The use of a segmental retaining wall with stabilized backfill was the solution for this tricky project.
From NCMA
Developers of a Zoom Zoom drive-in storage facility in a suburb of Toronto, Canada, needed a retaining wall in a tight, difficult, location. A segmental retaining wall system, along with state of the art engineering, steel pilings, a gradebeam, and stabilized backfill provided an elegant, cost-effective solution.
The job was a tricky one. The objective was to maintain access to an emergency exit at the base of the highrise next door. The construction zone was very narrow due to the highrise being right on the property line and the presence of utilities under the main access road onto the new site. A deep layer of uncompacted “junk” backfill right under the construction zone complicated the matter further, according to Glenn Herold, P.Eng., M.ASCE, Director Commercial Solutions with NCMA member Oaks Landscape Products, a division of Brampton Brick. In that capacity, he assists customers with a wide range of engineered solutions with segmental retaining walls and concrete pavers.
The owner of the project is Zoom Zoom Storage. Project management was supplied by Net Zero Construction, while Strybos Barron King supplied landscape architecture on the project. The wall itself was designed by A-D Engineering Group Ltd, with installation by TEKK Construction Group Limited.
SRW Wins Out Over Cast-in-Place Retaining Wall
Developers initially examined the idea of digging down to good native soil and constructing a cast-in-place retaining wall. ”The cost of that solution was just astronomical. So the design team came in and asked us if there was any other way,” Herold said. “‘Can we possibly figure out how to build a segmental retaining wall on the site?’
“And I actually had just finished a project up in Northern Ontario, where for a waterfront project over sediment deposits we drove steel pilings into the ground and put a concrete slab on top of that. Then we built the wall above that. So we decided to examine that approach,” Herold said.
The ultimate solution at the Zoom Zoom storage facility involved steel pilings topped with a grade beam — a reinforced concrete slab — along with stabilized backfill behind the segmental retaining wall. “The grade beam actually has to support not just the wall blocks, but also the reinforced zone behind the wall. If the reinforced zone behind the wall isn’t supported, and it starts to settle, and geogrid between the wall blocks and reinforced zone can fail — shear off.”
Stabilized backfill is a relatively new technique in segmental retaining wall construction. It allows segmental retaining walls to be built in areas where they wouldn’t otherwise be possible. The stabilized backfill material is very similar to conventional ready mix concrete. However, the mix contains less water (lower slump) and it doesn’t contain fines/sand, which makes it porous. The backfill material is placed directly behind the wall units at a designated thickness to serve as the drainage layer, and to increase the overall mass of the wall system. Geosynthetic reinforcement can be used to provide a mechanical connection between the wall and the stabilized backfill.
The wall blocks serve as the front “form,” for the pour, while the back of the excavation or removable wooden panels can be used for the back. Being a low slump product, it takes a little more effort than required by standard ready mix concrete to spread, but the material is solid enough to walk on even while placing which helps. Individual concrete pours should be no more than 2’ high to prevent hydrostatic loading on the wall” Herold said.
Large-scale Proprietary Units Get Mechanically Installed
At the Zoom Zoom storage facility, the wall itself was constructed with Oak’s Proterra™ retaining wall system which features a special M-Lock technology, helping ensure a stable structure, even in a true vertical installation. Made of dry-cast concrete on a paver machine, the units are 39.4 inches, or one meter, long, 7.28 inches high and 14.76 inches deep, and are available with both a split and a smooth face.
The combination of the massive units, the steel pilings, the gradebeam, and the stabilized backfill came together to help solve a complex design problem and help ensure that the wall at the Zoom Zoom facility will be standing for years, and years to come.
“The techniques that we used on this project can be helpful in many segmental retaining wall applications. The stabilized backfill, in particular, is something that is not that widely used currently, and not all that well understood yet. But it’s catching on for good reason. It makes SRW competitive in some very tricky design situations,” Herold said.
Find out more about the many segmental retaining walls and design and installation resources available on NCMA’s website.
