Glacial Soil and Foundations in the South Puget Sound

Washington DNR describes Vashon till as a gray, highly compact mixture of clay, silt, sand, gravel and boulders deposited directly by glacier ice. Above it, looser outwash sand and gravel drain quickly, while the till itself lets water through slowly.

Perched water. Where loose soil sits on dense till, winter rain can collect on top of the till and move sideways, sometimes right into a crawl space or against a basement wall. A curtain drain intercepts it.

Piers. Steel piers often stop on dense glacial soil. Ask the provider what they expect under your house and how refusal depth affects the price.