The transportation community is recognizing the importance of fish and wildlife passage.

By Bob Barrett, Al Ruckman, and John Steward

Serendipitous. That's how we describe the recent combinations of research findings in bridge abutments and scour prevention regarding structures that provide better fish and wildlife passage - and at lower costs.

Evolving needs often outpace technologies to fill those needs, and even more important, evolving needs often outpace paradigms that recognize those needs. Change seems to take forever, and it is typically restrained by paradigms. We tend to repeat our successes rather than improve our designs.

Fish passage. Open-bottom boxes. Elimination of riprap. Less-expensive bridges. These concepts are now gaining momentum. It is important that salmon reclaim former spawning grounds. It is important that bears and badgers have the option of walking under our highways.

Our engineering community is charged with developing and maintaining our world's best transportation infrastructure. This is an awesome responsibility. A key to top performance and product delivery is the allocation of funding. Our engineering culture (paradigm) developed in an era when folks were just thankful to "get out of the mud." Transportation managers continue to jealously protect the limited funding for least-cost construction and maintenance of our highways.

However, in getting out of the mud quickly and for the least cost, unfortunate shortcuts were taken. We see steep, poorly constructed cuts and fills that now are sliding, eroding, and polluting. We see streams and rivers narrowed and retrained with massive rock revetments, causing loss of valuable habitats. Pipes and culverts typically were sized with minimal openings, often resulting in inadequate accommodations for all the potential future customers, including fish and wildlife.

Roads restrict the ability of fish and wildlife to go about their daily business. Wildlife and fish have declined worldwide in part because of roads slicing their habitats into parcels too small to make a living. "It's analogous to separating your kitchen and bedroom - forever," explains Sandra Jacobson, a wildlife biologist who specializes in wildlife and highway interactions at the United States Department of Agriculture Forest Service's (USFS) Pacific Southwest Research Station. "Several legally threatened or endangered fish and wildlife species are seriously hampered by highways restricting their movement to breeding or feeding areas, with the result that millions of dollars are being spent on public lands and federal and state highways to facilitate their passage."

Many species of wildlife will use underpasses to cross highways, but only if the openings are large enough for them to feel safe and unconfined. "Imagine a moose ducking under a 6-foot culvert," says Jacobson. "They won't do it. Generally, bigger is better for wildlife because it provides more visibility and more appearance of a natural passage. Fish like bigger better also, because more of the natural functions of a stream can be accommodated with larger openings."

These passages work best when they are large and frequent, without travel-hindering riprap along otherwise suitable streamside pathways. Biologists now recommend large, open underpasses for fish and wildlife passage but typically meet resistance because of the cost. The most common reason underpasses fail to meet the needs of wildlife passage is that the structures are too small for the species they are intended to serve.

"Fish and wildlife prefer natural substrates too," notes Jacobson. "Imagine again the moose attempting a crossing within a corrugated metal pipe. The noise, as well as the size of the opening, is far too foreign to say, OIt's safe here.'"

A new paradigm that includes higher priorities for environmental elements is gaining acceptance. The major obstacle here is funding priorities. How do we equate highway safety with, say, restoration of fish passage? Would we cancel a project that includes critical safety features so that a small culvert can be replaced with a bridge to facilitate elk migration?

There's good news. In the case of underpassage, we can adapt new landslide and retaining wall technologies to build larger open-bottom boxes and short bridges for the same or lower cost as traditional concrete and steel boxes and bridges. This article provides an overview of what we can now expect from the transportation community as it implements these new concepts.

Geosynthetically Reinforced Soil

A GRS bridge abutment built in Jamaica on 90 ft. of "zero blow count" soil. It is not embedded.

Author John Steward and John Mohney of the USFS introduced a new concept to the US in the 1970s that employed geosynthetic sheets as earth reinforcement to construct retaining walls. Subsequent research by the Colorado Department of Transportation, the University of Colorado at Denver, and many others improved and expanded the science of geosynthetically reinforced soil (GRS). GRS structures have been shown to be easier to build, longer lasting, more earthquake resistant, and less expensive than traditional retaining walls and bridge abutments.

Wider implementation of GRS technology has been the goal of researchers and engineers in government and industry for many years. Large sums of money are being spent to foster acceptance and improve design criteria. Currently National Cooperative Highway Research Program (NCHRP) Project 12-59 is charged with developing design and construction guidelines for GRS bridge abutments. NCHRP Project 22-23 is evaluating backfills for this GRS composite. Yenter Companies of Arvada, CO, is the leader in private-sector GRS research and development. This design/build group has taken implementation of generic GRS technologies several steps beyond practices recognized in the major guidelines.

GRS retaining walls are becoming widely accepted even though some constructions use older, less efficient design criteria and construction methods. Use of GRS bridge abutments is lagging, although in many cases these structures are superior to traditional concrete abutments on deep foundations. This is particularly true in box culverts.

Open-Bottom Box Culverts

Open-bottom box for skier underpassage at Breckenridge, CO. This project was designed with a three-sided, precast concrete box culvert.

This open-bottom box across a seasonal wetland is founded on the ground and not embedded. This box value engineered to replace a concrete structure that was to be embedded 5 ft. into the soft soils.

State and federal aquatic passage design requirements in the Western US call for a "bankfull" or wider opening with natural inverts simulating adjacent natural stream bottoms. The 48-in.-diameter pipe that adequately passes the flow of water to protect the road might need to be replaced with a structure that has a 10- to 20-ft.-wide opening. Open-bottom culverts often best meet these requirements.

