MSE Walls and RSS
The design of mechanically stabilized earth (MSE) walls or other earth retaining systems, including Reinforced Soil Slopes (RSS), should be performed in accordance with applicable sections of AASHTO LRFD Bridge Design Specifications (latest edition) and Federal Highway Administration (FHWA) Publication No. FHWA-NHI-10-24, FHWA-NHI-10-25 and FHWA-NHI-132042.
GRS-IBS abutments are to be designed by latest Design and Construction Guidelines for Geosynthetic Reinforced Soil Abutments and Integrated Bridge Systems (FHWA-HRT-17-080 and constructed in accordance with the WVDOH Special Provision Section 626 Retaining Wall Systems.
For MSE walls, the external stability should be analyzed by a geotechnical engineer; the internal stability is the vendor’s responsibility. West Virginia requires that both factored bearing resistance and factored bearing pressure be calculated on a one-square-foot basis at the tallest section of the wall (as measured from the profile grade to the bottom of the leveling pad). The calculation for factored bearing resistance should also not deduct for eccentricity. This method of bearing check is more conservative than using the entire length and breadth of the MSE wall and ensures proper bearing near the toe area of the wall. It also satisfies the LRFD bearing check requirement.
When needed, select embankment or crushed aggregate is to be placed in an undercut centered on either side of the leveling pad. The undercut is to be sized based on a spreading of the factored pressure at ½:1 (H:V) basis on each side of the leveling pad.
MSE wall height (H’) should not exceed 40 feet, as measured from the top of the leveling pad to top of profile grade elevation. No more than thirty feet of height is preferable when an MSE wall is used for bridge abutments. A 4-foot wide berm should be provided at the base of the wall. Tiered MSE walls may exceed 40 feet tall provided no individual wall exceeds 40 feet. The horizontal offset between tiered walls should be at least one-half the height of the lower wall. A minimum offset of 10 feet should be provided at the base of the upper wall for maintenance and inspection access. Strap length may be reduced from 0.7H to 0.4H for MSE walls next to bedrock.
Both MSE and RSS structures must have a minimum design life of 100 years. RSS height should not exceed 50 feet and, for any RSS greater than 20 feet high, the reinforced backfill must conform to the same gradation limits specified for MSE walls. A drainage blanket that outlets to daylight should be placed between the reinforced zone and the retained zone. The outside slopes of an RSS must be designed to support vegetation. The secondary reinforcement, or geocells, should have a minimum design life of 75 years when exposed to daylight. Topsoil must be used in the outside 18 inches of an RSS and it must conform to Section 651 of the Standard Specifications. Two seedings must be performed and at least one seeding should consist of Type D seed mixture per Table 652.5 of the Standard Specifications. Good vegetation of the outside slopes is critical to extend the life of the geosynthetics to the 100-year design life of the RSS.
The reinforced soil backfill material must conform to the gradation limits included in Section 626.5.3 of the WVDOH Standard Specifications for both MSE and RSS structures (greater than 20 feet high) and must have a unit weight of 120 pcf and a maximum friction angle of 34 degrees for design, unless laboratory testing demonstrates otherwise. Retained soil properties must be defined in the geotechnical report but should not exceed a friction angle of 30 degrees for design unless laboratory testing demonstrates otherwise. When MSE and RSS walls are below the design flood, the reinforced backfill should be a free draining material such as size # 57 stone.
Both total and effective stress conditions can be considered as appropriate by the Geotechnical Engineer. Zero cohesion must be assumed for effective stress conditions. The placement of obstructions within the reinforced zone, such as catch basins, inlets, sign foundations, or culverts, must be avoided.
Foundation bearing resistance calculations, foundation types, minimum sizes, and depths are required in the geotechnical report per AASHTO LRFD specifications. For MSE walls, the report should include the factors of safety for base sliding, overturning, and global stability. These factors of safety and the soil parameters used in the calculations for MSE walls should be noted on the plans to assist the vendor in their internal stability. The RSS stability analysis and design of the reinforcement layers must be included in the report. The details of the RSS reinforcement must be transferred into the plans and verified by the geotechnical engineer.