Building New Slab Over Foundation Rubble

Kevin writes: In 2013 we decided to demolish our existing home (build in 1941) and build a new home in the same place. Because our neighborhood is built on 85-foot-deep clay over a glacial lake, the old cinder block foundation had failed in the basement and we had serious moisture problems as a result. To prevent a repeat, we chose to not have a basement in the new home. Our contractor initially told us that they would be digging out the old foundation and placing new footings under where the old footings were then backfilling the void with incompressible gravel.

However, what they ended up doing was knocking down the old cinderblock walls and placing the new footings literally on top of that material.  We observed just prior to spreading gravel on top of the old material, we observed whole cinder blocks and sections of wall filling the hole all the way to about 2 feet from grade. Thus I would estimate somewhere in the range of 4 feet deep cinder block fill. The new footings sit directly on top of the old remains. At the time that this was happening I tried to stop the contractor but he told me that his engineer told him that this was a better way to proceed. I asked to see a report on it and never received and they simply proceeded with the build.

My question is this: Does building a new foundation on top of the remains of an old foundation, where the old materials have not been crushed down to small pieces, introduce a risk for subsidence?

Steve Bliss, of, responds: I would have the same concerns as you about building on top of knocked-down block walls and other foundation rubble. It’s certainly not the ideal way to build and could lead to future problems.

A foundation relies on a proper subgrade that is well-drained and uniform in strength (bearing capacity). The subgrade should be free of voids, organic matter, and pockets of uncompacted fill. If there is excessive or uneven settlement, then there will be damage to the foundation and structure it supports.

To properly design a foundation, you need to know the bearing capacity of the soil. Weaker soils require larger footings and more steel reinforcement. To some extent, steel reinforcement in the footings, stem wall, and slab can bridge weak areas of soil, but this should be a planned and engineered solution. With unknown subgrade conditions, you are leaving a lot to chance.

It sounds like you have a lot of unknowns underground. Did you get any photos of the knocked-down walls and loose or broken blocks under your house? It’s possible that they were arranged in such a way and carefully filled with suitable fill (coarse gravel, crushed concrete, etc) that they make an adequate base. It’s also possible that they create voids and uneven support that will lead to uneven settling and foundation damage.

Building on this sort of base is not conventional practice – in which case an engineer should be consulted. In many areas, the building department would not approve this type of foundation without an engineer’s stamp. A smart builder would want the same assurance and liability protection.

One consideration is that true “cinder block” is much weaker than modern concrete block. Cinder blocks have not been widely manufactured for 50 years or more, but may be what you have. The more important factor, I would imagine is the configuration of the debris, and whether any effort was made to arrange it such that it was stable and free of voids. Many engineers would be reluctant to make this sort of judgement call.

Using two feet or more of well-compacted fill under the new slab is a good thing. While it is best to place footings on undisturbed native soils that have been well compacted by nature over the eons, footings can rest on appropriate fill. This usually means well-compacted crushed stone or course gravel.  Compaction requires that the material be placed in “lifts” of 6 in. to one foot, and then tamped with a plate vibrator or other compacting tool. Some types of gravel (e.g.,  ½ – ¾ inch smooth stone)  are considered self-compacting, but I would want an engineer’s opinion of this, not just the builder. The cost of failure is just too high to take any chances here.

It’s possible that the contractor spoke with an engineer and got a verbal OK, but it’s certainly reasonable for you to request (demand, if necessary) that he go back to the engineer to get this opinion in writing.

Another important issue is the difference between the contractor’s initial plan and the “as built” foundation. Did he commit in a written (or verbal) contract to remove the material and replace it with compacted fill? Which plan was approved for the building permit? Did you verbally or in writing agree to the revised plan? If not, you might want to file a complaint with the local or state contractor’s licensing board.

If there is no engineering plan or approval by the building inspector, you need to figure out how best to protect yourself in the event that there is a failure, which might not occur for several years. And, of course, you need to maintain a working relationship with this contractor, assuming you plan to continue working with him.

One option is to hire your own engineer to review the work done so far and provide his own independent analysis. Based on his opinion, you may need to take precautionary measures or redo part of the work. I think this a perfectly reasonable thing to do and you can blame any need for remedial work on the engineer.

Some other options to consider to protect yourself:

Ask the contractor for written description of the foundation and subgrade plan and a long-term performance guarantee of the foundation.

Write a letter stating the work done was not consistent with the original plan and without your consent.

Whatever happens, document as much as you can in photos, any written correspondence, and dated notes of conversations in case this ever ends up in a dispute process. I can’t say what the risk is of foundation failure down the road – there are too many unknowns and too many variables – but there is certainly some risk here.

See also
Building on Fill        Foundation Damage from Buried Debris       Building on Poor Soils



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