"Most wildlife are more sensitive to the width of an underpass than its height - assuming the minimum height for the animal in question is met," says Jacobson. "Wide underpasses allow animals to have a broad viewing area, which makes them feel less vulnerable." Wide underpasses are also more likely to provide a dry surface next to a stream and more riparian cover nearby. Because the dry edges of streams typically are used for travel corridors - even by aquatic species, such as some salamanders - a dry area alongside a stream is much more likely to be used than an underpass that is abutment-to-abutment water.

Environmental needs and demands for better passages for fish and fauna can be met with GRS bridge abutments as sidewalls and concrete panels as "lids" on top. These types of structures are significantly less expensive and much less intrusive. Construction costs to date show that GRS boxes can provide savings in the range of 25-50% compared to three-sided boxes and arches. An owner in Montana saved $700,000 in replacing deep foundations with GRS abutments on a Highway 191 single-span bridge.

Lower cost is only one of the benefits, however. These abutments or sidewalls can be built on very soft foundations without piling and without large excavations into the wetlands. Green concrete is not required. Time of construction often is less than half that of other methods. In some situations time savings is more important than cost savings; we have built boxes and small bridges in one day, avoiding expensive, disruptive detours. Minimizing collateral damage to the adjacent areas can also supercede cost savings.

GRS abutments do not produce the "bump" at the bridge. They are superior in seismic events, particularly with the new "earthquake wing" concept added to the design. Their major components, gravel backfill and high-end geosynthetic reinforcements, will outlast concrete and steel.

Scour Prevention

The soil nail launcher can insert steel bars into the ground at speeds exceeding 200-plus mph and into live streams, avoiding the need for diversion and pumping.

The one reservation or restriction to all open-bottom boxes is scour. Scour is a complex, poorly understood natural process in which the bed of the stream can become fluid during high-flow events. This temporary deepening of the channel can remove lateral support for nearby structures on shallow foundations.

Scour prevention is the primary reason concrete box culverts have a concrete floor, and it is a reason for embedding traditional arch culvert foundations and three-sided concrete box culverts deep into the streamside. Scour prevention is also the reason heavy rock (riprap) is placed on streambanks and shorelines.

With GRS abutments, closely spaced geotextile layers preclude a major failure caused by localized scour. Launched soil nails can be used to prevent more serious scour and eliminate the need for riprap and deep foundations.

In 1992, Steward, then the national chief USFS geotechnical engineer, headed a demonstration program that introduced the British soil nail launcher to the US. This tool launches 1.5-in.-diameter steel bars into the ground at speeds exceeding 200 mph. Launched nails can stabilize landslides for half the cost of traditional H-pilings or shot rock fills.

Authors Bob Barrett and Al Ruckman recently discovered that launched nails can be inserted in a close array at the mouth of GRS box culverts to prevent scour. These launched scour micropiles can eliminate the need for riprap and disrupting excavations. As with the GRS sidewalls, stream diversion and pumping can be avoided because the launched scour micropiles can be inserted through live stream flow. Costs and permitting hassles are also reduced.

Scour Platform

Even with launched scour micropiles installed to minimize scour, it is important to avoid any possibility of scour or erosion damage to the shallow foundations of the GRS sidewalls. We have developed a complementary adjunct called a scour platform, making use of a field-formed reinforced soil platform installed nominally 2 ft. into the streamside alluvium. This scour platform is constructed in a closed-sided trench and is not pumped. Construction is generally limited to the future disturbed area during this brief installation time.

Conclusion

A fortuitous combination of emerging technologies has been brought together to provide more environmentally friendly passages under highways and other manmade features. Traditional culverts, including pipes and four-sided concrete boxes, often act as barriers to many species of fish and fauna. Newer metal arch culverts and three-sided concrete underpasses are more expensive and require deep sidewall foundations, resulting in disruptive excavations and wet concrete in and near the stream. Traditional open-bottom passages also are susceptible to scouring that could result in failure, thus requiring riprap or piling to be incorporated in their design.

GRS technologies provide a new sidewall support that can sit on the ground beside the stream. A pair of these with a lid provides a cost-effective bridge. The span or opening width can be any dimension.

This merger of technologies and methods has produced a box culvert/bridge that can be built at a lower cost than traditional options and better meets environmental needs. Permitting is typically less complicated. Transportation managers now are looking at how to incorporate these new boxes in remedial and new constructions.

For more information on the issues surrounding wildlife and fish passage across highways and how biologists and engineers across the world have found innovative solutions, see the Wildlife Crossings Toolkit Web site at www.wildlifecrossings.info.

Note: The concept of using scour micropiles to prevent scour and the scour platform concept have been included in patent applications. The concept of launching soil nails has been granted a US patent.

Bob Barrett is manager of bridge design and construction for Yenter Companies, a Colorado-based geotechnical design/build firm. He is president of Soil Nail Launcher Inc. and panel chairman of NCHRP Project 12-59. Al Ruckman, P.E., is a civil/geotechnical engineer and former Colorado Department of Transportation engineer and researcher. He is president of Yenter Companies and is responsible for the design and construction of more than $200 million worth of reinforced soil constructions of all types. John Steward, P.E., is a consulting geotechnical engineer for PBS Engineering and Environmental in Vancouver, WA. He was chief geotechnical engineer for the USFS and an internationally recognized geotechnical innovator.

EC - November/December 2003