Earth pressure refers to the natural movement of the earth's soil. The movements will often apply pressure on existing structures. It is sometimes referred to as lateral earth pressure, which is measured by the amount of tension that occurs against basement or retaining walls. There are three types of forces, including at rest, passive or active earth pressure.
There is the common saying that a house creaks due to the fact that it is "settling," which is a direct reference to earth pressure. Natural movements and changes in the soil cause the foundation to move. Most of these changes and movements are slight. Despite the fact that most cases are slight enough to go undetected, there are instances where the amount of pressure is strong enough to result in visible cracks, leaning, crumbling or even complete relocation.
The effects of earth pressure may take time to accumulate before they become visible. Since the earth's soil is constantly evolving and changing at different rates during certain periods of time, many years may pass before any effects are seen at all. Some types of soils or landscapes are more vulnerable to change, meaning that structures built in those areas are more likely to fail or sustain damage sooner.
There are a few theories that attempt to explain the type and amount of active earth pressure. One of those is the Rankine Theory. It assumes that the soil and horizontal structures do not experience pressure. Rather there is only pressure in vertical walls. Failures occur within a certain parameter and manifest as a wedge in the structure as a result of a force that is parallel to the wall.
Another theory, which is called the Coulomb Theory, states that there is active pressure between a structure and the earth's soil. The pressure does not just occur within vertical structures, but horizontal ones as well. Pressure forces and friction from the soil occur in other areas that are not parallel to the structure, according to the theory.
Both of these theories involve mathematical calculations that can be used to determine the amount of soil pressure. The calculations are used to predict both active and passive earth pressure. With passive pressure, the wall or structure moves towards the soil. In active cases, structures are pushed away from the soil. In contrast, at rest cases indicate no movement, either due to insignificant amounts of pressure or no changes in the earth's composition.