Seismic Earth Pressure Coefficient
Seismic Analysis and Design of Retaining Walls
Incorporation of the effects of earthquakes is a 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 an extension of the Coulomb sliding-wedge theory taking into 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 a few assumptions. The following conditions should be satisfied to apply this analysis.
- Wall is a fee to yield sufficiently to mobilize the active pressure
- Backfill completely above the water table or below the water table. If the top surface is not horizontal then the soil can be partially saturated.
- Valid for cohesionless soil
- The surcharge is uniform and cover the entire area
Equations are given in the EM 110-2-2502 to find the active seismic pressure as follows.
(click on the image if can not see clearly) the above equations indicate the methods of calculating active seismic earth pressure and passive seismic earth pressure. Notations of the above equations are as follows.
If the vertical and horizontal ground accelerations and other parameters of backfill material are known, we can calculate the seismic effect on the wall.
The following figures indicate the soil wedges considered for deriving the 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.
For more information and method of calculating the earth pressure due to the seismic effects, refer another post in this blog – Seismic Design of Retaining Walls.
