Expert: Jeanette A - 11/26/2008

Question
What is really meant by active and passive earth pressure, and how can we apply Rankine's theory

Answer
Hey Sami,

There are three categories of lateral earth pressure (at rest, active and passive) and each depends on the movement experienced by the vertical wall on which the pressure is acting. The wall could be a retaining wall, basement wall, earth support system eg sheet piling or soldier pile.

Active earth pressure develops when the wall is free to move outward eg a typical retaining wall and the soil mass expands sufficiently to mobilise its shear strength.

If the wall moves into the soil, then the soil mass is compressed, which also mobilises its shear strength and passive earth pressure develops. This situation might occur along the section of wall located below grade and on the opposite side of the retained section of fill.

There are two widely used theories when discussing active and passive earth pressure; Rankine Earth Pressure Theory and Coulomb Earth Pressure Theory.

Rankine Theory states the following assumptions:

- No adhesion or friction between the wall and soil
- Lateral earth pressure is limited to vertical walls
- Failure in backfill occurs as a sliding wedge along an assumed failure plain defined by phi
- Lateral earth pressure varies linearly with depth and resultant pressure is located 1/3 of the height above the base of the wall
- Resultant force is parallel to the backfill surface

As for Coulomb Theory, it is similar to Rankine's Theory except:

- There is friction between the wall and soil and this is included in computations by the use of the soil-wall friction angle (delta)
- Lateral earth pressure is not limited to vertical walls
- Resultant force is not necessarily parallel to the backfill surface due to the soil-wall friction angle.


Hope that answers your question and gives you some more background.




Kind Regards,
Jeanette