If you drive on State Route 167 in Renton near the Interstate 405 interchange, you may have noticed an imposing tower of big white blocks stacked in the median. Contrary to popular belief, they aren’t actually giant sugar cubes. They are a material called geofoam that is critical to building one of the bridge approach ramps on the I-405/SR 167 Interchange Direct Connector Project.
|Nope, these aren’t giant sugar cubes. We’re using geofoam to build bridge approach ramps on our Direct Connector project.|
Why did we decide to use geofoam?
For those who aren’t familiar with the Direct Connector, it’s a new flyover ramp currently under construction that will connect the I-405 HOV lanes to the SR 167 HOT lanes.
To take drivers up and over the interchange, crews need to build what we call approach ramps on each side. On the I-405 side, crews are able to use compacted soil to build the ramp. But they can’t use this same method as easily on the SR 167 side because the underlying soils are very soft and would settle or sink over time under the weight of the compacted soil. In this case, our contractor decided to use geofoam blocks instead of compacted soil to keep the project moving on schedule. Since late April, crews have been installing more than 2,700 blocks between the concrete walls of the SR 167 ramp, and their work is now almost complete.
What are the benefits to using geofoam?
Although you may not realize it when looking at a finished construction project, geofoam is actually quite common. We have used this synthetic material on other construction projects, including the direct access ramp at the I-405/NE 128th Street interchange in Kirkland, on SR 519 near Safeco Field in Seattle, and at the I-405/NE 10th Street Bridge in Bellevue.
The most common other fill material that we use — good old-fashioned dirt — would require more extensive work to improve the ground. Crews would need to bring oversized piles of dirt to the work zone and dump them onto the ground – a practice we refer to as surcharging or preloading – causing it to settle. This method can take longer and have higher costs because crews have to wait until the ground has settled before removing a portion of that dirt and building a structure on top of it.
Using geofoam reduces the weight placed on the underlying soils, eliminates the need for that ground improvement process and speeds up construction. First, crews dig out a section of the ground that is greater than the weight of the geofoam. Then they place blocks of geofoam and stick them together with a quick-setting commercial roofing adhesive. Once all the geofoam is in place, crews tie reinforcing steel on top of it between the ramp walls and then pour concrete on top.
|Once the geofoam is in place, crews tie reinforcing steel on top before pouring concrete (above and below).|
How much does the geofoam weigh?
The geofoam weighs a fraction of the weight of normal soil, yet it is just as strong because it is manufactured to meet the same strength standards as other methods. The dirt that crews dig out before placing the geofoam weighs more than all of the geofoam being placed.
Each standard block of geofoam has a weight of 248 pounds. Once all of the blocks are installed, their total weight will be almost 700,000 pounds! If we used only dirt, it would weigh as much as 100 times more and have greater impacts on the ground in this area.
|Each standard block of geofoam weighs 248 pounds, and once they’re all installed the total weight will be 700,000 pounds.|
|Using innovating materials and techniques helps construction crews complete our projects as quickly as possible.|
We realize that construction is disruptive to traffic and to the communities nearby, and we are working to bring travel benefits to the public as soon as possible. That’s why we are working with our contractors to look for innovative materials and construction techniques — such as geofoam — to complete our projects as quickly as we can.