Design guidelines for bioengineering
Despite hundreds of years of practice, few objective design guidelines have been developed for soil bioengineering techniques. Therefore, when slope stabilization and erosion control measures are put in place to protect valuable assets such as highways and bridges, stakeholders are sometimes reluctant to support these techniques due to a lack of scientific and engineering support.
To develop these needed guidelines, Mn/DOT has funded research by principal investigator Omid Mohseni of the University's St. Anthony Falls Lab, co-principal investigator Bruce Wilson of the Department of Biosystems and Agricultural Engineering, and recent grad Jeff Weiss, now of Barr Engineering.
So-called "hard armoring," or structural, techniques on stream banks are not always as effective as previously thought, Weiss began, so alternatives are needed. "Soft armoring," using vegetation, has many advantages: it is flexible and self-repairing (unlike riprap), and plants will colonize an area. It also increases habitat and is aesthetically pleasing.
The trick is to choose appropriate techniques depending on site conditions, which is the goal of this study—to give "the numbers and equations," he said. The objective is to review current design procedures by groups such as the USDA's National Resources Conservation Service and the Army Corps of Engineers as part of an extensive literature search to understand both the history and the state of the art of soil engineering.
The basic principles of bioengineering are simple: fit the project into a site; retain existing vegetation and limit removal of vegetation; and stockpile and protect topsoil. In addition to their ability to adapt to the local climate, plants offer advantages such as low cost and lower longer term maintenance costs, plus improved water quality. And vegetation comes in handy for steep sites where machinery access is difficult.
There are some disadvantages, however. For one, the approach is labor intensive and can require special training. Also, plants are very vulnerable during maturation, so sites may need to use both structural and biological approaches. And once a site reaches maturity, there is still a risk of deterioration due to mismanagement. Conventional practices are still more accepted by society and contractors, and the installation season is limited to spring and fall.
The researchers have examined the fundamental physics behind soil bioengineering and have identified many areas of research that could contribute to the further development of design. In addition, they have explored several outdoor research sites for conducting the needed research.
The final report is due to be published later this year. For more information contact Mohseni at omohseni@umn.edu.
Source material adapted with permission from Omid Mohseni.