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GWMP Slope Repair

 

George Washington Memorial Parkway
Northbound Slope Repair

Project Dates: June 22 - July 30 2009



The Project

On December 13, 2002, a rockfall event occurred on a 240 ft long slope located along the northbound lane of the George Washington Memorial Parkway (GWMP), in Arlington County Virginia. The rockfall event involved several large rock blocks that damaged curb and pavement sections of the adjacent road. Initial cleanup activities included limited scaling of the rock slope with a backhoe to remove the most critically unstable blocks. However, there were concerns that potentially unstable rock blocks remained as a rockfall hazard. The geological engineer, Schnabel Engineering North, recommended that rock slope stabilization be performed using rock scaling, installation of rock bolts, and rock gluing techniques. The total area to be reinforced was approximately 2,500 ft.2.

Challenges


The GWMP runs through historic and preserved portions of the greater Washington, D.C. area. The road and adjacent land is controlled by the Federal Highway Administration (FHWA) and the National Park Service (NPS) respectively. As such, preserving the natural appearance of the rock wall post stabilization was of the upmost importance.

The GWMP has an average daily usage of 70-80 thousand vehicles per day. To avoid traffic congestion no lane closures were permitted during peak traffic times, which are in the mornings and evenings. Construction activities extending from the roadway could only be performed from 9:30 a.m. to 2:45 p.m. daily.

The location of the project was in an extremely environmentally sensitive area. Special priority was placed on the protection of trees and shrubs inside and outside of the construction zone. The close proximity of a nesting Bald Eagle also highlighted the importance of minimizing the impact of construction activities.

Safe removal of loose rocks and material through hand scaling with close proximity of the rock wall to the busy roadway.

The highest point of the rock wall that needed to be reached for drilling and grouting applications was over 45' high.

Access to treatment areas needed to be versatile because the rock wall surface was not uniform and contained many outcroppings and rock ledges.

Innovative Solutions


To preserve the natural esthetics of the rock wall, the injection of a Polyurethane Resin (PUR) grout was chosen as the primary means for stabilization. As a secondary source, rock bolts could be used but only if deemed necessary, and if used, the exposed hardware painted to match the surrounding rock.

A series of 33 primary and secondary grout holes were drilled 15' into the rock face at various locations along the wall. The polyurethane and resin were mixed at the injection point and pumped into the 2" holes under pressure. The PUR was pumped until a predetermined pressure was obtained or the grout became visible through cracks in the rock wall. Once completed the PUR setup and acted as the bonding agent that held, or glued, the loose rocks together, and provided a stable rock formation. The entries of the grout holes were then filled with dyed cement that matched the rock face, which concealed the method of stabilization.

The installation of three rock bolts was deemed necessary due to the rock wall condition in one area. The three rock bolts were installed and the exposed hardware was painted to match the surrounding rock.

Also, 20 horizontal drain holes were excavated to a point of 10' behind the planar sliding service to help relieve potential pressure build up from water.

To accommodate the small permitted work time with lane closures, equipment on the project was selected that allowed for work behind an existing barrier for much of the project. When lane closures were permitted, activities needing that access were performed.

With environment sensitivity in mind, all machine oils were switched to biodegradable oils in case of leaks or spills. Also, special attention was given during any refueling operations.

During the scaling of loose rock and material, special techniques and tools were utilized to control the removal of the material. These methods allowed for large boulders and rocks to be removed from heights exceeding 45'and land safely at the foot of the rock face.

The PUR injection points ranged from 5' off the ground to over 45' off the ground. In house design and fabrication of equipment allowed for easy access to drill the grout holes and to inject the PUR grout. The design of the equipment also had to take into account the jagged, un-uniform surface of the rock wall.

Conclusion

The challenge of reducing the rockfall hazard on the George Washington Memorial Parkway was met through the successful rock slope stabilization using slope scaling, installation of rock bolts, injection PUR grout and horizontal drain pipe installation. Equipment design innovations were made to accommodate the topical features and innovative construction techniques were adopted to alleviate risk of rockfall from unstable rock blocks. Scaling significantly reduced the potential for localized rockfall during reinforcement construction activities thereby protecting workers.� The result was a safe, successful project completed on time, on budget, and with minimal disruption to traffic or the environment.

 

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