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Best Practices for Segmental Retaining Walls Best Practices



Disclaimer


The intent of this material is to communicate the best practices for design of Segmental Retaining Walls (SRW) as determined by Allan Block Corporation based on 25 plus years of research, design and field experience. This is not meant to be a final authority as each project has its own set of unique situations.

The local engineer of record must use their best engineering judgment to account for those situations that present themselves and provide a safe and efficient design for the customer. At no time does the contractor or local building official have the authority to override the approved plans and specifications provided from the local engineer of record.

It is the recommendation of Allan Block Corporation that the local engineer of record work for and be paid by the project owner. It has been determined that the local engineer of record should be the Project Site Civil Engineer as they are best suited to take responsibility for the design, and how it affects the site, whether they do the design in-house or use an outside consultant to do the design for the project.

The Project Site Civil Engineer has control of several of the overall aspects of the project and therefore is most able to properly handle the integration and communication required to ensure the performance of the wall complies with the needs of the site. For wall design applications that are outside of the experience level of the Project Site Civil Engineer, a wall designer with the appropriate knowledge and experience should be contracted with by the Project Site Civil Engineer. It is recommended that the wall contractor not be responsible for securing the engineering.

last updated: 8/11/2015



Chapter 12: Above Wall Considerations

Above retaining wall Stability

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Click on the topics below to view more information on the best practices for Allan Block segmental retaining wall design for residental and commercial applications.

Design Guidelines Item: The term, "owner" refers to the property owner or their designated representative.

12.1   Minimum Grid lengths at the Top of the Wall

12.1   Minimum grid lengths.

  1. Top layers of grid should be lengthened to 90% of wall height or an additional 0.9 m, whichever is greater, in seismically active areas or with any type of loading above the wall including slopes and surcharges. In the event that an interference may exist with the top layer based on other structures in the reinforced zone it is acceptable to move this extended layer to a lower grid elevation at the top of the structure.
  2. The portion of wall above the top layer of geogrid should be reviewed as a gravity wall taking into account all soils and surcharge parameters.

12.2   Fences and Railings

12.2   Details for fencing and other potential lateral loads from objects located above or within the reinforced soil mass.

  1. Impact structures or fence posts should be positioned a minimum of 0.9 m from the back of top course to allow a properly designed load for local overturning.
  2. If fence posts must be placed within 0.9 m of the back of the wall facing, the designer must consider the localized top of wall overturning force into their design.
  3. Common lateral loads from railings are 0.74 kN/m along the top of the railing or 2.92 kN/m point load at the top of the post or both depending on the local requirements.
  4. Common post footings are formed by using construction tubes placed at desired on-center locations during construction. If posts are placed after construction is complete, holes must be hand dug as using a power auger will cause severe damage to the geogrid layers.
  5. If the post design calls for an engineered product to solve localized overturning forces, a product such as the Sleeve-IT System can be specified. Sleeve-IT is a Stratagrid product designed for use in segmental retaining walls to resist top of wall overturning forces from fences and railing. For more information on design capacities and installation, contact Stratagrid at www.geogrid.com.

12.3   Slopes Above the Wall

12.3   Details for placement and compaction of a slope above the wall structure.

  1. Proper lift and compaction recommendation should be followed from the geotechnical recommendations. See Chapter 6.0 Soils and Compaction.

12.4   Reinforcing the Slopes

12.4    Stability of slopes above the wall.

  1. There are limits to the steepness of a slopes above a wall that must be considered when designing any soil structure.
    1. In static designs, the maximum unreinforced slope above any wall cannot exceed the friction angle of the soil used to reconstruct the slope.
    2. In seismic designs, the maximum unreinforced slope above any wall cannot exceed the friction angle of the soil used to reconstruct the slope minus the seismic inertial angle. The seismic inertial angle is based on the horizontal and vertical seismic coefficients ( kh and kv ) determined by the soils engineer for that specific project site.
    If the desired slope above exceeds either of the two limits above, the designer must analyze the slope above in a global stability program and provide slope reinforcement as required.
  2. For any wall having a slope above greater than 3:1, and/or any slope with poor soils or walls with seismic requirements, it is recommended that the designer call for the slope to be reconstructed with stabilizing geogrid layers. These layers typically match the standard grid lengths in the wall along with their spacing.

12.5   Compaction Requirements for Slopes Above

12.5   Slopes above the wall must be compacted following the proper lift and compaction recommendation from the geotechnical recommendations. See chapter 6.0, Soils and Compaction.

12.6   Reinforcing Slopes Above Walls

12.6   Reinforcing the slope above the wall may be necessary to provide adequate stability to the wall system. Geogrid lengths typically matching the standard grid lengths in the wall along with their spacing are commonly used in walls with steep slopes above, greater than 3:1, and/or any slopes with poor soils or walls with seismic requirements.

12.7   Plantings

12.7   Details for plantings located above the wall structure.

  1. In general, plantings in the reinforced zone are acceptable and may in fact enhance the reinforced soil mass provided:
    1. Engineer should review with the owner the planting plans from the landscape architect to verify size of root balls and if any geogrid layers will be disrupted.
    2. Language can be placed in the project specifications.
    3. Augers should never be used within the reinforced grid zone.
    4. Consider the effect of excess irrigation. See Water Management, Chapter 3.0.