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Active Lateral Earth Pressures in Retaining
Walls
Active 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.
c) The result of this movement on the retained side is a reduction
in lateral earth pressures from the initial state.
d) If this lateral movement keeps increasing, the driving lateral
earth pressure reaches a minimum value, beyond which there is
no further drop 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.
While at a first glance this might seem unsafe, active earth
pressures take place at very small displacements, typically less
than 3% of the total excavation depth.
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 active earth pressures can be modified to include wall
friction, seismic effects, and surface inclination.
Do you have to include Active Pressures for Retaining Wall
Design?
Engineers typically design gravity walls for active 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.
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