Episode 9 - Mechanically Stabilized Earth Walls: Seismic Analysis

When designing MSE walls in seismic regions, it is tempting to treat them like regular retaining structures until the ground actually shakes.

FHWA guidance reminds us that seismic loads must be included when evaluating MSE wall stability. That includes two main forces during an earthquake:

1. Seismic Earth Pressures – additional lateral loads caused by shaking. These are often estimated using the Mononobe Okabe equation to calculate the seismic earth pressure coefficient, which accounts for the peak ground acceleration, soil friction angle, and wall geometry

2. Inertia Forces – the reinforced soil mass itself has weight, and during shaking that mass generates horizontal forces acting through its center of gravity.

Both of these forces must be considered when checking:

• External stability such as sliding, overturning and bearing resistance under seismic loads

• Internal stability to make sure reinforcements can carry the increased tensile demand

• Global stability through slope or compound failure analyses that include seismic forces

One important observation is that the inertial force of the reinforced soil zone might govern the seismic design. Ignoring it can lead to a large underestimation of loads.

Although sometimes overlooked, these checks are critical especially in zones with moderate to high seismic risk. Walls designed without considering seismic loads may underperform or even fail when the ground moves.