The foundation pit, also known by its French name caisson, is a type of semi-deep foundation, used when the soils are not suitable for shallow foundations because they are soft. Foundation pits are also frequently used to cement bridge piers in river channels when it is not possible or advisable to create a partial or total diversion of the river.

The peculiarity of the foundation pit is that it is built as it sinks into the ground. The cross section of the caisson is generally circular, but there are also square, rectangular or elliptical sections.

The construction process, when it is carried out on land, begins with the construction of the head, generally made of reinforced concrete, provided on its external generator with a vertical blade, a steel sheet solidly anchored to the head. Next, once the head has been placed in the position in which it is to be sunk into the ground, the first meters of the body of the foundation pit are built. The body of the foundation pit is about 5 to 10 cm smaller than the external perimeter of the head, in order to reduce the friction of the wall with the ground.

When the excavation of the earth inside the well begins, the weight of the well will begin to sink it. As it sinks, the wall that still protrudes out of the ground progressively increases, until the desired depth is reached, a few dozen meters. Once the desired depth is reached, a concrete plug is built at the base, so that the weight of the structure to be built on the foundation is distributed evenly over the entire section of the well, which can be up to 12-15 m in diameter, if it is circular, and have a length of more than 20 m if it is rectangular.

Once the well is sealed at the base, it can be filled with inert material or left empty. In the latter case, if the water table is higher than the bottom, over time the water will balance outside and inside. Leaving the well hollow, or partially hollow, helps reduce the total weight of the structure.

When the foundation pit must be placed in water, its construction is generally completed on the bank of the river or lake and then it is towed to its final location, where it is sunk to the desired depth. If the depth is considerable, it may be convenient for the body of the well to have a closed chamber where the workers in charge of sinking work.