Active Pressures in Deep Excavations
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.
Fig. Cantilever Excavation - Active and Passive Soil Pressures - DeepEX Software
DeepEX Software can Calculate all Soil Pressure Types for Deep Excavations
Active/Passive, At-Rest, Peck Apparent, FHWA Apparent, Custom Trapezoidal, AASHTO 17 and more!
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.