We're not talking about a beach here. Sand underground with overburden pressure is quite stable, to the point that piles driven to it are treated as bearing pi

We're not talking about a beach here. 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.