noun
on sand; built on sand
Often used in the phrase 砂上の楼閣 (a castle built on sand) to describe something unstable or without a solid foundation. Can also be used literally for things placed on sand.
その計画は砂上の楼閣にすぎない。
That plan is nothing but a castle built on sand.
砂上に足跡が残っていた。
Footprints were left on the sand.