MSE Walls and RSS
MSE Walls and Reinforced Soil Slopes (RSS)
The design of Mechanically Stabilized Earth (MSE) walls and Reinforced Soil Slopes (RSS) is critical for maintaining the structural integrity of West Virginia’s highway infrastructure. All earth retaining systems must be designed in accordance with the latest AASHTO LRFD Bridge Design Specifications and applicable Federal Highway Administration (FHWA) publications, including FHWA-NHI-10-24, FHWA-NHI-10-25, and FHWA-NHI-132042.
Additionally, GRS-IBS abutments must follow the latest Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (FHWA-HRT-17-080) and be constructed under WVDOH Special Provision Section 626.
MSE Wall Stability and Bearing Resistance
Ensuring the stability of an MSE wall requires a dual-analysis approach: a geotechnical engineer analyzes the external stability, while the vendor remains responsible for internal stability. WVDOH maintains conservative bearing requirements to satisfy LRFD standards and ensure long-term performance.
| Requirement | Specification Detail |
|---|---|
| Calculation Basis | Factored bearing resistance and pressure must be calculated on a one-square-foot basis at the wall's tallest section. |
| Measurement Point | Measured from the profile grade to the bottom of the leveling pad. |
| Eccentricity | Factored bearing resistance calculations should not deduct for eccentricity to ensure robust bearing near the toe area. |
| Undercuts | If required, select embankment must be centered on the leveling pad. Undercut size is determined by spreading factored pressure at a 1/2H:1V basis per side. |
Dimensions, Tiering, and Strap Lengths
Height restrictions and geometric offsets are implemented to prevent structural overstress and provide adequate room for ongoing maintenance and inspection.
- Maximum Height (H’): General wall height should not exceed 40 feet (leveling pad to profile grade).
- Bridge Abutments: A maximum height of 30 feet is preferred for walls serving as bridge abutments.
- Berm Requirements: A 4-foot wide berm must be provided at the base of the wall.
- Strap Length: Standard length is 0.7H, but this may be reduced to 0.4H when constructing next to bedrock.
Tiered Wall Systems
Tiered walls may exceed an aggregate height of 40 feet provided no individual wall segment exceeds that limit. They must adhere to the following offsets:
| Feature | Required Minimum Offset |
|---|---|
| Horizontal Offset | At least one-half the height of the lower wall. |
| Maintenance Access | 10 feet required at the base of the upper wall for inspection access. |
Reinforced Soil Slopes (RSS)
RSS structures are engineered to provide a 100-year design life. Unlike vertical walls, these slopes require specific attention to vegetation and secondary reinforcement to prevent surface erosion.
- Height & Backfill: RSS height should not exceed 50 feet. For slopes over 20 feet high, reinforced backfill must meet the same gradation limits as MSE walls.
- Drainage: A drainage blanket must be placed between the reinforced and retained zones, outletting to daylight.
- Vegetation: Outside slopes must support growth; secondary reinforcement (geocells) must have a 75-year design life. Topsoil must be a minimum of 18 inches thick per Section 651.
Material and Design Parameters
Standardized soil properties ensure consistency in stability modeling across all WVDOH projects.
| Material | Design Parameter (Standard) |
|---|---|
| Reinforced Backfill | Unit weight: 120 pcf; Max Friction Angle: 34 degrees (unless lab-tested). |
| Retained Soil | Max Friction Angle: 30 degrees (unless lab-tested). |
| Flood Conditions | Use free-draining material (e.g., #57 stone) for walls below design flood elevation. |
| Obstructions | Placement of culverts, inlets, or sign foundations within the reinforced zone is prohibited. |
Reporting and Documentation
The geotechnical report serves as the foundation for the internal design performed by vendors. Reports must consider both total and effective stress conditions, assuming zero cohesion for the latter.
- MSE Reports: Must explicitly state factors of safety for base sliding, overturning, and global stability. These must be noted on the plans.
- RSS Reports: Must include full stability analysis and reinforcement layer design, with details verified on the final plans by the geotechnical engineer.
Accessibility Statement:
The West Virginia Department of Transportation will, upon request, provide reasonable accommodations including auxiliary aids and services necessary to afford an individual with a disability an equal opportunity to participate in our services, programs and activities. Please contact us at (304) 558-3931. Persons with hearing or speech impairments can reach all state agencies by calling (800) 982-8772 (voice to TDD) or (800) 982-8771 (TDD to voice), toll free.
