An oyster shell slab foundation is a slab foundation that was made with oyster shell aggregate in the concrete. For far too many years, oyster shell was dredged from Galveston Bay and used for many things: concrete, chicken and cattle feed, aluminum, textiles, fertilizer, soap and many other things. Five thousand cubic yards of Galveston Bay oyster shell was used in the construction of the Astrodome and the adjacent parking area.
Concrete for residential foundations and flatwork – driveways, patios, and sidewalks – was made with oyster shell aggregate for many years. Shell dredging in Galveston Bay continued until at least the 1960’s.
Where you find oyster shell foundations?
You can find oyster shell foundations in virtually every older neighborhood in and around Houston. They are most common in older homes built between 1950 and 1970. The towns adjacent to Galveston Bay are filled with oyster shell foundations. I have seen them in older homes in every area of Houston.
How can you identify an oyster shell foundation?
In many cases there is no obvious sign that signals that the foundation either is or is not an oyster shells flat. In other cases it will be quite obvious whether a foundation is an oyster shell foundation or not.
The first thing to do is to determine approximately when the house was constructed. If it was built before 1960 it is highly likely that it is an oyster shell slab If it was built after 1970 it is highly unlikely that it is an oyster shell slab.
Next look at the driveway and any sidewalks in the yard. You’re looking for an area with the top surface of the concrete has worn and you can actually see the aggregate. If what you’re saying or pieces of oyster shell then it is more likely than not that the slab may be in oyster shell slab. If you see small pieces of gravel there’s a good chance that the foundation is not an oyster shell slab.
Now walk around the house looking carefully at the exposed portion of the slab. In some cases you will see aggregate that is open to view in the concrete. If what you see is oyster shell yellow oyster shell slab; if what you see is gravel is not an oyster shell slab. It is possible that the aggregate is a mix of oyster shell and gravel but in my experience this is very rare.
Are oyster shell foundations a problem?
Although oyster shell slab or weaker than hard rock aggregate slabs, most of them perform about as well as a hard rock aggregate slab.
However, they do not perform nearly as well in many cases as a hard rock aggregate slab if foundation repair is done. Many of these slabs are simply too weak to withstand the loading that would be placed on them during a re-leveling process.
I have personally seen situations where an oyster shell slab was severely damaged when it was lifted during a foundation repair process.
Can oyster shell foundations be repaired?
In many cases, the answer is no. This is especially true, in my experience, when the slab was constructed around 1955 or earlier.
If the slab appears to be in good condition and measurements are consistent with the slab that has not experienced excessive or high bending moments. But I always advise homeowners that, if a slab is an oyster shell slab, the risk involved in trying to lift it goes up.