For a Seattle road over marshland, the contractor chose a Tensar International Corp. geogrid for soil stabilization.
By Alec Anderson, P.E., Regional Sales Manager, Tensar International Corp.
When soft soils are encountered on a jobsite, many contractors automatically turn to the same solutions they’ve always used. However, their go-to method may not be the cheapest or fastest and could cost them more in material costs or lost time on the jobsite. Take for example this recent project in Seattle, Wash.
A contractor was building an access road over historic marshland when the crew encountered a high water table and 10–15 feet (3.0–4.6 m) of soft soils. The original solution to stabilize the subgrade used a nonwoven geotextile and 2–3 feet (0.6–0.9 m) of rock. But the roads quickly rutted and became unserviceable for dump trucks. Time was of the essence since the access road was critical for transporting a necessary drill rig. Lost productivity was the last thing they needed.
Many assume that geotextiles are a great solution to stop rutting because they equate high tensile strength with better in-ground performance. However, when it comes to soil stabilization, meaning the ground will endure intermittent loads applied from above (like heavy dump trucks), tensile strength won’t get you very far. It has been widely documented that the ability to confine granular fill materials is the key to higer performance.
The contractor contacted Tensar International Corp., which offered a soil stabilization option using geogrid. A stabilization geogrid has apertures (openings) that allow granular materials to interlock. This interlock confines the granular particles to keep them from spreading laterally when loads are applied from above. Therefore, a stabilization geogrid is recommended for applications such as roadways; a stiffer stabilized layer resists rutting and improves bearing capacity. A geotextile can’t provide the necessary interlock, which is why performance suffers.
The geogrid’s design not only reduced the required aggregate thickness but also allowed the use of 0.75-inch (19-mm) aggregate instead of the expensive ballast rock needed with geotextiles.