Special Considerations for Tall Walls 

Special Considerations
for Tall Walls

Taller walls often have special concerns that are not significant issues for shorter walls. Given their height, taller walls will influence and be influenced by a much larger portion of a site, so project design professional(s) must pay careful attention to site conditions well beyond the location of the SRW wall face and well below the SRW system. Layout issues, such as the wall batter and geosynthetic reinforcement lengths become more significant with tall walls that lose more space and need more space for longer reinforcement lengths.

One of the primary structural concerns for taller walls is the post-construction settlement of the reinforced soil (infill). Even well-compacted, high-quality granular backfill will experience some post-construction settlement. Even if the percentage of backfill settlement to fill height is less than one percent, this can yield significant settlement in a 30, 40 or 50 ft (9.14, 12.19 or 15.24 m) high wall. Total settlement of wall backfill is an issue for the performance of any top of wall structures such as pavements. Also, the possible differential settlement between the wall face, which is made of uncompressible concrete SRW units, and the wall backfill soils also is an issue for taller walls because the differential settlement increases with height. The backfill, and the geosynthetic layers within the fill, may be pulled down relative to the SRW units due to this differential settlement, possibly causing damage to the geosynthetic or overloading of the SRW unit-geosynthetic connection.

Design professionals’ typical strategies to address these settlement issues for taller walls may include:

  • Increasing the relative density compaction requirements to 95 percent Modified Proctor or 98 percent Standard Proctor.

 

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  • Encouraging higher levels of consistent compaction quality, including higher levels of quality control and quality assurance. More frequent compaction testing may be needed than for shorter walls. Installation practices that provide adequate performance in shorter walls, such as providing little compaction of the gravel fill or not strictly adhering to leveling and alignment tolerances, may need to be specifically addressed and improved to insure acceptable results for taller walls.
  • Thickening the minimum width of the gravel fill behind the SRW unit face up to 3 ft (1 m) to assist in graduating any differential settlement between the units and the reinforced backfill soils. Sometimes the thickness of the gravel fill is also graduated throughout the wall height. For example, for a 45 ft (13.7 m) wall the gravel fill may be 3 ft (1 m) thick gravel fill for the bottom 15 ft (4.5 m) of wall, 24 in. (610 mm) thick for the middle 15 ft (4.5 m) of wall, and one foot (305 mm) thick for the top 15 ft (4.5 m) of the wall.
  • Decreasing the plasticity index of the fine fraction of the backfill soils down to PI < 5 to 10.
  • Requiring select granular backfill in the reinforced zone that has no more than 5 to 15% fines.
  • Providing special attention to internal and surface drainage.
  • Breaking a single tall wall into two tiered walls with the upper wall set back no more than a few feet (m). This does not significantly change the loads on the walls or the reinforcement requirements but it does allow the wall contractor an opportunity to reset the wall face alignment and reduces the differential settlement between the upper SRW units and the wall backfill.

Whether any or all of the suggestions are needed, as well what value in these criteria ranges should be used, depends on the height of the wall, the on-site soil and fill soil types available, the accuracy of the site and materials data, local experience, anticipated quality control of installation, and the wall design engineer’s and project geotechnical  engineer’s judgment.

As an example of the range of judgment, taller walls backfilled with on-site, fine-grained soils are commonly successful in some regions, while in other regions the native soils properties make fine-grained soils unsuitable as fill for even 10 ft (3.05 m) high walls.

Finally, from a technical point of view, the height to which SRWs can be built is limitless. From a practical point of view, however, experience with very high (>50 ft (15 m))retaining walls is limited. Although SRWs have been successfully built in excess of this height, the knowledge and experience with the behavior of these structures at these heights is continued to be collected. The retaining wall designer should be aware that new and unique challenges are confronted at these heights. CMD