DHARMLATERAL EARTH PRESSURE AND STABILITY OF RETAINING WALLS 513
the arching active case but not the totally active case. In this case, the assumption of
a triangular pressure distribution is incorrect; the actual pressure distribution is
statically indeterminate to a high degree, but is roughly parabolic.
A common example of the arching-active case is the pressure distribution on the
sheeting of trenches.
III. If a retaining wall with a cohesionless backfill is not attached to any adjacent struc-
ture, it can yield considerably without harm to the structure; in such a case the
totally active case is attained. (In fact, even a yield of 0.5% of the wall height is
adequate for this condition). The design of such a wall on the basis of active pressure
and triangular distribution of pressure is rational.
IV. In the case of a retaining wall with a cohesive backfill, the totally active case is
reached as soon as the wall yields but, due to plastic flow within the clay, there is a
tendency for a continuous increase in the pressure on the wall, unless the wall is
permitted to yield continuously. The continuous yield, although slow, may lead to a
large movement over a period of years. In such cases, one can either design for a
higher pressure or design for a totally active pressure if the wall is considered capa-
ble of withstanding any movement without harmful effects; for this latter basis,
which is a commonly used basis for design, the probable life of a wall with a cohesive
backfill may be relatively short.
According to these principles any wall capable of yielding without detrimental results
may be designed on the basis of active thrust and triangular distribution of pressure; however,
the actual thrust on the wall may be more and the pressure distribution may not be triangular.
This need not cause alarm to the geotechnical engineer, since any wall must have a margin of
safety and will be designed to withstand thrusts greater than the calculated values. This mar-
gin of strength may prevent the wall from ever yielding sufficiently to give active conditions.
Furthermore, the moment the wall is subjected to an increased thrust it merely yields a small
amount, which immediately reduces the pressure. This interdependency of yield and pressure
is thus a saving factor which partly takes care of any uncertainties in the theory.
13.8.4Choice of Appropriate Earth Pressure Theory
The choice of appropriate earth pressure theory for a given situation will become easy if one
remembers the various assumptions in the development of the earth pressure theories dealt
with.
For example, if the retaining wall has a vertical back and is smooth, Rankine’s theory
may be considered appropriate. One may use Rankine’s theory even for an inclined back of the
wall with a slight wall friction, provided the resultant thrust obtained by combining the thrust
on an imaginary vertical plane through the heel of the wall and the weight of the additional
wedge of soil standing on the back of the wall, does not have an obliquity greater than the wall
friction angle.
If the backface of the wall is plane and the wall friction is not inconsiderable and the soil
shows a tendency of slide along the back of the wall, the use of Coulomb’s theory is appropriate.
Coulomb’s wedge theory with plane rupture faces should not be used for the estimation
of passive resistance, especially in the case of structures such as sheet pile walls, wherein