They are called foundations or foundation to the set of structural elements whose mission is to transmit the loads of the building or elements supported on it to the ground, distributing them so that they do not exceed a series of maximum values of the support terrain. Because the resistance of the soil is generally less than that of the pillars or walls it will support, the contact area between the soil and the foundation will be much larger than the supported elements (except in very coherent rocky soils).
The foundation is important because it is the group of elements that support the superstructure. Special attention must be paid since the stability of the construction depends to a large extent on the type of terrain.
We can have different types of foundation or foundations. In general, we can classify the foundations as superficial or direct and deep. We can establish a particular intermediate type: the semiprounda foundation. Within each type of foundation there are several elements, although the vast majority of structures use almost always the same.
Surface or direct foundation.
They are those foundations that rely on shallow or shallow layers of the soil, for having this sufficient capacity for endurance or because they are constructions of secondary importance and relatively light. In this type of foundation, the load is distributed in a horizontal support plane. In important structures, such as bridges, foundations, including surface foundations, are supported at sufficient depth to ensure that no damage occurs. The support elements that connect the supports with the ground are called shoes. When many foundations are foreseen in the design of the foundation and next to each other, they are replaced by a continuous element called the foundation slab.
Semi deep foundations.
When the terrain where we are going to support the structure is not very resistant, and important forces are foreseen, it is necessary to deepen more until finding a layer of soil with sufficient guarantees of stability. The so-called semi-deep foundations are then executed.
They are the foundation wells; intermediate solutions between the superficial and deep foundations. They are usually concrete, but there are systems that can be found, mainly in non-contemporary buildings such as brick arches.
When the poor quality of the terrain forces us, it is necessary to resort to deep foundations. As the simple compression support is not valid, they are based on the vertical friction effort between the foundation and the ground to support the loads, so they need to be located more deeply, in order to distribute over a large area, an effort large enough to bear the load. Some methods used in deep foundations are piles and screens.