Retaining Walls: The Inside Story – Part 3

Oct. 29, 2011

Canton Marketplace
Canton Marketplace, in Canton, GA, is a mixed-use site being developed for condos, retail space, and possibly single-family residences. As Joe Bailey of Tensar International explains, “It’s a very large project—about 75,000 to 80,000 square feet for the retaining wall—and there are some interesting features to the project. The height of the walls reaches over 50 feet in spots, and they have a broken section in which to plant trees for architectural interest. Some sections have trellises attached so that ivy can grow up on them.”

Tensar supplied geogrid for the project, while Shelby Block provided the retaining wall blocks. Contour Mesa Retaining Wall Systems handled the engineering and construction work.

Whereas a geogrid might normally extend just a few feet back into the retaining wall backfill, for Canton Marketplace it was extraordinarily long. “It’s probably a 40- or 50-foot grid length—there’s a lot of grid in this project,” says Bailey. But no anchoring was required. “They don’t need to be anchored; they don’t need to distribute stresses that way. They interlock with the soil and they distribute stresses back through the soil.”

Controlling movement of soil behind a retaining wall is crucial. “You’re assuming a wedge of soil is going to move up the face of the wall,” says Bailey. “Sometimes you have a very poor foundation and you can have stability issues. If you have a poor foundation, you can also have sliding issues. But with a proper foundation and with the geogrid, you capture a certain amount of soil. Between the frictional force it creates with the foundation and the sheer weight of it, the weight will resist overturning, and that frictional force will resist the sliding.”

For this project, the retaining wall is near vertical: “Just a tiny bit of batter,” according to Bailey. “I’ve seen the project, and it’s very impressive. It’s a good project, and it hasn’t moved. Structurally, it seems sound.”

Brookings Water Tank
When the town of Brookings, OR, decided to put in a new water tank, it had an unusual design requirement.

“The southern Oregon coast is in a similar seismic zone as San Francisco and those areas along the San Andreas Fault. It’s a pretty similar earthquake magnitude risk,” explains William Galli, senior principal with The Galli Group, the design geotechnical engineer for the project. “The project is a 1.6-million-gallon reservoir tank that they sited on a hillside. They wanted it close to the zone it would serve, and they wanted it to be at a certain elevation in order to meet pressure demands of the different zones in their city system. They found this lot—it’s not a very large lot—and it was sloping, and the project had to be designed for a 2,500-year seismic event, which comes out to about a magnitude 8.5 earthquake. It created a really interesting situation.”

Furthermore, he says, “There was an irregular rock layer an average of about 8 to 10 feet below the surface, and the soil was like a silky sand. We had them excavate all of that out from beneath the tank and for a distance below the slope of the tank and replace it with a big drainage system so they wouldn’t get any groundwater. If the groundwater comes up, it lowers the strength of the retaining wall. They put in a complete dewatering system underneath and then backfilled under the tank with 4-inch angular rock. There was a high friction angle with the rock. This continued a distance down the slope, and this was all interlocked into the rock underneath. Where the rock had an adverse slope—sloping in a downslope direction—we had them cut keys into it, so that the compacted rock above would interlock with that rock. We didn’t want any predefined slip points. Then we had the retaining wall designed so that it is well embedded down into this new rock layer that we created.”

To get the elevation needed, Galli explains, “The downslope edge of the tank needed to be about 9 feet in the air above the grade. They wanted a 12-foot-wide access road, so they ended up with an 11-foot-high retaining wall around the downslope edge.”

For backfill, angular crushed rock was used.  Galli describes the special drainage system: “We put a drainage system behind the retaining wall, although a lot of it was handled by the subdrainage system that was underneath the rock. We added some more drainage behind the walls as a precaution. They get really heavy rains on the coast—sometimes they can have 8 inches in one night. So we wanted to make sure there was no buildup of groundwater at all. The retaining wall itself was designed by an engineering firm working with Lock+Load. We stipulated that they use the geogrids behind the wall. We also used several layers of those down into the fill below to help tie the rockfill behind the wall with the rockfill in front of the wall. We wanted everything to resist as a unit. Our analysis indicated that if the wall stayed intact, it was going to withstand the load.”

It was the responsibility of The Galli Group to handle all the seismic design recommendations and the loads that the structural engineers had to work into their design.

“We did the construction control,” Galli says, “and we had people onsite when they excavated down into the rock to verify that they got the keys right and to verify the placement and compaction of all the backfill and the geogrids. A lot of people are surprised to hear what high seismic risk we have along the coast. There’s a Cascadia subduction fault off the coast where two plates meet, and one is subducting under the continental plate. That creates the potential for a large seismic event.”

Galli also describes the access issues he faced. “It was a very tight site; there was very little staging area, because the tank took up about 90% of the site. At least it seemed like it—maybe not quite that much. There was a two-lane road coming in, and it was residential, so we had to be careful about residences right around the tank. We had to be careful about noise and related issues. Also, down below the tank, across the road, was a slope that fell away very steeply about 40 feet below to a bunch of houses. So one of the big issues is that it was a high-hazard area, because if the tank failed, the people in those houses below are going to take the impact of the water. There was the relation between hazard and risk—if you have high hazard, you have to have very low risk.”

Especially when your water tank has to survive an 8.5-magnitude earthquake.

About the Author

Steve Goldberg

Steve Goldberg writes on issues related to erosion control and the environment.