There are thousands, perhaps hundreds of thousands, of concrete slab-on-ground foundations in and around Houston that have oyster shell for aggregate. Oyster shell is very weak compared to hard rock aggregate. In spite of that, most of these oyster shell slabs have performed adequately and continue to do so.
The back story
After WWII the demand for single-family homes was off the charts. Once Ready-mix concrete was available, concrete slab-on-ground quickly blew the competition away. The demand for concrete translated into a high demand for sand, cement, and aggregate.
In Galveston Bay and its sister bays, there were hundreds, if not thousands, of oyster reefs. Many were dead. To people in the concrete business, those dead reefs were mountains of oyster shell that could be used in place of hard rock aggregate.
The use of oyster shell was controversial from the beginning. Sports fishers were disorganized but mostly opposed. In the end, the State of Texas issued permits for dredging oyster to several companies. The dredging continued until at least around 1970 and perhaps to near 1980. Even today you will notice Ready Mix concrete trucks that are owned by Houston Shell and Concrete.
If you are interested in the history of oyster shell dredging in Galveston Bay, this is an interesting article.
Where are oyster shell slabs?
Based on my experience, most of these foundations are on the east end of Harris and Galveston counties. I have seen them as far west as the Myerland area.
How do you identify an oyster shell slab?
You may see oyster shell debris in exposed honeycombing in the perimeter grade beam. If you see remnants of oyster shells, it is what I call an oyster shell slab. If you see rock aggregate, it’s not an oyster shell slab. If you cannot find any honeycombing, then it’s an open question.
I suppose the aggregate might be a mix of hard rock and oyster shell, but I have never seen that. So far as I know, it is always one or the other.
What is the most important thing to know about oyster shell slabs?
Remember that oyster shell is not nearly as strong hard rock aggregate. In many circumstances, it can perform as well as a hard rock aggregate slab. Where is at a distinct disadvantage, is how it may react to foundation repair.
When an oyster shell slab is lifted, it is at a higher risk of structural damage to the foundation as compared to a hard rock aggregate slab. How much risk is hard to say. It would depend on the number, size, and condition of the rebar.