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Passive Lateral Earth Pressures in Retaining
Walls
Passive pressures what is it and why?
When we excavate on one side of a retaining wall an unbalanced
load condition is created. As it is very obvious, the retained
side wants to move into the recently excavated zone. However,
we engineers boldly introduce our retaining walls in the middle
to take care of the unbalance. At a closer glance what is happening
is this:
a) The excavated zone gets unloaded, the zone above subgrade
is removed and therefore there is zero lateral pressure on the
recentrly excavated depth.
b) As a result the retained soil, which is initially at an "At-rest"
state starts moving towards the excavation, while the soil on
the excavated side gets further compressed into the excavation.
c) The result of this movement on the excavation side is an increase
in the resisting lateral earth pressures from the theoretical
initial state for the current excavation level.
d) If this lateral movement keeps increasing, the lateral lateral
earth pressure reaches a maximum value, beyond which there is
no further increase in lateral pressure. Essentially, this condition
is a limit state or in other words a failure condition. This means
that the soil has fully mobilized its strength.
This condition, if left uncontrolled can be unsafe, partly because
passive earth pressures take place at large displacements.
As all effective horizontal stresses, active earth pressures
are defined as a ratio of the effective vertical stress times
a coefficient of lateral pressure. For "active" conditions,
this coefficient is typically defined as:
Ka = [1+Sin(friction angle)]/[1-sin(friction angle)]
Lateral passive earth pressures can be modified to include wall
friction, seismic effects, and surface inclination.
Do you have to include Passive Pressures for Retaining Wall
Design?
Engineers typically design gravity walls for passive earth pressures
and then apply a safety factor in the overall wall design. Over
many years this practice has proven safe given that the retaining
wall is allowed to experience small lateral displacements and
that the passive earth pressure is used to determine a safe wall
embedment that is later multiplied by a safety factor. In Eurocode
7 the toe embedment safety factor is incorporated in soil strength
reduction factors. Another approach is to divide passive pressures
by a safety factor when designing a retaining wall.
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