Episode 7 - Mechanically Stabilized Earth Walls: Where Do the Largest Geogrid Forces Really Develop?

Most of us are used to the idea that the bottom reinforcement layers of an MSE wall should carry the highest tensile forces, because soil pressure increases with depth. That is why stronger geogrids or larger wire mats are usually placed at the base.

 But reality is a bit more interesting.

In practice, some of the largest tensile forces often show up in the middle reinforcement layers rather than at the bottom. Why? Because two opposite mechanisms interact along the wall height:

1. Lateral wall displacements – these are largest at the top of the wall, pulling the geogrids outward. Some studies even suggest the peak displacement occurs just below the top, depending on how the facing units connect to the reinforcement and how the numerical model handles interfaces.

2. Confining stresses – these increase with depth due to overburden. At the bottom, confinement is high and lateral movements are very small, so reinforcement forces remain low.

 When you combine these two effects, the result is that maximum tensile stresses occur around mid-height of the wall. The upper layers move more but are lightly confined, while the bottom layers are heavily confined but hardly move at all. The middle zone gets just the right balance of displacement and confinement to mobilize the largest forces.

Of course, applying a significant surcharge at the top changes the picture, increasing the demand in the upper geogrids.

 This is why finite element modelling or detailed monitoring often reveals force distributions that challenge the assumptions of traditional limit equilibrium analysis.

 It made me stop and think. In practice, we often reduce reinforcement strength toward the top of the wall and many walls have performed well this way. Still, the analysis showed me that the behavior can be more complex than we usually assume.