Q: My question stems from the fact that (in my area) there still seems to be confusion over when a global/slope stability analysis is required and who is responsible. A contractor might ask for some budget pricing for retaining wall design. Generally, this is in regards to a segmental retaining wall design. The challenging part is that I believe the global/slope stability is the responsibility of the project geotechnical engineer, so I wouldn’t include this as part of our scope. We would only do internal design checks. I make sure they are aware of this, and this seems to be the norm from what I understand. However, I wanted to get a better feel to see if there was a different understanding of this situation. Also, please advise when a global/slope stability might be required (really any code requirements and software recommendations for the calculations).
A (a): A geotechnical engineer is responsible for global stability analysis (GSA). Global stability analysis is often described in terms of the factor of safety against failure along a failure surface. The factor of safety is defined as the ratio of the soil shear strength (resistance) to driving shear stress acting along the failure surface. The analysis is a crucial part of the design process for mechanically stabilized earth (MSE) structures. It should be done in all cases and particularly when there is a boring taken (i.e., a geotechnical engineer is involved) and plans and specifications are stamped. In addition, whenever the groundwater is within 0.66H of the bottom of the wall, 2/3 of the wall height, GSA needs to be done. This will ensure an adequate factor of safety exists for deep failures that pass behind the geosynthetic reinforcement and for compound failures that pass partially through the reinforced zone.
A (b): All walls need global stability analysis. From experience, we have seen relatively low walls (about 4 feet [1.2 m] tall) fail and cause much loss, risk and damage.
A (c): We have always used the computer codes from ADAMA Engineering Inc., www.GeoPrograms.com or adama@GeoPrograms.com. ADAMA’s ReSSA, MSEW and ReSlope computer codes are excellent and highly recommended and will incorporate global stability analysis in the total design package. Please note that you will have to supply the proper input for the design to be relevant.