contractors designers

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 9: General Global Stability

Global Stability

Pdf    dwg

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.

9.1   Wall Embedment with Toe Slope

9.1   Wall embedment depth should be determined by the wall design engineer based on typical industry standard and specific site requirements.

  1. Walls with slopes below must have additional buried course for stability. For walls with slopes below, local ordinances commonly reference to bury enough blocks to have a 1.5 m to 2.1 m level bench in front of the wall, and then the 2.5 cm of depth per 30 cm of wall height rule is used beyond that point, Figure 9-1. After the wall is complete, the level area can be backfilled to continue the appearance of a continuous slope.
  2. In general when a slope below the wall is present, there is need for a global stability analysis to confirm the overall stability of the site.
Toe Slope Embedment

Figure 9-1: Toe Slope Embedment

9.2   When to Analyze for Global Stability

9.2   If a global stability analysis is not included in the wall designer's scope of work, establish that when a global stability analysis is required by the wall designer, using their best engineering judgment, then the owner will contract with a geotechnical engineer for overall site stability. Global stability analysis should be run for a variety of conditions including:

Design Envelope ICS

Figure 9-2: Design Envelope ICS
  1. For those walls with slopes below or above.
  2. Walls built in sites with clays, silts, poorly graded sands, expansive clays and/or soils with a PI greater than 20 and or a LL greater than 40.
    1. The infill soil used must meet or exceed the designed friction angle and description noted on the design cross sections, and must be free of debris and consist of one of the following inorganic USCS soil types: GP, GW, SW, SP, GP-GM or SP-SM meeting the following gradation as determined in accordance with ASTM D422.
  3. For walls where more than 50% of the Internal Compound Stability (ICS) slip arcs fall at the back of the wall design envelope when ICS is run in AB Walls.


Sieve Size Percent Passing
1 in (25 mm) 100-75
No. 4 (4.75 mm) 100-20
No. 40 (0.425 mm) 0-60
No. 200 (0.075 mm) 0-35

We have identified a Design Envelope for the wall designer that defines the extents of an ICS evaluation within that envelope. An ICS analysis does not take into account a slope below the wall, as the bottom slip arc is located at the base of the bottom block. Therefore it will not provide guidance for what happens below the wall. One easy to identify guide for additional global evaluation may be found by reviewing where the ICS slip arcs with the minimum factors of safety originate. When a concentration of ICS slip arcs originate at the back of the Design Envelope, a complete global stability analysis should be conducted, See Figure 9-2.

9.3   Increasing Global Stability Options

9.3   For walls with global stability concerns, it is common to increase the stability by:

  1. Increasing the length of the geogrid layers to force the minimum slip arcs deeper into the hillside.
  2. Increasing the depth of buried block to force the minimum slip arcs deeper into the hillside.
  3. Increasing the strength of the geosynthetic reinforcement layers to force the minimum slip arcs deeper into the hillside.
  4. Increasing the friction angle of the infill soil will increase the soil’s shear resistance which will increase stability.
  5. If the cause for the global stability problems is weak foundations soils, it may be necessary for the geotechnical engineer on the project to do foundation improvements before the retaining walls are built.