Publication Number: FHWA-HRT-11-010
Date: January/February 2011
Printable Version (.pdf, 1 mb)

An Innovative Solution for Rock Slope Stabilization

How do you stabilize surrounding rock slopes and protect drivers from rockfall hazards while preserving the scenic beauty of Virginia's George Washington (GW) Memorial Parkway? For the Federal Highway Administration's (FHWA) Eastern Federal Lands Highway Division office and the National Park Service, the innovative answer proved to be "rock glue."

After rocks from a 10.7-m (35-ft) high, 73-m (240-ft) long rock cut slope located along the GW Parkway in Arlington, Virginia, fell onto the road's shoulders and travel lanes in 2002, damaging sections of the curb and pavement and disrupting traffic, FHWA contracted with Schnabel Engineering to both investigate the slope failure and develop slope stabilization design alternatives. As traditional stabilization methods such as rockfall netting, rock anchors, and rockfall control barriers can have an unsightly impact on the visual appearance of rock slopes, something new was needed.

At the same time, FHWA's Central Federal Lands Highway Division office was investigating a technique known as rock gluing to achieve slope stabilization. Used since the 1960s in tunneling, mining, and dam projects to control water seepage and stabilize tunnel crowns, among other objectives, the technique involves injecting polyurethane resin (PUR) grout into a rock mass. The PUR hardens and adheres to the rock, bonding discontinuous individual rocks into a bigger, more stable, continuous mass and preventing water intrusion into the cracks. Any excess rock glue can be cleaned off, resulting in a natural-looking rock slope with greater structural integrity and slope stability. The Central Federal Lands Highway Division office's application of the technique was the first designed use of rock gluing for slope stability. More information on the technique can be found in FHWA's Polyurethane Resin (PUR) Injection for Rock Mass Stabilization (Pub. No. FHWA-CFL/TD-08-004), which is available online at http://www.cflhd.gov/programs/techDevelopment/geotech/PUR.

A view of a rock cut slope on the George Washington
Memorial Parkway in Arlington, VA.

"This is not a new method but it was new to the transportation field," says Khalid Mohamed of FHWA. "It was particularly effective in this case because of the aesthetics of the roadway. The National Park Service did not allow the use of rockfall netting or barriers on the GW Parkway because of concerns with the historic and aesthetic nature of the Parkway." The rock gluing was supplemented by rock anchors in one location and horizontal drains that were installed to relieve groundwater pressure that could contribute to slope failure. The glue injection holes were spaced 3 m (10 ft) apart and drilled 4.6 m (15 ft) deep behind the rock face. Schnabel determined the spacing and depth based on the spacing of the rock mass discontinuities and expected size of rocks in a rockfall.

"The glue used was high in tensile strength," says Mohamed. The technique not only improves the appearance of the rock slope but better supports the entire rock mass and prevents the potential for localized rockfall and raveling between rock anchors. While a high-strength PUR grout has a higher unit cost, use of the glue can help reduce overall project costs. The GW Parkway project costs included approximately $315,000 for rock stabilization measures, with the use of PUR accounting for about $150,000 of that.

The technique has also been used by Federal Lands for a project in Great Smoky Mountains National Park in Tennessee and two projects currently under construction in North Carolina and Pennsylvania. "Its use has been very effective," says Mohamed. "Since the technique is new to transportation applications, however, quality control and quality assurance are very important."

Completed in 2009, the GW Parkway project was honored by the Association of Environmental and Engineering Geologists (AEG) as its 2010 Outstanding Environmental and Engineering Geologic Project. AEG cited the project for representing a "significant advancement for the engineering geology profession."

For more information on the LTBP program, visit https://www.fhwa.dot.gov/research/topics/infrastructure/bridges/subindex1.cfm (select "Long Term Bridge Performance"), or contact Hamid Ghasemi at FHWA, 202-493-3024 (email: ltbp@dot.gov).

A close-up view of installed rock bolts on a rock cut slope overlooking
the George Washington Memorial Parkway in Arlington, VA.

Two workers prepare a rock cut slope for injection of polyurethane resin
or "rock glue" on the George Washington Memorial Parkway in Arlington, VA.

Back to Articles in this Issue