The San Diego Regional Airport Authority operates the busiest single-runway commercial airport in the United States. For this project, it sought bids to relocate and expand two employee surface parking lots to a different section of the airport campus.
The challenge
The client needed a construction strategy for Lot 6 and Lot 8 that would be affordable, efficient and reliable despite poor subgrade conditions.
The airport is located next to San Diego Bay. The subgrade soils consist of clay and sand with pockets of shallow groundwater with a resistance value of 10.
Standard construction methods would have entailed installing an 11-in.-thick aggregate base in combination with an asphalt top layer. The owner decided not to pursue this approach because of concerns over cost, construction scheduling and life cycle performance.
The solution
The authority instead decided to use the Spectra roadway improvement system, a mechanically stabilized layer incorporating Tensar TriAx TX7 geogrid, as a lower-cost, longer-lasting alternative to using a thick base layer.
“The TriAx Geogrid is a breeze to install,” said Project Superintendent Larry Miller of Hazard Construction, the project’s general contractor. “We would normally install one foot of aggregate. With this geogrid, we were able to install 1 million sq ft of parking on just 6 in. of compacted aggregate. We had areas with Jell-o-like subgrade and had no problems bridging that stuff.”
Hazard Construction began the installation by removing the subgrade soil and leveling and rolling the subgrade. Next, it covered the subgrade with TriAx TX7 geogrid. Workers overlapped each row by 18 in. and loosely secured them together using standard zip ties. To prepare the base, they installed 6 in. of Class 2 aggregate base and vibrated it to 95% compaction. With the base prepared, they were able to install two 2-in. lifts of hot asphalt mix. In areas with significant groundwater and soft soils, they excavated the saturated material, installed an additional layer of TX 7 geogrid and covered it with aggregate to return the subgrade to the required profile depth.
“The TriAx geogrid’s rib has a higher profile, and the grid has a unique hexagonal shape that gives 360-degree load distribution,” said Tensar International Corporation Regional Manager Lars Nelson. “These properties allowed the project engineer to optimize the pavement section, reducing the aggregate base layer by 45% while still providing superior base confinement and strength. This advantage allowed the contractor to install a single lift of aggregate base material on top of the TriAx geogrid, which saved approximately 15,000 cu yd of aggregate base material and 1,200 aggregate base trucks on the project while also increasing the speed of construction.”
Miller said he was skeptical about using TriAx geogrid at first.
“I thought it wouldn’t work,” he said. “But a year later, the parking lots look great. The stuff is amazing.”
