A.Washed sand, with particles that have been rounded, acts differently than "sharp" sand, when it is wet. It may be necessary to support the footing on piles to prevent movement with the extreme changes in soil moisture content at your site. This is an instance when the services of a soils engineer are well worth their cost. Without knowing detailed information about the soil, including the shape of the sand particles and the type of subsoil, there can be no definitive answer.
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