The geosynthetic reinforced soil integrated bridge system (GRS-IBS) is a relatively new bridge construction method that has been deployed and promoted by the Federal Highway Administration (FHWA) and become more widely used across the nation in recent years, particularly in Florida by the Florida Department of Transportation (FDOT).
GRS is an engineered, well-compacted granular fill with closely spaced (one block maximum) layers of geosynthetic reinforcement. IBS was initially developed in the mid-1990s at FHWA. It is a short-schedule, cost-effective method of bridge support that blends the roadway into the superstructure using the GRS technology. These abutments are shallow foundations constructed using a combination of gravel and the layers of geotextile. The approach to the bridge is integrated into the GRS abutment in lieu of using an approach slab.
One example of the more widespread use of GRS abutments is in Polk County, Florida, where FDOT’s Cow Camp Road bridge project has recently been completed. The project involved new roadway construction and a new bridge to replace the existing deteriorating structure on Cow Camp Road in Lake Kissimmee State Park in Polk County. The area is rural with lowland terrain. One reason GRS-IBS with Keystone’s Compac III system was an optimal choice for the bridge design is that it works well for these type of sites, where the bridge is smaller, and where lighter, more transportable equipment can be used.
Other advantages using GRS-IBS and Keystone Compac III is a high degree of cost savings, averaging 25 to 60 percent below standard bridge construction. Material costs are also reduced, and maintenance costs will likely also be reduced since the system has fewer parts than a conventional bridge system and is jointless.
Keystone was selected after FDOT determined the new bridge was eligible for federal funding using the GRS-IBS system; Compac III suited the design parameters and has been approved as a “segmental block FDOT wall type.”
The Compac III unit is a 12-inch-deep unit with a typical face width of 18 inches by 8 inches high. Units weigh from 67 to 89 pounds each (varying with local manufacturing and aggregates). The dual pin hole configuration allows 1-degree, 4-degree, and 8-degree setbacks. For design purposes, the in-place density of the aggregate-filled unit is 120 pcf. Face styles are Straight Split and Tri-plane. In some markets, Hewnstone, Victorian, and Regency are available.
Keystone’s engineering department provided the shop drawings, which were based on the project plans prepared by the FDOT structures office. The total square footage of wall/abutment area is 2,314 sf, with a height of 15.33 feet. The combined length is 77 feet. This is the first GRS-IBS project Keystone has done in Florida, with two more pending.
Todd Dollar with American Construction Experts, the general contractor on the project, was more than satisfied with the Keystone abutments. “What my crew and I loved most working with the Compac III product made by Tremron was block consistency and quality. Support from the Tremron/Keystone sales and production teams was also solid.”