A soil mass is stable when the slope of the surface of the soil mass is flatter than the safe slope. At some locations, due to limitation of space, it s not possible to provide flat slope & the soil is to be retained at a slope steeper than the safe one. In such cases, a retaining structure is required to provide lateral support to the soil mass.

A retaining structure is a permanent or temporary structure which is used for providing lateral support to the soil mass or other materials. Some of the examples of retaining structures used in soil & foundation engineering are: Retaining wall, Sheet piles, Anchored Bulkheads, Sheeting & Basement wall, etc. Generally, the soil masses are vertical or nearly vertical behind the retaining structure. Thus, a retaining structure maintains the soil at different elevations on its either side. In the absence of a retaining structure, the soil on the higher side would have a tendency to slide and may not remain stable.

The design of the retaining structure requires the determination of the magnitude & line of action of the lateral earth pressure. The magnitude of the lateral earth pressure depends upon a number of factors, such as the mode of movement of the wall, the flexibility of the wall, the properties of the soil, the drainage conditions. For convenience, the retaining wall is assumed to be rigid & the soil structure interaction effect is neglected which arises due to the flexibility of the wall.

The lateral earth pressure is usually computed using the classical theories proposed by Coulomb (1773) & Rankine (1857). The general wedge theory proposed by Terzaghi (1943) is more general and is an improvement over the earlier theories. The general equations developed for both theories are based on the fundamental assumptions that

the retained soil is cohesionless (no clay component), homogeneous (not a varying mixture of materials), isotropic (similar stress-strain properties in all directions or in practical terms, not reinforced), semi-infinite (wall is very long and soil goes back a long distance without bends or other boundary conditions), and well drained to avoid consideration of pore pressures.

The pressure or force exerted by soil on any boundary is called the earth pressure. When the earth pressure acts on the side (back or face) of a retaining wall, it is known as the Lateral earth pressure. The magnitude of the lateral earth pressure depends upon the movement of the retaining wall relative to the backfill & upon the nature of the soil.