Chapter 1 — Introduction¶
1.1 Objectives¶
Scope. This manual covers the design, construction, and monitoring of two families of reinforced-soil structure used on highway projects:
- MSE walls — mechanically stabilized earth walls, in which soil is reinforced and the reinforced mass presents a vertical or near-vertical face.
- RSS — reinforced soil slopes, in which soil is reinforced and the reinforced mass presents a sloping face (typically steeper than the soil could stand unreinforced, but flatter than a wall).
Both are built from the same three ingredients — frictional fill, tensile reinforcement, and a facing — and both rely on the same soil–reinforcement interaction. They differ in geometry, in the failure modes that govern them, and therefore in how they are designed.
Source documents. The manual consolidates and updates a line of earlier FHWA and AASHTO work, principally:
- Reinforced Soil Structures, Vol. I (Design and Construction Guidelines) and Vol. II (Summary of Research and Systems Information) — FHWA-RD-89-043.
- Geosynthetic Design and Construction Guidelines — FHWA-HI-95-038.
- Design and Construction Monitoring of Mechanically Stabilized Earth Structures — FHWA, 1994.
- Guidelines for Design, Specification, and Contracting of Geosynthetic Mechanically Stabilized Earth Slopes on Firm Foundations — FHWA-SA-93-025.
- AASHTO Bridge Design Specifications, Section 5.8 (and its interims), which supplies the load and resistance framework the design chapters follow.
This manual is itself the update of FHWA-SA-96-071 (Demonstration Project No. 82, Ground Improvement).
Terminology. The manual fixes a vocabulary that is used consistently throughout:
| Term | Meaning |
|---|---|
| MSE wall (MSEW) | A reinforced-soil mass with a vertical or near-vertical facing |
| Reinforced soil slope (RSS) | A reinforced-soil mass with a sloping face |
| Reinforcement | The tensile elements — steel strips, bar mats, welded wire grids, geotextiles, geogrids |
| Reinforced fill | The frictional soil placed within the reinforced zone |
| Retained fill / retained soil | The soil behind the reinforced zone, which the structure holds back |
| Facing | The visible element that retains the face of the reinforced fill — concrete panels, modular blocks, wrapped geosynthetic, or vegetated wrap |
| Foundation soil | The in-situ soil beneath the reinforced mass |
| Extensible / inextensible reinforcement | Whether the reinforcement strains enough to redistribute load (geosynthetics) or is effectively rigid relative to the soil (steel) |
| Design life | The period over which the structure must perform, including the durability of the reinforcement |
1.2 Historical development¶
Reinforced soil as an engineered system begins with Henri Vidal's Reinforced Earth patent in the 1960s: the observation that a frictional fill reinforced with closely spaced, frictionally bonded strips behaves as a coherent, anisotropic mass with apparent cohesion. The first major applications were in France and then the United States, and the concept spread in two directions:
- Metallic reinforcement (steel strips and grids) matured into standard proprietary MSE wall systems, with published design methods and decades of performance records.
- Geosynthetic reinforcement (geotextiles and geogrids) followed, first for slopes and then for walls and abutments, and opened the technique to a much wider range of fills and site conditions.
In parallel, the design methods converged on the framework this manual uses: external stability of the reinforced mass treated as a rigid body, internal stability of each reinforcement layer against tension and pullout, and global stability of the whole slope or wall system. The manual's contribution is to state those methods uniformly for both metallic and geosynthetic reinforcement, and to carry them through to specifications and field inspection.
1.3 Key takeaways¶
- MSE walls and reinforced soil slopes share principles but not failure modes — design them differently.
- The reinforcement is gripped by friction, not bolted to a rigid structure.
- The manual follows AASHTO's load-and-resistance framework and states design uniformly for metallic and geosynthetic reinforcement.
- The structure's design life and the reinforcement's durability are design inputs, not afterthoughts — see Chapter 3.