Suppose you have to support a mobile home in a northern area. The supports, called the piles or piers, have to travel below the frost line. The purpose is to provide a stable surface, free of frost heave. Frost heave lifts and depresses the ground, leading to a tilted mobile home. The engineer drills a hole into the ground sufficiently deep to travel past the frost line. Then the engineer fills the hole with iron reinforcement rods, called rebar. Afterward, the engineer fills the hole with concrete. Many buildings are also supported in this manner. It's up to the building engineer to design the size and depth of the pile.
Before an engineer specifies the depth of the bore, she runs small test boreholes to extract the subterranean soil. If after analysis, the soil is too weak to support a load, then the engineer may have to extend the borehole down to the bedrock. To keep the hole open the engineer inserts a large steel tube, called a case, into the hole. After the hole reaches the required depth, rebar and concrete fill the tube, which is left in the hole. Foundation Specialists calls the tube left in place a sacrificial casing.
Sometimes, the pile borehole has to travel through the subterranean water table. If this has to occur, then the engineer normally employs a casing. The engineer uses a the casing to keep the hole open, and free of water. If the engineer didn't use casing, then the hole would simply fill up with water and collapse. The casing can either be removed after the concrete hardens, or sacrificial. Sometimes, its cheaper to just leave the casing in place, as opposed to expending the time and labor to remove the casing.
Bridges also have foundations. For many bridges, bored piles are used as the foundation. Drilling a hole into a river bottom has its own set of difficulties. First, the hole must not be allowed to fill with water. A caisson is used to keep water out of a work area. A caisson is a large tube, which workers and equipment work in. Caissons are, in essence, large retaining walls. After the caisson is erected and the water pumped out, the hole boring can begin. Erecting the caisson is actually far more work than boring the actual hole.