Explain why moist sand is typically stronger than either dry sand or saturated sand.

There are three main types of sand based on their moisture content: Dry sand, Moist sand, and Saturated sand. We will briefly describe each type: 1. Dry sand: It is sand with little to no water content in it. 2. Moist sand: This type of sand has some water content in it, making it damp. 3. Saturated sand: In saturated sand, the water content is so high that it occupies all the voids within the sand. Now, let's analyze the properties and forces in each sand type to understand their strength.

To understand the strength of moist sand, we need to look at the role of water within the sand. Water in the sand creates a meniscus - a curved surface - between the sand particles due to its surface tension. This meniscus allows capillary forces to act between the sand particles, creating extra cohesion.

In dry sand, the sand particles are only held together by gravity, which makes them relatively weak. The forces acting between sand particles are low, making it more susceptible to shearing and displacement. On the other hand, saturated sand has too much water, which acts as a lubricant between the sand particles. This lubrication reduces the friction between them, making the sand lose its internal strength and become unstable. Finally, moist sand exhibits a balance between the two extremes mentioned above. It has enough water to create capillary forces between its particles, which enhances its strength.

Capillary action is the phenomenon where a liquid travels through porous spaces due to adhesion, cohesion, and surface tension. In the case of moist sand, it occurs because water is attracted to the surface of sand particles. The capillary action and surface tension in moist sand create additional forces between the sand particles, making it stronger. Surface tension in moist sand is what makes the sand stick together, increasing the sand's overall strength and resistance to shearing. In summary, moist sand has higher strength than dry or saturated sand due to a combination of factors that include the presence of water, capillary action, surface tension, and the right balance of water content allowing for additional cohesion between the sand particles.