Ask the Builder: Settling soil causes bizarre porch problem

DEAR TIM: We have a problem with the front porch of our 44-year-old home. The outside corner of the poured concrete porch is curving up, lifting the post that’s supporting the roof. There’s no tree or roots that could be to blame. The brickwork under the porch that connects back to the house looks fine with no cracks. Gravity should be pulling the concrete down, not going up in the air. Every contractor we have talked with is clueless. What is going on? How did this happen? How could it have been prevented, and how do we repair it? – Sami B., Toledo, Ohio

DEAR SAMI: I’m sorry you’re having this problem, and it’s a shame you can’t locate a knowledgeable contractor to tell you what’s going on. I was able to determine the cause of your problem in seconds after looking at the excellent photo you provided.

You’re so right about gravity’s pull; that’s exactly what’s in play here. What you didn’t take into account is what you discovered in high school physics class about a simple first-class lever. (A good example is a playground see-saw.)

Your front porch slab probably has some reinforcing steel bars in it, and this monolithic slab is acting as a giant lever. The low brick wall under the slab is the fulcrum. Gravity is causing the slab to sink at the rear of the porch and move up at the front under the roof-support post.

I was able to tell all of this by looking at the photo. You can see clearly that the back of the slab, where it touches the front wall of the house, has dropped. The edge of the slab is no longer parallel to the brick mortar joint, and you can see small traces of concrete from the slab that got onto the brick all those years ago when the house was built.

In a nutshell, the ground under the concrete porch has settled. My guess is that the builder just took some of the soil that was dug out of the ground to install the foundation and used it to fill under the porch. The trouble with this practice is that soil gets fluffed up when you dig it. That introduces all sorts of air into the soil, and its volume increases.

The soil should have been compacted before the slab was poured, although that’s not easy to do. There are special machines that can be used to do it, and on a small scale it’s labor intensive. It’s very common for substandard builders to cut corners like this.

There were other ways to prevent this problem. Self-compacting fill could have been used, but the builder has to buy this. Rounded pea gravel is a great example of this product. Concrete block piers could have been extended up from the house footer to support the slab. Poured concrete haunches could have been incorporated into the house foundation to support the slab.

You have several options for fixing the problem – providing you have a specialty contractor in your city that can do the job.

You could tear out the concrete porch and start over. This method guarantees that the porch slab will never drop a fraction of an inch, but it’s expensive. If you do this, you have to make sure the new slab will be held in place. You could dig down to the house foundation footer and extend 8-inch-diameter concrete or concrete block piers up to where the bottom of the new slab would be. If you do this every four feet on center, you’ll have created legs that the new slab will rest upon. This is not much different than how your dining room table stays floating in midair.

You could also carefully remove every other brick under the row of brick that would start at the top of the new slab. The concrete would flow into the void were each brick used to be. At this location you should also have a 5/8-inch steel bar that extends back into the void and out to the other side of the porch slab where it crosses the low brick wall. This steel rod needs to be surrounded with the new concrete; it prevents the new concrete slab from falling again under the influence of gravity.

You could also create a mat of reinforcing steel within the slab with the steel rods parallel with the house 2 feet on center as they march toward the low brick wall under the porch-support posts.

The concrete slab should be no less than 5 inches thick and be no less than 4,000-pounds-per-square-inch in strength. Be sure you put compacted fill on top of all the dirt that has settled under the new slab. Before adding the fill, water the dry dirt for a few days to encourage further settlement before adding the new fill.

If you don’t want to remove the old slab, you could find a company that can lift the existing slab back into its original position. This may be a cheaper option, but there’s no guarantee the slab will stay in place.

This work has been done for years in large industrial settings and at airports, where concrete slabs settle like yours for any number of reasons. These companies drill 2-inch-diameter holes into the slab back near the wall of your home. A slurry of Portland cement and fine sand is injected under significant pressure under the slab. As more and more slurry is pumped under the slab, as if by magic, the concrete slab starts to lift and float back into its original position. Once the slab is in place, the slurry under it hardens and all is well.

The only problem is that if the fill under the slurry continues to settle, then the slab starts to drop once again.

So you have to ask yourself a question (doing my best Clint Eastwood impersonation): Do you feel lucky? Well, do ya?

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