Engineering best practices for low-volume roads

Gordon R. Keller, geotechnical engineer, US Forest Service, introduces a unique field guide and training programme to promote road engineering with a holistic perspective

Low-volume roads have long been known to contribute to substantial erosion and sediment production, as well as create other problems such as channel modifications or slope instability. 60-70 per cent of the roads in most countries are “rural” and often unsurfaced or with only a gravel surfacing. Roads are a basic part of rural infrastructure and needed for development and access to critical areas such as schools and clinics, as well as movement of goods and services, etc.

Roads are also relatively expensive. Thus, design and maintenance practices should be used that help prolong the useful life of the road and minimise problems. Roads Best Management Practices, or BMPs, have been drawn up and are particularly useful for developing long-term, cost-effective designs for roads – designs that incorporate mitigation measures to minimise adverse environmental impacts, protect water quality, minimise the need for maintenance and make roads more resistant to the impacts of storms.

Good road engineering
Good road engineering today involves a blend of three basic components:
1. Application of Basic Engineering and Design Concepts: including good planning and location; drainage analysis; good road surface drainage; proper use and installation of culverts, fords and bridges; building stable slopes and use of slope stabilisation measures; proper use of roadway materials; and appropriate road maintenance.

2. Environmental Awareness and Application of Practical Environmental Mitigation Measures: such as erosion and sediment control; water quality protection; fish passages and wildlife crossings; and invasive species control.

3. Use of Appropriate, Innovative Technologies: to facilitate work and make it more cost-effective, such as GIS mapping; use of geosynthetics; trenchless technology; mechanically stabilised earth structures; biotechnical erosion control and slope stabilisation measures; and simple in-situ site characterisation tools. Many useful references exist today to help promote and guide road work in an “environmentally friendly” and technically sound manner.

The forest service of the US
Department of Agriculture (USDA) has an extensive low-volume road system across the United States, and operates in a very environmentally conscious world. Thus, it has developed many useful design techniques and mitigations, and has presented Roads Engineering Best Practices training to numerous agencies and groups worldwide – introducing the subjects of “road engineering” and environmental protection with a “holistic” perspective. Much of this information has been summarised in a recent publication, ‘Low-Volume Roads Engineering Best Management Practice Field Guide’, written by G. Keller and J. Sherar. It is currently available in English, Spanish and Portuguese and this technically based and environmentally conscious training for low-volume roads is unique. Courses have typically been from two to five days long, and ideally include time in the field evaluating local roads.

Some of the key objectives of Roads BMPs are to:
• Produce a safe, cost-effective, environmentally friendly and practical road design that meets the needs of the users
• Protect water quality and reduce sediment into water bodies
• Avoid land use conflicts
• Protect sensitive areas and reduce ecosystem impacts
• Maintain natural channels, flows and passage for aquatic organisms
• Minimise ground and drainage channel disturbance
• Control road surface water and stabilise the roadbed driving surface (see lack of water control in Figure 1, and measures to control and prevent the concentration of water in Figure 2)
• Control erosion and protect exposed soil areas
• Implement slope stabilisation measures where needed
• Avoid problematic areas
• Stormproof and extend the useful life of the road.

Storm damage repairs
Millions of dollars are spent annually in the US and other countries on storm damage repairs. Most of this work is to repair existing roads and transportation facilities, many of which are not well designed or maintained.  Agencies cannot afford to build roads to be 100 per cent storm resistant, or “stormproof”. They can, however, make them  more storm resistant, and measures can be taken to reduce the risk of storm damage from any given event, particularly through the application of Roads Best Management Practices.  

Storm damage risk reduction measures include many maintenance, drainage improvement and structural items. Roadway surface drainage structures, such as ditches, cross-drains and rolling dips need to be clean, properly armoured and properly spaced to prevent concentration of water. Drainage crossing structures, such as bridges, fords and culverts, need to have adequate capacity; or at least be clear of debris, well-armoured, scour resistant and functioning properly. Trash racks can be added. Marginally stable road cuts and fills can be stabilised with retaining structures, drainage and modified slopes, and reinforced with vegetation or soil bio-engineering treatments, etc. (see figure 3).

Since low-volume or rural roads are very much needed, but expensive, it is important to build roads in a manner that minimises their long-term cost and that maximises their usefulness, while limiting negative environmental impacts. The application of BMPs helps to achieve those goals. The Low-Volume Roads Engineering Best Management Practice Field Guide is available at the following Forest Service International Programs website /www.fs.fed.us/global/topic/welcome.htm#8

The above article is extracted from Volume 1 of the recently published IRF Bulletin on Rural Transport. A second volume on the same theme will be available in Spring of this year and both complement the earlier companion Bulletin devoted to Urban Mobility. All volumes may be freely downloaded in PDF format from the www.irfnet.org website.

Throughout the years, the IRF Bulletins have proven a great success in bringing the work and experiences of IRF members and other specialists around key themes to the attention of leading international stakeholders.

The next in this lively and informative series will be devoted to the Environment. The call for contributions for this issue is now open. Articles should be around 500-1,500 words in length and should include illustrations and photographs. They may be submitted through the IRF Geneva office. For more information visit www.irfnet.org or e-mail to info@irfnet.org

For more information:
Gordon Keller, geotechnical engineer
E-mail: gkeller@fs.fed.us