Paw Paw Tunnel Slope Stabilization Project

Our Client’s Challenge

The serene landscapes of the Chesapeake & Ohio (C&O) Canal National Historical Park are home to some of the most impressive feats of civil engineering in U.S. history. The C&O Canal, one of the first large-scale civil works projects funded by the U.S. Government, stretches 184.5 miles along the Potomac River from Washington, D.C., to Cumberland, Maryland.

Instead of constructing the canal six miles along the Potomac River, the C&O Canal Company constructed a 3,118-foot-long tunnel and open cut through interbedded shale and siltstone. This marvel, known as the Paw Paw Tunnel, facilitated boat passage, transporting coal from Maryland to Washington D.C. Completed in 1850, the Paw Paw Tunnel is a historic structure and a vital piece of park infrastructure beloved by hikers, bicyclists, and tourists for its unique charm and scenic beauty.

However, years of natural wear and erosion threatened the safety of the tunnel’s slopes. The rock formations became increasingly unstable, posing significant risks to the structure and park visitors. Frequent rockslides and erosion compromised the structure’s safety and integrity. Critical intervention was necessary to stabilize the slopes while preserving the area’s historical legacy and environmental essence.

The Solution

The National Park Service (NPS) selected Gannett Fleming as the construction management firm overseeing the ambitious Paw Paw Tunnel Slope Stabilization Project. Our team collaborated with the NPS, the design-builder, historians, environmental scientists, and local stakeholders to develop a comprehensive project plan.

Extensive planning and analysis were necessary to develop a sound construction strategy. During the design phase, the design-builder conducted detailed geological surveys and stability analyses to understand the slope composition and behavior. These surveys were crucial in designing a stabilization plan that would effectively mitigate the risks without causing undue harm to the surrounding rock.

Rock bolts made from high-strength steel and high-tensile strength netting were installed at strategic points to stabilize loose rocks, prevent future slides, and capture any dislodged debris. The project also included drilling holes to install new drainage systems that redirect water away from the slopes, reducing water-induced erosion and instability. The rock bolting and drainage improvements dramatically increased the structural systems’ long-term slope stability.

Vegetation is an often overlooked component of slope stability. While certain vegetation types can help hold the rock and soil together, invasive species can disrupt and destabilize the ecosystem. The team removed invasive species from the slopes and planted native vegetation, enhancing soil stability, geological conditions, and ecological value.

Like any large-scale project, the Paw Paw Tunnel Slope Stabilization faced its share of challenges. One significant hurdle was working within the constraints of a historical building site. Our construction oversight ensured all interventions were reversible and did not permanently alter the tunnel’s historic fabric. The project:

Reduced landscape visual effects by painting all new rock bolts and internal rock drains to match the existing geology, coloring installed rock fall mesh to blend with the adjacent rock hues, and sculpting and coloring 15 shotcrete shear keys to blend with the adjacent geologic features.
Minimized rare, threatened, and endangered species impacts, including those found in the very rare Central Appalachian shale barrens ecosystem.
Utilized articulated dump trucks and helicopters for material deliveries, in addition to conventional heavy equipment, due to the tight site constraints.

As the construction manager, we ensured stabilization efforts were practical, minimally invasive, and respectful of the tunnel’s historic significance. This multifaceted approach balanced modern historical preservation engineering solutions with preserving the tunnel’s historic integrity, demonstrating that infrastructure projects can enhance safety and heritage through careful planning, collaboration, and respect for history.

Key Features

Strategically installed painted rock bolts to support slopes and stabilize loose materials.
Draped earth-toned, high-tensile strength netting over the rock masses to catch any dislodged debris.
Installed new painted drainage systems to redirect water from the slopes, reducing the risk of water-induced erosion and instability.
Removed invasive species and planted native vegetation.
Combined drilling, grouting, and anchoring to reinforce rock slope stability.

Outcomes

Created a safer park environment, ensuring it remains a cherished destination for future generations.
Established visual landscape effects that blend with existing geologic features.
Enhanced the tunnel’s ecological health.
Minimized impacts on rare, threatened, and endangered species in various habitats.