Six-Foot Retaining Wall Under a Fire Lane
A 180-ft-long segmental retaining wall is planned along the edge of a new commercial parking lot. The exposed height is 6 ft, with a 90-degree outside corner; the wall supports a fire access lane. The retained soil is a silty sand with a plasticity index of 12, and the geotechnical report gives an allowable bearing pressure of 2,000 psf. The contractor proposes a gravity wall using 60-lb, 8-in.-high by 18-in.-long units with a 6-in. compacted aggregate leveling pad, 12 in. of free-draining gravel behind the block, filter fabric, and no geogrid. The submittal says the block is rated to 6 ft based on the manufacturer's chart for level backfill with no surcharge. The wall will be built in August with temperatures near 95 F.
1. Is the proposed unreinforced gravity wall appropriate for this site? Explain the controlling issue.
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No. The manufacturer's chart gives a gravity-wall height for level backfill with no surcharge. This wall carries a fire access lane, which means vehicle loading behind the wall, and a vehicular surcharge can be on the order of 250 psf or more depending on the governing code. A 6-ft wall with that surcharge will almost certainly fail the sliding and overturning checks at the required factors of safety, typically 1.5 for sliding and 2.0 for overturning, with only the block weight resisting. The 90-degree corner also concentrates lateral load and must be detailed. The correct submittal is a geogrid-reinforced wall designed to the NCMA SRW design manual or an equivalent method, with geogrid lengths and vertical spacing shown, and with the geotechnical engineer confirming bearing and global stability.
2. What design checks must the reinforced submittal include?
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The design must cover three groups of checks. External stability: sliding, overturning, bearing against 2,000 psf, and global stability of the wall and slope system. Internal stability: geogrid tensile capacity with reduction factors for installation damage, creep, and durability; pullout from the retained soil; and connection strength between geogrid and block. Facing stability: block-to-block shear and the ability of the facing to resist local loads. The design must also include drainage, including a perforated toe drain and outlet, not just gravel; seismic where required by the site; and separate consideration of the corner and any tiered sections. The submittal should state the design assumptions: unit weight, friction angle, surcharge, and geogrid properties.
3. What construction controls should the inspector enforce during installation?
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The leveling pad should be compacted aggregate or concrete as specified, and the first course should be fully buried to the required depth. Geogrid must be laid flat and taut, with the strong direction perpendicular to the wall face, on compacted fill, with no folds and no construction traffic, and with the specified embedment length behind the block. Backfill should go in lifts of about 8 in. and be compacted to the specified density, commonly 95% of standard Proctor, with equipment that does not disturb the geogrid. The gravel drain and filter fabric must be kept clean, and the wall should be capped. In 95 F weather, block and mortar can dry quickly, so mortar joints should be tooled promptly and grout should not be retempered with water.