The federal Endangered Species Act drives much planning for how to avoid impacts to species listed as threatened or endangered. Floodplains are protected by federal Executive Order 11988, the Rivers and Harbors Act, and the Clean Water Act. State laws and community expectations impose additional requirements. State DOTs are developing many creative approaches to meet these demands and their own environmental stewardship commitments. DOT and state DOT-DNR cooperative initiatives to identify and develop standards and methods for improving fish passage are discussed in Chapter 3.5 on Culvert and Fish Passage Design Practices. Other enhancement practices for bridges are discussed in the following section, 7.3.

Impact assessment is a standard part of the NEPA process, as well as compliance with other environmental laws. A few DOTs have taken this a step further by systematically identifying opportunities for avoidance, minimization, and/or environmental enhancement on projects or across classes of projects.

Oregon DOT is preparing an Environmental Baseline Report for every bridge requiring improvement over the next 20 years, to inform design teams of all opportunities to avoid or minimize impacts. Environmental Performance Standards serve as a single, common set of terms, conditions and design targets that apply to all bridge projects and form the basis of programmatic or batched permits from multiple agencies. A Comprehensive Mitigation and Conservation Strategy (CMCS) that integrates wetlands mitigation with habitat conservation, through a series of regional banks.

As bridge owners, DOTs continually seek ways to build more durable structures, safely, and in an environmentally sound fashion. In sensitive environments such as stream channels and floodplains, resource agencies often have an interest in confining construction to certain windows of time, to minimize impacts. There are times of the year when the effects of pollution from bridge maintenance and repair would cause the most damage and times when the damage would be minimal. The exact timing depends upon the site and the species involved. Practices may include:

• Scheduling bridge maintenance to avoid egg incubation, juvenile rearing and downstream migration periods of fish.
• Calling upon state DOT fish and wildlife specialists or local fish and wildlife agencies for assistance in scheduling to avoid aquatic impacts.

For example, TxDOT has modified timing of maintenance modifications to protect bats in bridges, including postponing tree trimming and/or bridge maintenance work until outside of bat season.[N]

DOTs and resource agencies may find common ground in the interest in fast construction, if resources can be mobilized and the project completed in the time allowed. To meet these needs and reduce the time to construct a bridge while maintaining or improving quality, prefabricated structures, more rapidly constructible details, and self-propelled modular bridge transporters are becoming more common.

Using prefabricated bridge elements and systems makes construction less disruptive for the environment. As traffic and environmental impacts are reduced, constructability is increased, and safety is improved because work is moved out of the right-of-way to a remote site, minimizing the need for lane closures, detours, and use of narrow lanes. Prefabrication of bridge elements and systems in a controlled environment without concern for job-site limitations can increase quality and lower costs, especially where use of sophisticated techniques would be needed for cast-in-place, such as in long water crossings or higher structures, like multi-level interchanges. NCHRP Synthesis 324 recently concluded that while prefabricated bridge components are more expensive in some cases, environmental impacts are reduced, quality is generally higher, and costs may fall as standardization increases in the industry. [N]

Precast bridges consisting of pretensioned girders, posttensioned spliced girders, trapezoidal open-box girders, and other types of superstructure members are becoming more common due to their potential for accelerating construction and solving constructability issues in certain cases. Handling and shipping limitations often control the span capability of pretensioned girders. Spliced girder design has provided a solution where a one-piece pretensioned girder could not otherwise have been used. [N] Tennessee is developing bridge systems for steel bridges that can be erected similarly to precast, prestressed beams made continuous, with cranes of the same or lower lifting capacity, and can be fabricated at a reduced cost to be competitively priced. [N]

In February 2003, FHWA and AASHTO sponsored a conference showcasing the successful uses and benefits of prefabricated bridge elements and systems. Along I-40 in Oklahoma where the cost of labor and materials to replace a collapsed bridge across the Arkansas River was $11.8 million, while the cost to control traffic and detours around the construction zone was $12 million, the higher upfront costs of prefabricated materials were offset by the amount of money saved by reduced construction times. In San Juan, Puerto Rico, construction crews were able to minimize traffic delays and reduce construction times by using prefabricated bridge modules, while building four bridges at a congested intersection of a four-lane arterial. Crews completed the first bridge within 36 hours, and construction of each of the other three bridges took only 26 hours, using prefabricated elements. The conference profiled several new technologies and construction techniques, including new specifications for high-performance concrete and steel and self-compacting concrete, and contracting procedures that ensure that bridges are built within one day and charge high penalties for any delays.[N]

Figure 7 : Prefabricated Bridge Construction - Hawaii DOT

Using prefabricated substructure elements reduces the amount of heavy equipment required and the amount of time required on-site for heavy equipment, causing less disruption to sensitive environments.[N] For example, the Hawaii DOT's Keaiwa Stream Bridge minimized environmental disruption because deck topping did not require shoring or falsework in the streambed, and minimized traffic disruption because precast planks were fabricated during pier construction. [N]

