DHARM512 GEOTECHNICAL ENGINEERING
Suppose, however, that the wall, starting from the at-rest condition, yields outward by
horizontal translation, the face of the wall remaining vertical, until active thrust conditions
are achieved. This is illustrated in Fig. 13.52 (f). In this case the wedge collapses somewhat as
shown in (f), are failure and the φ-obliquity condition occur only in a thin zone in the vicinity of
line BC. The major portion of the wedge is not appreciably distorted and, therefore, the lateral
pressure on the upper portion of the wall remains very much similar to that in the at-rest
condition. In spite of this the total thrust on the wall in (f) is approximately the same as it was
in (c). This is evident from a consideration of the force triangle shown in Fig. 13.52 (e). In both
cases the weight of the wedge ABC is W, which must be in equilibrium with intergranular
reaction, R, on the sliding surface and the wall thrust Pa. Force W has the same magnitude and
direction in (c) and (f), and the other two forces have orientations that are same in (c) and (f).
Thus the thrust Pa, representing the equilibrant of W and R, shall be essentially the same in
both cases. If follows that the pressure distribution on the wall in (f) must be roughly as shown
by the curved line AJ, approximating to a parabolic shape in Fig. 13.52 (g). The high pressures
that occur near the top of the wall and on the upper portion of the surface BC constitute an
‘arching action’ and has been referred to as the ‘arching-active’ case by Terzaghi (1936) and is
described in detail by him; the conditions are described briefly, but excellently by Taylor (1948).
This type of yield condition leads to a situation approximating to the wedge theory, the centre
of pressure moving up to 0.45 to 0.55 H above the base.
The differences between the pressure distributions may be observed better by superim-
posing all in one figure as in Fig. 13.52 (h); line AF represents the at-rest pressure, line AG
represents the totally active pressure, and curve AJ the arching-active pressure. The total
thrust in the second and third cases is the same, but is somewhat smaller than that in the first
case.
Terzaghi Observes:
(i) If the mid-height point of the wall moves outward to a distance roughly equal to
0.05% of the wall height, an arching-active case is attained. (According to another school of
through, the top of the wall must yield about 0.10% of the wall height for this purpose). It is
immaterial, whether the wall rotates or translates; however, the exact pressure distribution
depends considerably on the amount of tilting of the wall.
(ii) If the top of the wall moves outward to a distance roughly equal to 0.50% of the wall
height, the totally active case is attained. This criterion holds if the base of the wall either
remains fixed or moves outward slightly.
Based on these concepts, the principles of design for different conditions of yield of the
wall are summarised by Taylor (1948) as follows:
I. If a retaining wall with a cohesionless backfill is held rigidly in place by adjacent
restraints (e.g., if it is joined to an adjacent structure), it must be designed to resist
a thrust larger than the active value; for the completely restrained case it must be
designed to resist the thrust relating to the at-rest condition. However, this case
will not occur often, in view of the relatively small yield required to give the case
given in II.
II. If a retaining wall with a cohesionless backfill is so restrained that only a small
amount of yield takes place, it is likely that this movement will be sufficient to give