Cambridge Turnpike Improvement Project

The Town of Concord, MA is a residential institutional, commercial, and cultural center west of Boston with a significant place in American history. The Cambridge Turnpike is a major corridor for commuters (8,000 vehicles/day) connecting Lexington Road and Crosby Corner (Routes 2 and 2A), but also serves residences and institutions and portions are within the important “American Mile” from our nation’s origins.

Since its original construction in 1805, the Cambridge Turnpike has continuously settled into the two marsh areas where it crosses Mill Brook, subjecting it to chronic flooding. At these two marsh crossings, there are significant deep peat and other organic materials in the subgrade that compress with any new vertical loading. As a result, the roadway had settled continuously with repeated attempts to build it back up to mitigate the flooding. This chronic flooding took the corridor out of service annually or more frequently and posed significant safety concerns. Design followed the Town completing a FEMA 100-yr flood level modification which supported raising the elevation of the roadway and bridge/culverts in the marsh areas by up to feet.

Investigations revealed that over 1,400 feet of marsh crossings have deep peat and organic layers that have negligible bearing strength and required extensive and innovative geotechnical design of the raised roadway including the bridge and precast box culverts.

The innovative geotechnical solution at both marshes included “deep ground improvements” utilizing a grid of GeoConcrete Columns (GCC) rigid inclusions and modular precast retaining walls along the entire marsh area. The figure depicts the system superimposed over the previous roadway. Over 1,400 GCCs were installed.

Reconstruction of the Cambridge Turnpike required an extensive wetland permitting process. The vertical modular retaining wall system greatly limited the horizontal impacts of raising the roadway, limiting to the maximum extent the environmental footprint of the project.

Working with the Concord Historical Commission, the project team re-designed a dangerous intersection at historic Lexington Road, while enhancing the grounds of the Emerson House and Concord Museum.

Reconstruction included context sensitive design, with a multi-modal commitment, including sidewalks for the entire corridor, traffic calming measures and other provisions for pedestrian safety.

The project included replacement of all public and private utilities prior to roadway reconstruction, including relocating overhead utilities underground in the historic district, and a rigorous ongoing public program involving stakeholders including residences, businesses, and institutions.

Design and construction were phased to include separate contracts for more conventional roadway construction, and the more complex “deep ground improvements” and bridge and culverts. The phasing shortened the construction schedule and attracted appropriate contractors for the different types of work.

Historic Sensitivity

Landscape design at Lexington Road was developed to enhance historic properties while making intersection safety improvements integrated with pedestrian movements. The project team worked with the Concord Historic Commission to develop enhancements, including expansion of a pedestrian park in front of the Emerson House.

Complexity

The project included many varied elements and individual challenges. Conventional roadway construction was combined with challenging geotechnical considerations with structural elements. In addition to individual complexities of certain parts of the projects, the integration of all these elements added to overall project complexity.

Design to manage the poor bearing subsurface conditions across the two marsh sections was innovative. Complex geotechnical design used a system of GCC’s for deep ground improvements and precast retaining systems to support roadways, utilities, and culverts. This approach provided the means to significantly limit the extent of wetlands impacts of elevating the roadways.

Social, Economic, & Sustainable Development Considerations

• Commitment to minimizing wetlands impacts is an application of sustainable design.
• Use of modular precast concrete systems greatly reduced the horizontal impacts of raising roadway elevations, and significantly reduced the area of disturbed wetlands.
• Historic sensitivity was applied to all elements of design and added to the social appropriateness of the project.