SRW History Articles series: SRW Design
Today, it is estimated that about 50 percent of all segmental retaining wall (SRW) units are sold through retail outlets for smaller, gravity wall applications—walls that do not rely on internal soil reinforcement. The characteristics and design of some of the most common segmental retaining walls used by the construction industry vary depending on the site conditions and wall geometry.
Retaining walls are designed to resist the soil and surcharge loads supported by the system. Designing a retaining wall involves balancing the resisting forces with the driving forces to create a stable mass with a margin of safety against failure. When designing a segmental retaining wall, designers should follow established National Concrete Masonry y Association (NCMA) or American Association of State Highway and Transportation Officials’ (AASHTO) methodologies.
NCMA vs. AASHTO
While AASHTO requirements are more conservative than NCMA’s, the two are similar. There are some key differences between the two:
- Minimum Geogrid Length:
NCMA 4ft (1.2 m) or 0.6H whichever is greater
AASHTO 8ft (2.4 m) or 0.7H whichever is greater
AASHTO specifications based the minimum 8 foot (2.4m) length on experience and ability to get construction equipment in behind the wall to compact the soil. The 0.7H was also based on experience with steel reinforcing. NCMA designs are based on geogrid reinforcing with 100 percent coverage (better pullout capacity), thus the 0.6H has worked well for common design scenarios.
- Load on the Reinforcement, TMAX While NCMA always uses the Coulomb Earth Pressures approach, factoring the slope, wall friction and wall batter into the equation, thus reducing the load (Tmax) in the reinforcing layers compared with an AASHTO design that only uses Coulomb Earth Pressures for walls above 10˚ face batter and Rankine Earth Pressures below 10˚.Internal Compound Stability Analysis ExampleReinforced Fill Soil Types.NCMA allows for the use of materials having a maximum 35 percent passing the #200 (75 µm) sieve, and suggests that materials with greater percentage of fines may be used when a geotechnical engineer is involved. AASHTO restricts use of materials for the reinforced fill zone and requires granular materials with less than 15 percent passing the #200 (75 µm) sieve.
NCMA uses the maximum geogrid load (Tmax) with factor of safety of 1.5 and compares this to the Peak
Connection Capacity between the geosynthetic and SRW Unit.
AASHTO also uses a factor of safety of 1.5 on Tmax; however, AASHTO also requires an additional reduction factor to be applied to the Peak Connection Capacity that is derived from long-term, sustained load connection testing. This is a very conservative assumption and not justified by performance or data from instrumented structures that indicates the conservatism inherent in current SRW design methods.
NCMA has suggested a factor of safety of over turning (ability of the structure to resist rotating for ward) of 1.5 for gravity walls and 2.0 for MSE structures. In AASHTO LRFD (2012), eccentricity is specified (the location of the load resultant with respect to the center of the footing, L) as e/L < 0.33. The resultant should be within the middle 1/3 of the base. e/L yields about the same design as a factor of safety of 2 would using the NCMA methodology.
Both NCMA and AASHTO methods use the Mononobe-Okabe methods for pseudo static design. Both also assume the total dynamic stress is evenly distributed over all the layers of reinforcing. NCMA, however, promotes the use of seismic design in seismically active zones, where AASHTO does not consider seismic design mandatory in zones 1-3 unless liquefaction induced lateral spreading seismically induced slope failure, due to the presence of sensitive
Forces and geometry for external stability analysis of conventional SRWs clays that lose strength during seismic shaking, may impact the stability of the wall or if the wall supports another structure that is required by code or specification to be designed for seismic loading and poor seismic performance could impact the structure.
This article is excerpted from the “SRW History Article Series: SRW Design.”