The sand sculpture festival at Blankenberge
Photograph: PETER DECONINCK/AFP/Getty Images
By compacting, the base are of the sandcastle until it is hard, flat and level, you are creating a strong base for your beach fortress. The same goes for a retaining wall; you must have a solid base that is compacted well to hold the wall. Without a good foundation your wall can sink, sag, tumble, or fall.
Reinforcement is just as important. How many times have you built a sandcastle only to have it topple over or shear in half, thwarting your chances of winning the title of Sand Castle Master? In retaining walls we use a synthetic mesh material called geogrid to reinforce and stabilize the soil and add strength to the wall. By layering this within the wall and compacting the soil around it, a very strong structure is created.