UMass Amherst Serves as National Test Site for Evaluation of Soil-Nailed Walls

AMHERST, Mass. - For anyone who has wondered about those large piles of dirt along the western edge of the University of Massachusetts campus, next to the wastewater treatment plant, consider the mystery solved.

That particular site is a National Geotechnical Experimentation Site, one of only five in the nation established by the Federal Highway Administration (FHWA) and the National Science Foundation NSF) to evaluate techniques for building safer and more economical highways and bridges. According to Carlton Ho, associate professor of civil and environmental engineering, the work currently under way at UMass is part of a research project testing the construction of soil-nailed walls, a type of retaining wall used as temporary and permanent shoring for a vertical soil cut.

Ironically, says Ho, the goal of the UMass experiment is to have the wall collapse – which it did – so as to learn more about why these structures might fail during an actual construction project.

Ho is overseeing the UMass portion of the research, which is a cooperative project with Thomas Sheahan, associate professor of civil engineering at Northeastern University. FHWA and the Association of Drilled Shaft Contractors are providing the funding.

While soil-nailed walls are used during excavation – in building a deep basement, for example, in close proximity to another building or a street – they are also used as a permanent construction, such as along a highway. Primarily because of soil conditions, they are presently more commonly used in the western U.S. and in the mid-Atlantic states than in the Northeast, where there is a large amount of rock. They are, however, currently being utilized as part of the Central Artery project, or the "Big Dig," in Boston.

In the construction of soil-nailed walls, explains Ho, six feet of soil is excavated and long, specially threaded steel bars are inserted into the wall in pre-drilled holes and secured in place with grout. This provides support to the soil mass in much the same fashion as reinforcing steel bars in concrete. Shotcrete, a mixture of sand, cement, and a gravel-like substance, is then applied to the surface of the wall over wire mesh to support the vertical cut. The procedure is repeated at five-foot intervals to the depth intended.

The soil-nailed wall constructed at UMass failed according to plan, says Ho.

"This is a controlled failure," says Ho. "Though these walls are conservatively designed and rarely fail, we’re hoping to determine how they perform when they occasionally do fail."

In the UMass experiment, the soil shifted and a large cavity began to appear behind the wall, which started to sag and eventually began to develop a huge crack. Sophisticated instrumentation has been installed around the site to help in analyzing the failure. "What we learn will hopefully help to determine a better design for soil-nailed structures," says Ho.

Ho is hopeful that the project findings will contribute to the wider use of soil-nailed walls. "This is an exciting project," he says, "and one that is important nationally." The research findings will be published and widely disseminated by FHWA, according to Ho.

Analysis will continue for about another month at the site, which will be totally restored to its former appearance at the conclusion of the research.

The site has also been used for past experiments. Most recently, during the summer of 1994, it was used to conduct research on buried pipes. According to Ho, other parts of the site are being used for research in embankment construction, shallow foundations, and soil anchors.

Other participants in the current project are Construction Drilling Inc., of North Adams, Mass.; Dywidag Systems International, of Arlington, Texas; Foundation Services Inc., of Greensboro, North Carolina; and Rich Goettle Co., of Cincinnati, Ohio.