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Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

CHAPTER 1. GEOSYNTHETIC REINFORCED SOIL INTEGRATED BRIDGE SYSTEM

1.1. INTRODUCTION

The Geosynthetic Reinforced Soil (GRS) Integrated Bridge System (IBS) provides an economical solution to accelerated bridge construction. Employing this technology will help agencies save both time and money in planning and executing projects. This interim implementation manual and its companion document were developed to assist deployment of this promising technology as part of the Federal Highway Administration's (FHWA) Every Day Counts initiative.⁽¹⁾ The purpose of this manual is to provide a framework for GRS–IBS design and construction that is safe and consistent with the policies and procedures of FHWA and the American Association of State Highway and Transportation Officials (AASHTO), except where the behavior of this technology relieves itself of those requirements.

GRS–IBS was initially developed by FHWA during the Bridge of the Future initiative to help meet the demand for the next generation of small, single span bridges in the United States. GRS–IBS can be built with lower cost, faster construction, and potential improved durability and can be used to build bridges on all types of roads, on or off the National Highway System.

GRS–IBS is a fast, cost–effective method of bridge support that blends the roadway into the superstructure to create a jointless interface between the bridge and the approach (see figure 1 ). It consists of three main components: the reinforced soil foundation (RSF), the abutment, and the integrated approach. The RSF is composed of granular fill material that is compacted and encapsulated with a geotextile fabric. It provides embedment and increases the bearing width and capacity of the GRS abutment. It also prevents water from infiltrating underneath and into the GRS mass from a river or stream crossing. This method of using geosynthetic fabrics to reinforce foundations is a proven alternative to deep foundations on loose granular soils, soft fine–grained soils, and soft organic soils.⁽²⁾ The abutment uses alternating layers of compacted fill and closely spaced geosynthetic reinforcement to provide support for the bridge, which is placed directly on the GRS abutment without a joint and without cast–in–place (CIP) concrete. GRS is also used to construct an integrated approach to transition to the superstructure. This bridge system therefore alleviates the "bump at the bridge" problem caused by differential settlement between bridge abutments and approach roadways.

Figure 1 . Illustration. Typical GRS–IBS cross section.

The riding surface of GRS–IBS can be maintained as if it is part of the roadway pavement. No special attention to joints or the bridge deck is required. Unlike a traditional integral abutment, IBS is unique in its use of GRS to support the superstructure. This method of accelerated bridge construction is as easy as 1–2–3: (1) a row of facing blocks, (2) a layer of compacted granular fill, and (3) a layer of geosynthetic reinforcement. The 1–2–3 process is repeated until the required abutment height is reached.

GRS–IBS has many other distinct and innovative qualities. GRS technology is extremely durable and can perform well in earthquakes if constructed as outlined in this manual. GRS abutments can be built with readily available material using common construction equipment without the need for highly skilled labor. Construction of the abutment is contained within its footprint for a reduction of environmental impact as well as a reduced work zone. Additional benefits are convenience and design flexibility, as GRS–IBS can be built in variable weather conditions and can be adapted easily in the case of unforeseen site conditions.

This manual addresses the design and construction of GRS–IBS. In–service performance, inspection, maintenance, and repair are also described, along with special requirements for hydraulic and seismic conditions. Finally, procedures for quality assurance (QA) and quality control (QC) (including necessary construction documents) are provided. The ultimate purpose of this manual is to allow designers and contractors to effectively design and construct a durable GRS–IBS.

1.2 BENEFITS OF GRS–IBS

Based on constructed demonstration projects, GRS–IBS is more cost–effective than traditional bridge construction, utilizes common materials and construction techniques, and provides a safer work environment for personnel in work zones.⁽³⁾ GRS–IBS bridges can be built in less time (in weeks, rather than months), which translates into less congestion; fewer road closures, disruptions, and shutdowns around work zones; and lower materials and labor costs. The method of construction is such that the abutments are built from the inside out, reducing the exposure of personnel to potential roadside hazards. In addition, the technology is environmentally sensitive and results in minimal environmental impacts. The technology produces a reduced construction and carbon footprint, eliminates the need for installation of a deep foundation or CIP concrete, and can be adapted to fit the site–specific environmental needs.

The cost to build a GRS–IBS bridge is potentially 25–60 percent less than traditional methods, depending on the standard of construction. The savings is attributable to the simplicity and flexibility of the design, speed of construction (which is less dependent on weather conditions than CIP abutments), use of readily available materials and equipment, and elimination of the deep foundation and other construction details associated with the approach way to the bridge. Furthermore, this method has the potential for reduced maintenance costs because it eliminates the bump at the end of the bridge, creating a smoother and safer transition. Also, the application of GRS technology in other facets of earthwork (e.g., walls, culverts, foundations, slope stability, rock fall barriers, etc.) has the potential to result in significant cost savings and more effective use of transportation funding.

In summary, the benefits of GRS–IBS include the following:

• Reduced construction time.
• 25–30 percent lower cost than standard pile cap abutments on deep foundations with 2:1 slopes for off–system bridges.
• 50–60 percent lower cost than standard department of transportation bridges.
• Construction that is less dependent on weather conditions.
• Flexible design that is easily field–modified for unforeseen site conditions.
• Easier maintenance due to fewer parts.
• Construction with common equipment and materials.
• Better quality control.