalk lifted a couple of inches with the porch foundation and required void filling. Notice the injection pipe is set deep for soil stabilization:
As the pressure builds, the pipe is raised to fill the void under the sidewalk. Grout began flowing along the edges of the sidewalk when the void was full. Some gray colored grout can be seen in the shadow under the bush:
Injection Hole Sealing
The injection holes are sealed with fast curing hydraulic cement. I asked the workman to leave a pocket so I can cap the hole with buff colored cement to match the porch:
Hydraulic cement in the sidewalk hole. The cement will weather to match the sidewalk:
Now all that’s left to do is reinstall the porch rail, reset some loose flagstones and fill the grout injection holes with buff cement.
Reinstall the Porch Rail
I removed the porch rail before the foundation was raised to free the porch post. Fortunately I build with wood screws making it simple to remove the panel. Having raised the foundation a couple of inches, the corner post had to be re-plumbed (set vertical) which moved it towards the house. The result is the rail is too long and also needs to be mounted lower against the post. It was simple job to measure, saw, level and remount the rail with screws. Notice the two grout injection holes in the porch:
Thanks for reading,
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