Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 7 RETAINING WALL
The concept of lateral earth pressure acting on a wall can be explained based on the basic of the
wall deformation. Consider an element of soil within a dry coarse-grained cohesionless soil mass.
The geostatic effective stress on an element at any depth, z. would be as shown in Figure 7.3(a).
Since the ground is not disturbed without any deformation, it is regarded as ‘at-rest’ condition. The
coefficient of lateral pressure for this condition is termed as K0.
Assume that a hypothetical, infinitely thin, infinitely rigid “wall” is inserted into the soil without
changing the “at rest” stress condition in the soil as shown in Figure 7.3 (b). Now suppose that the
hypothetical vertical wall move slightly to the left, i.e., away from the soil element as shown in
Figure 7.3(c). In this condition, the vertical stress would remain unchanged. However, since the
soil is cohesionless and cannot stand vertically on its own, it actively follows the wall. In this event,
the horizontal stress decreases, which implies that the lateral earth pressure coefficient is less than
Ko since the vertical stress remains unchanged. When this occurs the soil is said to be in the
“active” state. The lateral earth pressure coefficient at this condition is called the “coefficient of
active earth pressure”, Ka.
δ a
δ p
p o p o p o p o
p h =K o p o
p h =K o p o p h =K a p o p h =K p p o
Figure 7.3 State of Stress on a Soil Element Subjected to Stresses Induced by Wall Deformation (a)
In-situ vertical and horizontal stresses (b) Insertion of hypothetical infinitely thin and infinitely rigid
(c) Active contition of wall movement away from retained soil (d) Passive contition of wall
movement toward retained soil
Now, instead of moving away from the soil, suppose the hypothetical vertical wall move to the right
into the soil element as shown in Figure 7.3 (d). Again, the vertical stress would remain unchanged.
However, the soil behind the wall passively resists the tendency for it to move, i.e., the horizontal
stress would increase, which implies that the lateral earth pressure coefficient would become
greater than Ko since the vertical stress remains unchanged. When this occurs the soil is said to be
in the “passive” state. The lateral earth pressure coefficient at this condition is called the “coefficient
of passive earth pressure,” Kp.
The relationship between Ka, Kp, and Ko can best be illustrated graphically by Figure 7.4 below.
March 2009 7-3