Seismic Earth Pressure Coefficient

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Seismic Analysis and Design of Retaining Walls

Incorporation of effects of earthquakes are vital aspect in present designs due to the prevailing conditions in the world. Therefore knowing the facts adequately will help engineers to think about these when designs are carried out.

Different literature can be found for finding the effect of the earth pressure on retaining walls. One of famous analysis is Mononobe - Okabe analysis. This analysis is a extension of the Coulomb sliding-wedge theory taking in  to account horizontal and vertical inertial forces acting on the soil. This analysis is described in detail by Seed and Whitman (1970) and Whitman and Liao (1985).

Mononobe - Okabe analysis is based on few assumptions. Following conditions should be satisfy to apply the this analysis.

  • Wall is fee to yield sufficiently to mobilize the active pressure
  • Back fill completely above the water table or below the water table. If top surface is not horizontal then soil can be partially saturated.
  • Valid for cohesionless soil
  • Surcharge is uniform and cover the entire area
Equations given in the EM 110-2-2502 for find the active seismic pressure as follows.

  
(click on image if can not see clearly)
Above equations indicates the methods of calculating active seismic earth pressure and passive seismic earth pressure. Notations of above equations are as follows.
If the vertical and horizontal ground accelerations and other parameters of back fill material is known, we can calculate the seismic effect on the wall.
Following figures indicate the soil wedges considered for deriving above equations and the forces on the wedge.
Usually, vertical acceleration, wall friction angle and wall inclination are zero. The above equations can be simplified as follows.

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