From the point of view of the source from which the retaining structure derives its stability, we have basically two types of retaining structures, viz., the gravity and the non-gravity types.
The gravity type structures are normally ‘rigid’ and the non-gravity type, ‘flexible’.
The gravity type of retaining structure derives its stability mainly from the self weight of its components, while in the case of the non-gravity type, the factors contributing to stability are other than gravity or self-weight forces.
Old masonry type of retaining walls (Fig.-1) and the comparatively new reinforced concrete retaining walls – both of the cantilever (Fig.-2) and the counterfort (Fig.-3) types – are examples of gravity type of retaining structures, but with the difference that, whereas in the case of the stout masonry wall, the self weight of the wall alone is the main source of stability, what contributes to stability is not only the weight of the thin R.C. structural elements, but also that of the soil on the base slab, in the case of the latter.
There is a host of types of retaining structures which derive their stability from sources other than gravity. The foremost example of this category is the sheet pile wall which is too thin, whether in steel, reinforced concrete or timber, for any stability to be derived from its self weight. While in the case of the cantilever wall of this type (Fig.-4), the only source of stability is penetration into the soil below, penetration and anchorage together contribute to the stability of anchored bulkheads (Fig.-5).
Diaphragm walls (Fig.-6) and bored pile walls (‘contiguous’ and ‘secant’ types – Fig.-7) are thin structures which are invariably anchored into the side soil using ‘prestressed ground anchors, when they are called upon to function as retaining structures. Hence in this state, their stability comes mainly from anchorage.
The most modern type of flexible retaining structure is the reinforced earth, where a thin ‘facing skin’ is held in position by a large number of thin ‘reinforcing strips’ tied to it and running through the backfill (Fig.-8). This type of wall owes its retentive action to the mechanical friction between the reinforcing strips and the backfill soil. This in a sense one may look upon it as the facing skin anchored into the backfill, even though the facing skin has a very minor role to play in this system. A major difference, however, between the anchoring action in the case of bulkheads and diaphragm walls on the one hand, and reinforced earth on the other, is that whereas the former two can be described as examples of ‘terminal anchorage’, the latter represents a case of ‘continuous friction anchorage’.
Among the non-conventional types of retaining structures must be mentioned crib walls and gabions both of which are predominantly gravity structures, but to a degree flexible, in nature. Even ‘tetrapods’, laid along coast lines as a protective measure against sea erosion, can be considered as falling under the broad category of retaining structures.