Sustainable solutions for bridge and roadway construction

Floating structures and vessels are an example of temporary bridging structures used for open, navigable waters. Photo Credit: Florida Department of Transportation

One year ago, the collapse of an eight-lane bridge in Minneapolis—a disaster that killed 13 people and injured 145—brought about immediate calls for bridge repair across the United States. Equally important is the public outcry and a collective moral conscience for engineers, architects, and designers to fly high the flag of sustainability for each new project. Consequently, bridge repair can and should be combined with an environmentally responsible engineering effort that avoids, or at least minimizes, impacts to waters and wetlands during construction.

Temporary bridge techniques
Bridge projects over waterways, whether for repair or new construction, typically involve traffic detours that are subject to environmental permitting. The permitting process, as well as a project’s resulting mitigation efforts, inevitably involve significant costs to design and construct. However, there is an alternative to temporary construction access consisting of riprap and culverts that discharge fill into wetlands.

Temporary bridging techniques such as floating or prefabricated bridging can protect rivers, wetlands, and other waters from the environmental impacts of bridge and roadway construction and can save project expense. Several types of temporary bridge alternatives exist, each with its own set of potential impacts, benefits, and limitations.

Overarching factors to consider before selecting the best temporary bridge alternative for a particular project include bridge length, site environment, temporary structure duration, access and right-of-way constraints, permit considerations, temporary bridging of wetlands for other operations, and cost of various alternatives.

Following are descriptions of six of the most common types of temporary bridging, including the potential environmental benefits and impacts of each.

Panel bridges—Originally designed for military use (commonly known by the brand name, Bailey bridge), panel bridges can span distances as long as 450 feet. This application can be launched from side to side. Panels can be leased or rented to lower capital investment. However, panel bridges may require placement of construction equipment in wetlands if intermediate bents are required, causing potential for short-term shading of plant communities.

Contractor-designed structures—The span length of this widely used method is typically 25 to 60 feet. The design involves placing timber mats on steel girders supported by temporary pile bents. These structures are customizable and may not depend on vendors since contractors can design structures using materials they have on hand. With minimal environmental impacts, this "top-down" method limits disturbance to wetlands as a result of pile placement.

Floating structures and vessels — Appropriate for open-water situations, this method is suited for areas classified as navigable water that require permits from the U.S. Coast Guard. Potential impacts include the shading of submerged aquatic vegetation and reduced plant growth/productivity. Construction noise may also decrease the number of fish and aquatic birds and animals that feed on nearby plants.

Causeways—State and federal environmental agencies must approve construction of temporary causeways, and time and mitigation requirements are significant. Although construction materials are usually readily available and cost effective, the potential environmental impacts are high. Vegetation and substrate habitats are eliminated, construction equipment may crush species dwelling in the area, and hydrologic patterns could be negatively impacted.

Railroad flats—This type of temporary bridge is constructed from retired railroad flatcar units with the running gear removed. Although shipping can be costly, installation is quick if site conditions are suitable. Potential impacts include the shading of vegetation under the structure, as well as wetland disturbance from pile placement and driving.

Mats on the ground—Mats are used as temporary bridging to support construction equipment in areas not prone to flooding or tidal influence. This method is the least costly and most time-efficient overall. However, mats damage or destroy native vegetation and can smother small animals.

With each method of temporary bridging, regulatory agencies may require mitigation to offset the environmental impacts. For each type, it is imperative to consult a licensed professional engineer and environmental professional with the appropriate competencies to determine applicability to specific projects and to site conditions.

Bridging success
In 2006 and 2007, a comprehensive research study was conducted to investigate how to use temporary bridge alternatives to lessen the impact on wetlands, as well as to amass beneficial information for decision-making stakeholders. The study, Temporary bridging to avoid or minimize impacts on waters and wetlands during construction, was sponsored by the National Cooperative Highway Research Program (NCHRP) and conducted by Mulkey Engineers & Consultants and the Center for Transportation and the Environment.

The study’s findings were presented at the Southeastern Association of State Highway and Transportation Officials conference in August 2008. For further information about the research study, the presentation, or temporary bridging, visit www.mulkeyinc.com/SASHTO.

The research resulted in a thorough examination of the benefits and limitations of temporary bridging and created a decision-making matrix based on the conclusions. The information has now been developed into a comprehensive guidebook from NCHRP (Project 25-30) to assist state, federal, and private decision makers. Go to www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=765 to learn more.


George Gibson, P.E., is principal of Mulkey Engineers & Consultants, based in Raleigh, N.C. He can be reached at ggibson@mulkeyinc.com or at 919-858-1806.