The Adaptability Of Geogrid: Heavy Trucks, Complex Interchanges, And Earthquakes

The Challenge

The Carroll Canyon Road Bridge crosses I-805 just twenty miles north of the Mexican/U.S. border of Otay Mesa – California’s busiest commercial border crossing, facilitating 1.4 million truck crossings each year. As just one phase of an area-wide, $607 million, four-phase project, the project required a ramp and bridge system that could hold up against year-round usage by heavy trucks. The ramps not only had to be functional, but also aesthetically pleasing. In addition, the permanent structure had to withstand potential earthquakes, a common occurrence in the area.

California Department of Transportation (Caltrans) specified a wire faced reinforced soil slope for the approach ramp with a 3H:8V batter (69-deg). The proposed plan provided the contractor with several options for the rock facing. The project contractor, Diablo Contractors, preferred to utilize on-site river rock and the StrataSlope system.

The Solution

The adaptability and strength of geogrid, combined with Strata’s engineering expertise, allowed flexibility to accommodate all demands of the job. Strata provided several engineered solutions for Diablo Contractors to evaluate. After much consideration, they determined that screening the on‑site dry riverbed soil was the most economical. The riverbed contained an abundant amount of 4” to 8” rounded stone, which Diablo could use inside the wire basket facing to provide the aesthetics required by Caltrans. The remaining screened soil would be used for structural fill to construct the slope.

Once the backfill material was lab tested, the soil properties were provided to Strata for the final design of the StrataSlope system. The soil properties required Strata to increase the geogrid strength to SG600 from the original proposal in order to provide the adequate factors of safety. StrataTex180 was also used for separation between the structural backfill and the rock-filled facing to prevent soil migration. The hot-dipped, galvanized wire baskets were W4 wire with 4”x4” aperture and measured 1.5’x1.5’x10’. Seven wire struts per basket added additional stiffness to support the large rock infill. Caltrans specifications required a mechanical connection between the geogrid reinforcement and the wire basket. Strata developed a special connection bar and tested the connection at SGI laboratory in Atlanta. The finished structure stands almost 70 feet in height with 39,235 SF of face area.

The Result

Strata’s design provided a very aesthetic and economical solution, allowing the structure to be constructed under budget. This phase is expected to be completed in July of 2014. This wire-faced, reinforced earth slope was one of the first for Caltrans, and they were very pleased with the results. Caltrans fully expects to utilize these types of structures in the future.