DHARM502 GEOTECHNICAL ENGINEERING
the concept of wall friction. In Rankine’s theory, plastic equilibrium inside a semi-
infinite soil mass is considered, pressures evaluated, a retaining wall is imagined to
be interposed later, and the location and magnitude of the total earth thrust are
established mathematically.
(iv) Coulomb’s theory is more versatile than Rankine’s in that it can take into account
any shape of the backfill surface, break in the wall face or in the surface of the fill,
effect of stratification of the backfill, effect of various kinds of surcharge on earth
pressure, and the effects of cohesion, adhesion and wall friction. it lends itself to
elegant graphical solutions and gives more reliable results, especially in the deter-
mination of the passive earth resistance; this is inspite of the fact that static equilib-
rium condition does not appear to be satisfied in the analysis.
(v) Rankine’s theory is relatively simple and hence is more commonly used, while Cou-
lomb’s theory is more rational and versatile although cumbersome at times; there-
fore, the use of the latter is called for in important situations or problems.13.8 Stability Considerations for Retaining Walls
A retaining wall is one of the most important types of retaining structures. The primary pur-
pose of a retaining wall is to retain earth or other material at or near a vertical position. It is
extensively used in a variety of situations in such fields as highway engineering, railway engi-
neering, bridge engineering, dock and harbour engineering, irrigation engineering, land recla-
mation and coastal engineering.
When designing retaining structures, an engineer often needs to ensure only that total
collapse or failure does not occur. Movements of several centimetres are often of no concern as
long as there is assurance that larger motions will not suddenly occur. Thus the approach to
the design of retaining structures generally is to analyse the conditions at collapse and to
apply suitable safety factors to prevent collapse. This is known as limit design and requires
analysis of limiting equilibrium conditions such as the active and the passive states, which has
been the subject matter of this chapter till now.
13.8.1Types of Retaining Walls
The common types of retaining walls have been listed in Section 13.1. Each type will be seen
now in a little more detail.
Gravity Retaining Wall
A gravity retaining wall is typically used to form a permanent wall of an excavation whenever
space requirements make it impractical to simply slope the side of the excavation. Such condi-
tions arise, for example, when a roadway is needed immediately adjacent to an excavation. In
order to construct the wall, a temporary slope is formed at the edge of the excavation, the wall
is built and then backfill is dumped in to the space between the wall and the temporary slope.
In earlier days, masonry walls were often used. Today, most such walls are of plain concrete
(Huntington, 1957).
The lateral earth pressure is resisted by the this type of wall primarily by its weight;
hence the name ‘gravity type’. It is, therefore, of thicker section in contrast to a few other
types. A schematic representation of a gravity retaining wall is shown in Fig. 13.43.