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

Chapter 7 RETAINING WALL
At-Rest condition may be appropriate for heavily preloaded, stiff wall systems. However, at-rest
conditions are not typically used for flexible wall systems such as steel sheet-pile wall, where the wall
undergoes some lateral deformation and designing to a requirement of zero movement is not
practical.
7.4.2 Active and Passive Lateral Earth Pressures
Active earth pressure (condition in Figure 7.3(c)) occurs when the wall moves away from the soil
and the soil mass stretches horizontally sufficient to mobilize its shear strength fully, and a condition
of plastic equilibrium is reached. The ratio of the horizontal component or active pressure to the
vertical stress is the active pressure coefficient Ka.
Passive earth pressure occurs when a soil mass is compressed horizontally, mobilizing its shear
resistance fully. The ratio of the horizontal component of passive pressure to the vertical stress is
the passive pressure coefficient, Kp.
The amount of movement necessary to reach the plastic equilibrium conditions is dependent
primarily on the type of backfill material. Some guidance on these movements is given in Table 7.1
Table 7.1 Wall Displacements Required to Develop Active and Passive Earth Pressures
Soil Type and Condition
Necessary Displacement
Active
Passive
Dense Cohesiveless 0.001H 0.02H
Loose Cohesiveless 0.004H 0.06H
Siff Cohesive 0.01H 0.02 H
Soft Cohesive 0.02H 0.04H
Note : H = Wall Height
(Source: Wu, 1975)
There are two well-known classical lateral earth pressure theories i.e. Rankine’s and Coulomb’s.
Each furnishes expressions for active and passive pressures for a soil mass at the state of failure.
7.4.2.1 Rankine’s Theory
Rankine’s Theory is based on the assumptions that the wall introduces no changes in the shearing
stresses at the surface of contact between the wall and the soil. It is also assumed that the ground
surfaces is a straight line (horizontal or inclined straight line) and that a plane failure surface
develops.
March 2009 7-5