Chapter 5 — MSE Walls with Complex Geometries¶
5.1 Bridge abutments¶
An MSE abutment carries the bridge bearing loads in addition to the earth pressure, and those loads enter the reinforced mass at the top — where the reinforcement is least able to redistribute them. This is the most demanding MSE application, and the manual treats it in detail.
MSEW abutments on spread footings. The footing bears on the reinforced fill (or on the foundation behind it), and its loads (vertical, horizontal, and moment) are carried into the reinforced mass and the foundation. The design must check:
- Bearing under the footing, including eccentricity from the bridge loads.
- Internal stability with the footing load added as a large surcharge — the tension in the upper reinforcement layers rises sharply.
- Sliding of the reinforced mass under the horizontal bridge load.
- Global stability through the abutment and its foundation.
- Settlement of the footing and the approach, and the tolerance for differential movement between the bridge seat and the approach slab.
MSEW abutments on pile foundations. Where the foundation is too weak for a spread footing, the abutment is carried on piles, and the MSE wall is built around them. This decouples bridge settlement from wall settlement but creates its own problems: the piles interrupt the reinforcement, the pile cap generates concentrated loads, and the design must ensure the reinforcement is not damaged by pile driving or by the pile cap's movement.
5.2 Superimposed (terraced) walls¶
Where two or more MSE walls are stacked with a berm between them, the lower wall carries the surcharge of the upper one. The manual gives design rules for superimposed walls: the offset between walls, the reinforcement lengths in each, and the way the upper wall's load is spread onto the lower. Ignoring the interaction — designing each wall as if it stood alone — is a classic error.
5.3 Walls with uneven reinforcement lengths¶
Where the reinforcement length must vary along the wall (because of obstructions, geometry, or variable loads), the design must still provide a continuous reinforced mass and check the transition zones where the length changes, because the shorter sections attract less pullout resistance and can act as weak points.
5.4 Back-to-back walls¶
Where two walls face each other with a narrow soil mass between them, the reinforced zones may overlap. The manual distinguishes cases by the ratio of the separation to the wall height:
- Wide separation — each wall is designed independently.
- Narrow separation — the reinforced zones overlap and the walls act as a single composite; the design must treat the mass as one.
- Very narrow separation — the two facings effectively form a single wall with reinforcement crossing the full width.
5.5 Details¶
Abutment-specific details include the bridge seat, the approach slab connection, the bearing pad arrangement, and the drainage behind and through the abutment — all of which must tolerate the wall's outward deformation without distress.
5.6 Design example, bridge abutment¶
The manual works a full abutment design — geometry, bridge loads, external stability, internal stability with the footing surcharge, and the resulting reinforcement layout — by hand and by computer program.
5.7 Key takeaways¶
- An abutment is a wall plus a concentrated top load — the load path into the reinforcement and the foundation must be explicit.
- Piled abutments decouple settlement but interrupt reinforcement; detail the interface carefully.
- Superimposed and back-to-back walls interact — design the system, not the individual walls.
- Uneven reinforcement lengths create transition zones that must be checked.
- Tolerate deformation and differential settlement at the bridge seat, or the approach will crack.