A Sandbag is a sack that has been filled with sand or dirt. Gamewise at Version 34.28, there is no code differences between a gravel bag and a Sandbag.
A heavy but very sturdy bag of sand that can withstand gunshots. Stacking three of these bags together will create a protective, bulletproof, sandbag wall that can be hopped over by holding E or Climbing Through with the right-click menu.
By having a shovel equipped, Empty Sand bags can be used to pick up and spill dirt or sand on the ground to make farms on otherwise unfarmable tiles. Filling a sack with sand will transform it back to a normal sandbag which can be used as a barricade.
Usually found in wooden storage crates, which are mostly found in warehouses.