Being a loose,sandy site,describe to ensure that foundation trenches arent filled with sand when wind blows?

Answer 1

There's always going to be a bit of back-filling when the wind blows sand around. You could try to reduce it by covering the trenches. Grab a roll of vapour barrier from home depot and roll it out over the trench. Hold it in place with bricks or lengths of 2X4's or something a bit heavy to hold the plastic down.
Good Luck!

Answer 2

Planting vegetation is the best way to stop loose soil / sand from blowing away. Check out the beaches for tips!