Dry sand poured from a bucket readily collapses into a slope. Contacts and friction between its grains can support compression, but the pile has little ability to resist forces that pull the grains apart. A small amount of water changes that.
Rather than filling every pore evenly, water forms tiny capillary bridges where neighbouring grains meet. Curved water surfaces create a pressure difference, while surface tension acts along each interface; together they draw the grains towards one another. When many bridges form a network, the loose grains gain cohesion and can resist some tension and shear. An upturned bucket of sand can therefore retain walls and support further layers. Water has not simply become glue: the strength belongs to a structure made jointly by sand, water and air.
More water is not always better. As pores fill, separate bridges merge. Near saturation, there are fewer local water–air interfaces able to maintain the same pattern of attraction, so the sand may soften or flow. The useful moisture range also depends on grain size, shape and surface properties. “Wet sand is stronger” is therefore conditional: sand that is too dry lacks bridges, while sand that is too wet loses an effective bridge network.
https://www.nature.com/articles/srep00549
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