To understand this problem one needs to consider two ideas : friction and surface tension. If you put a small block of wood on a table you can slide it around easily, but if you put a heavy weight on top of the block you will need to apply a bigger force to move the block.
This is a simple example of friction. The bigger the force holding the block against the table, the bigger the force holding the block against the table, the bigger the force needed to make it slide.Sand consists of lots of small, hard grains of rock that can slide over each other.
If the forces pushing the sand grains together are small, then the grains slide easily over each other. This is the case for the dry sand in the bucket. The sand grains are only pushed together by the weight of any grains, above them, so when you tip the bucket upside-down they slide easily until they form a conical pile. The angle of the sides of the pile is sometimes known as the angle of repose.
When the sand is damp, each grain is coated with a thin film of water, which tends to collect at the points where the grains touch.
Surface tension acts at the surface of the water, producing the same result as if the water were covered by a stretched skin that is always in tension. You can see another example of surface tension if you fill a glass of water a bit too full-a little water can be held in place a millimetre or two above the rim of the glass by surface tension.
Where the droplets of water adhere to the sand grains, the tension is applied to each grain, and this effectively pulls each grain against its neighbour, providing quite a strong force between them even if there is no weight of sand above. This force is enough to provide plenty of friction so that your sandcastle stays together.
Interestingly, if you try to make your sandcastle under water with wet sand, again there are no surfaces between water and air in the sand mix, so there are no surface tension forces and again your castle will flop into a conical pile. This proves that it is not water gluing the sand together that allows you to make castles with damp sand.
When water is added to sand, it forms pendular bridges between the grains, which hold them together. This force is stronger than gravity and prevents walls of sandcastle from falling down. These bridges work through the water's surface tension. The surface of the liquid bridges are concave, and so generates a `capillary action' which helps to hold the grains of sand firmly together.
If you keep adding water to the sand mix the pendular bridges start to merge and you pass through the so-called `lunicular' and reach the "capillary" state. The concave liquid surfaces continue to generate a capillary action which holds the sand grains together.
However, if you add even more water the surface curvature becomes convex and the capillary suction pressure dissappears. This is known as the `droplet' state. The water no longer creates any attractive force between the particles and the walls of the castle will begin to slump and flow as a liquid slurry.
Source : The Hindu