In North Carolina, to avoid placement of heavy equipment in a sensitive environment on the Blue Ridge Parkway, the Linn Cove Viaduct on the Blue Ridge Parkway was built in one direction from the south abutment to the north almost entirely from the top down. The only exceptions to the top down method were construction of the initial span on falsework and construction of a temporary timber bridge that enabled the micropile foundation drilling machine to prepare several of the foundation sites ahead of the superstructure erection. Precasting each segment of the bridge allowed construction workers to assemble the bridge with little impact to the most environmentally sensitive section of Grandfather Mountain. This bridge proved that a design could be environmentally sensitive in addition to being utilitarian and economical.[N]

The Wolf River Bridge in Fayette County, Tennessee, crosses sensitive wetlands and carries the only east-west route through its geographic region. TDOT designers selected precast prestressed beams to facilitate speedy construction and allowed optional stay-in-place precast prestressed concrete deck forms. TDOT and the contractor developed details for precasting bent caps in two pieces to suit staged construction. Construction of the 1,408-foot long, 46-foot wide bridge was completed in eleven months without putting any equipment in the surrounding wetlands. Photo Credits: Tennessee Department of Transportation. [N]

TxDOT developed two new bridge superstructure systems that have maximum span lengths of 115 ft and a total superstructure depth of only 38 in. and are totally prefabricated: a steel tub girder and a prestressed concrete pretopped U-beam. The steel tub-girder system uses a conventional prefabricated trapezoidal steel girder, which is topped by a concrete slab before transport to the bridge site. To achieve the shallow superstructure depth of 38 in., shoring the beams during slab placement makes them composite for all loads. After slab placement, the beam is hauled to the bridge site and erected on the bridge piers. A simple cast-in-place closure pour joins the deck girder sections after they are in place. The prestressed concrete pretopped U-beams use a portion of the existing Texas U-beam form system. Each beam is fabricated as a closed U-beam and hauled to the contractor's yard, where a 4-in. topping is placed before beam erection. A cast-in-place closure pour joins the deck girder sections after erection. Texas DOT anticipates that these two systems will be used over the next 10 years for the rapid construction of nearly 150 bridges that cross I-35 in central Texas. Construction of the first four such structures begins in spring 2005. It is expected that girder erection and closure-pour placement will take less than 24 hours and that bridges will open to traffic after as few as 4 days.[N] In 2006, WSDOT is beginning research on Precast Systems for Rapid Construction of Bridges that will build on TxDOT's experience and experimentally verify that precast systems can be constructed to pass rigorous seismic standards.[N]

FHWA's segmental concrete bridge technology website offers resources and best practices in SCBT bridge use, a method of joining multiple cast-in-place or precast bridge elements to form a continuous span. Precast segmental concrete construction is especially useful and efficient in difficult construction sites, which have included both urban and natural areas. FHWA's website addresses engineering issues and construction methods, and features a photo gallery and an archive of "Ask the Experts" questions that have been submitted by site users and answered by team members. Geosynthetic Reinforced Soil (GRS) abutments and walls can help reduce the access needed for large equipment.[N]

Prior to the use of self-propelled modular transporters (SPMTs), rapid bridge replacement techniques were often limited to prefabricated bridge components assembled in the field. SPMTs now allow erection of entire bridge superstructures, which can then be replaced quickly within timing restrictions and without cranes, temporary or extensive detours, or traffic delays. In addition, construction of bridges in a controlled environment can maximize the quality of the finished bridge. Hydraulic, self-propelled platform trailers, supporting structures, jacking systems, barges, and project-built solutions, bridges with a weight of up to 6,000 metric tons (6,000 tonnes) have been moved transversely and longitudinally into precise position. The method is becoming increasingly common in Europe, for both highway and railway bridges.

An international scan ( Japan, Netherlands, Belgium, Germany, France) was conducted in April 2004 to learn how other countries are using prefabricated bridge components to minimize traffic disruption, improve work-zone safety, minimize environmental impact, improve constructability, improve quality, and lower life-cycle costs. The top implementation recommendation from the scan team was the use of self-propelled modular transporters to move bridges into position in hours rather than the typical months required for conventional bridge construction.[N]

Geosynthetic Reinforced Soil (GRS) abutments and walls can reduce the space needed for access by large equipment in building bridge abutments. FHWA has completed a substantial amount of research on this technology and feels it has great potential for future application. (See www.tfhrc.gov, geotechnical programs, "Performance Test for Geosynthetic-Reinforced Soil Including Effects of Preloading, FHWA-RD-01-018" and "Effects of Geosynthetic Reinforcement Spacing on the Behavior of Mechanically Stabilized Earth Walls, FHWA-RD-03-048.)"