By Jeff Dietz
Stantec
Last year, I attended a presentation by a professional liability insurer. The presentation gave a summary for claim dollars they had paid for various design professionals and various types of projects. The summary showed that, for the period their data had been taken, more than 20 per cent of claim dollars for geotechnical engineers, and 4 per cent of claim dollars for civil engineers, were related to retaining walls.
Retaining walls are commonly used when two areas on a property are at different elevations and there is a desire to transition from one elevation to the other in a short distance—shorter than can be used if the land was just sloped.
As a geotechnical engineer, I’ve participated in design and construction monitoring of retaining walls on many projects. I’ve even participated in the redesign and rebuild of retaining walls that turned out to not be designed and/or constructed properly in the first place. I’ve seen issues crop up in project after project. So, here’s my list of the top five mistakes made during design and construction of retaining walls — and how you can avoid them.
MISTAKE #1
Leaving the Design to the Bidding Contractors
I’ve seen many projects that include a retaining wall as part of a much larger project, where the design drawings show the wall location with both top of wall and bottom of wall elevations at various points along the wall. To save time and money, the owner or civil engineer doesn’t have the retaining wall design completed prior to having contractors bid on construction for the site.
The contractor must bid on the retaining wall portion of the project based on that limited information. What is the contractor going to do? They’ll go to a manufacturer of retaining wall components, who will give them a design for free. What is the manufacturer going to do? They’ll provide a design based on the configuration provided and typical design soil parameters. Are those design soil parameters applicable to the site? Possibly. Possibly not.
Leaving the design to the bidding contractors can also lead directly to the next four mistakes below. If the retaining wall is properly designed prior to the contractors bidding, you’ll know they’ve bid on a wall that will meet the project needs. You reduce the potential for delays — if it’s discovered that the wall isn’t appropriate for the site, which is often the case — and extra claims by the contractor.
MISTAKE #2
Not Having Soils Information at the Time of Design
I know. Shocking. The geotechnical engineer — who specializes in soils investigations — is recommending having soils information prior to design.
Well, if there hasn’t been some form of investigation prior to design, how are you going to know if the soils can support the forces produced by the wall?
Are there soft or organic soils that have to be removed or improved prior to construction? How do you do the global stability analysis to make sure the whole thing doesn’t rotate? If a soils investigation is completed first, you’ll have the information you need to properly design the wall.
So, design the wall for the soil conditions. It’s harder, and often impossible, to change the soil conditions to fit the wall that was designed.
MISTAKE #3
Not Doing Global Stability Analysis Prior to Construction
I’ve seen many retaining wall designs that leave the check of global stability to “others.” Global stability is the mechanism by which the whole wall, as well as the soil behind and below the wall, rotates.
If the global stability check finds that there isn’t an appropriate factor of safety in the design, the most appropriate way to fix that is to change the design. This can lead to a lot of back-and-forth between the designer and the other professionals who are analyzing global stability. If the global stability analysis isn’t done before the wall construction begins, and the analysis finds an issue, then there could be a need to remove the part of the wall already constructed and start again. Make sure global stability analysis is done as part of the design of the wall.
MISTAKE #4
Leaving Insufficient Space to Build the Wall as Designed
I’ve seen many retaining walls designed to be right up against a property line, with grades on the adjacent property being higher than the grades on the property owned by whoever wants the retaining wall. But you need space to construct the wall. When you excavate to construct the wall, you can’t leave the excavated slope vertical. There will have to be some sloping to ensure safety — so that the soil slope doesn’t fail during construction.
If the wall design calls for geogrid reinforcement to hold the wall back from falling forward, then that geogrid is extending into the neighbouring property — often by several metres. With any of these scenarios, the owner of the neighbouring property may have issues with your excavations extending onto their property, especially if they have any structures, landscaping, trees, or fences near or on the property line. Make sure that there is enough room to construct the wall you want to build when you design it. If you want construction to extend into a neighbouring property, make sure you have that neighbour’s buy-in.
MISTAKE #5
Insufficient Quality Control
In the province of Ontario, the building code requires continuous review during construction of most retaining walls. Even if quality-control inspection isn’t a requirement for the wall, it’s still a good idea. Aside from identifying soil conditions that are not as good as the designer assumed and coming up with solutions, the wall installer can make mistakes, such as choosing the wrong material, using the right material in the wrong way, or using material of the wrong size.
I remember talking to a contractor who had installed uniaxial geogrid — which is a geogrid that is significantly stronger in one direction — in the wrong direction for some fairly long walls. He said he had been doing it that way for years, and he probably thought he was doing it correctly. With the geogrid in the wrong orientation, the wall could move more than anticipated — possibly even fail. A proper quality-control program during construction can catch small errors so they can be corrected early on.
Avoiding these five mistakes can help steer your retaining wall project in the right direction and help you avoid insurance claims on projects involving retaining walls.
Jeff Dietz is the team leader for the geotechnical practice in Stantec’s southwestern Ontario offices. https://www.stantec.com/en