Soil Engineering: Wet Mounds

Why does water allow sand castles? Again, two words: Adhesion and cohesion. The addition of water adds two more forces to the mix (other than gravity and friction). Adhesion is the attraction of unlike materials (like adhesives, glue, tape, etc.) and involves the attraction of the water for the BBs, sand grains, and soil particles. Cohesion is the attraction of water to itself.
Wet BBs can be stacked because the water holds to the BBs by adhesion and to other water molecules by cohesion. These forces create a negative pressure, or pull, that helps the particles hold together.	However the stack cannot be very big, or the force of gravity overcomes the attractive forces of adhesion and cohesion.	We can build sand castles because the water is attracted to sand by adhesion and to other water molecules by cohesion. The sand grains add some friction, but the primary forces allowing the castle are adhesion and cohesion. Sand castles can be taller than BB stacks because sand grains are smaller, and therefore have more surface area, and more adhesion.
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Updated 06-23-2008. Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.