>or more typically until they hit the first layer of sand, which is stable.
I think this is the first time I have ever seen someone claim that a layer of sand is stable. But hey, NOLA is one of the craziest places, so sure, sand is stable there.
Sand underground with overburden pressure is quite stable, to the point that piles driven to it are treated as bearing piles (supported primarily on the end/tip), not friction piles (supported on sides by friction).
It's non-cohesive, so doesn't shrink or swell, and once it's compacted/consolidated, it's fixed volume and quite stable.
I think this is the first time I have ever seen someone claim that a layer of sand is stable. But hey, NOLA is one of the craziest places, so sure, sand is stable there.