Implementation of Geosynthetic Reinforced Soil- Integrated Bridge System (GRS-IBS) Technology in Alabama

Abstract

Geosynthetic reinforced soil-integrated bridge systems (GRS-IBS) utilize mechanically stabilized earth (MSE) systems to support single-span bridges. A typical GRS-IBS is made up of three components: the reinforced soil foundation (RSF), geosynthetic reinforced soil (GRS) abutments, and the integrated approach. Each abutment sits directly atop a reinforced soil foundation, which consists of compacted granular material encapsulated in woven geosynthetic. However, a concrete pad can be constructed in lieu of a traditional RSF if the native underlying material is competent rock or very dense sand. Abutments are constructed from the ground up; open-graded or well-graded gravel is placed in lifts, compacted, and overlain by layers of geosynthetic material. This process is repeated until desired roadway elevation is met. Closely spaced layers (typically less than 12 in.) and internal reinforcement that is frictionally (rather than mechanically) attached to the facing material are two unique features that distinguish GRS structures from traditional MSE walls. The integrated approach blends the bridge superstructure into the surrounding geology, resulting in a smooth transition between the roadway approach and the bridge pavement. This relatively new technology provides a cost-effective alternative to traditional bridge foundation systems, as well as ease of constructability. In 2011 the Federal Highway Administration created an Every Day Counts Initiative to accelerate implementation of these bridges. GRS-IBS has been successfully implemented in several states, with Alabama being the latest. Alabama’s first GRS-IBS was constructed in Marshall County, spanning across Turkey Creek in the northeast area of the state. The robust sandstone geology and low scour potential of this site provided a conservative option for a test case study. Two 12 ft. tall, 33 ft. wide abutments support seven 1.75 ft. thick, 4 ft. wide, 52 ft. long reinforced concrete beams, pavement, and traffic. Earth pressure and pore-water- pressure vibrating-wire sensors were installed within the abutments, and reflective prisms were placed on the corners of the abutments to monitor lateral and vertical displacement. Earth pressures reached 1800 psf after the concrete beams were placed, and pore-water-pressure has remained near zero- signifying no significant buildup of pore water pressure due to flooding or rainfall within the abutment. Periodic surveys showed that settlement (z) and lateral movement (x,y) of the bridge were minimal. Information pertaining to typical building practices and construction specifications were gathered from multiple state Departments of Transportation. This information was compiled into a draft Special Provision for the Alabama Department of Transportation.