Stabilizing Transportation Infrastructure with Minimal Carbon Impact

The California Department of Transportation (Caltrans) needed to stabilize a steep slope that separates the main section of an interstate and a maintenance station. The existing slope was 70 feet high with a gradient of 1.7 Horizontal: 1 Vertical (1.7H: 1V) and in some locations 1H: 1V.

A 20-foot section at the top of the slope had previously been reinforced with geosynthetic materials. However, due to overland flows, snow loads, and poor soil conditions, the unprotected section of the slope developed shallow plane failures that were approximately 3.5 feet deep. Caltrans was concerned that the slope would slough, affecting both the interstate and a road used to access the maintenance station.

ARMORMAX® Engineered Earth Armoring System was used to stabilize the slope. ARMORMAX consists of PYRAMAT® 75 High-Performance Turf Reinforcement Mat (HPTRM) secured with Engineered Earth Anchors to lock soil in place and provide immediate and permanent erosion protection for up to 75 years. For this project, both six-foot and nine-foot anchors were used. The anchors are corrosion resistant and designed to provide resistance to shear and lateral forces.

An analysis showed that using ARMORMAX improved the factor of safety over the unreinforced condition, and significantly reduced the probability of failure for the given soil parameters. The factor of safety is a ratio of the forces resisting movement to the forces driving movement, or the forces that initiate soil mobilization associated with slope failure.

For more than a decade, ARMORMAX has been used to reinforce embankments and slopes to protect transportation infrastructure. While it has most commonly been used to protect roadways, this same technology can be used to protect railways from slope instability.

One reason that ARMORMAX was selected by Caltrans is that it has a lower carbon footprint than methods traditionally used for slope stabilization such as rock riprap. One square meter of its HPTRM has a cradle-to-grave carbon footprint of 2.7 kgCO2e. Comparatively, the carbon footprints of concrete-based alternatives are up to 10 times higher, and rock riprap is up to 30 times higher. The carbon footprint of the HPTRM is verified by an independent third party, which certified that it meets the criteria for The Greenhouse Gas Protocol (World Resource Institute), PAS 2050:2011, and ISO 14064-3:2006.

Transportation requirements are a main factor contributing to the difference in carbon footprint. Projects that utilize ARMORMAX require significantly less truckloads of material, reducing transportation emissions by up to 95 percent.

While rock is a natural material, strip mining and crushing it for use in engineering projects is often done without regard for environmental impact. ARMORMAX is manufactured in a facility that is ISO 14001 certified. This international certification recognizes companies that are actively measuring environmental impact and continuously looking for ways to reduce their carbon footprint.

Once ARMORMAX was installed on the slope, trees were planted within the HPTRM and vegetation was established. The ARMORMAX system is engineered with a patented trilobal design that locks seeds and soil in place to promote rapid root development for long-term vegetation. This helps to decrease sedimentation and pollutants and encourages infiltration of water back into the groundwater table. These are two reasons why the Environmental Protection Agency (EPA) has identified systems that utilize HPTRMs like ARMORMAX as a Best Management Practice (BMP) for improving water quality. Conversely, rock does not promote vegetation and offers poor filtering and pollutant removal capabilities.