Broad Street Parkway
Broad Street Parkway
Like main roads in many cities and towns, Broad Street in Nashua, New Hampshire begins in the western suburbs and extends to the westerly edge of the downtown district. Along the way it intersects the F.E. Everett Turnpike (US Rte. 3), a heavily-traveled toll highway running from the Massachusetts border and Boston, to the south, to Concord, the state capital to the north. The combination of suburban and interchange traffic between the Turnpike and downtown causes congestion, increases pollution in Nashua’s Central Business District, and impacts public safety by reducing mobility of emergency vehicles. The recently completed "fix" is the Broad Street Parkway, the largest municipally-managed public works project in New Hampshire. Part of a long-term transportation strategy to improve air quality conditions in Nashua, construction of Broad Street Parkway also provides an economic benefit by enabling redevelopment in a long underutilized section of the city.
There was only one possible alignment for the Parkway as it departs its starting point at Broad Street and heads for downtown: through a narrow space between the adjacent railroad and an oxbow in the Nashua River, flowing more than 50 ft. below. Based on New Hampshire DOT (NHDOT) design standards, project plans prepared by Fay, Spofford & Thorndike, LLC (FST), of Bedford, NH, called for a Mechanically Stabilized Earth (MSE) retaining wall to handle the grade differential and a sheet pile wall ("cofferdam"), located behind the MSE wall, to support the excavation and the railroad. Addressing the railroad’s self-protection requirements while keeping the MSE wall and sheet pile wall sufficiently back from the river was a complex task, made more difficult by concerns for global stability of the overall hillside. FST specified the required wall systems, but left it to the contractor to design those wall systems and address the global stability challenge.
R.S. Audley, Inc., of Bow, NH, the project’s general contractor, had a plan. It began with asking The Reinforced Earth Company (RECo) to supply a Reinforced Earth® retaining wall for the MSE structure, because Audley knew they could rely on RECo for a quality-engineered product, a timely response and responsive local field service. The contractor’s plan also included moving the sheet pile wall farther away from the railroad, not only to minimize the influence of railroad loading but also to prevent the anchors of the sheet pile wall from extending under the railroad. So Audley worked closely with RECo Vice President and Regional Manager Peter L. Anderson, P.E., to optimize the combination of the two earth retention systems – the tied-back sheet pile excavation support to protect the railroad and the MSE wall to carry the Parkway between the railroad and the river.
In a typical Reinforced Earth wall design, the reinforcing strip length is 70% of the design height of the structure from top to bottom of wall. For the central portion of the Broad Street Parkway wall, the 70% length is 32 ft. But to address anticipated global stability issues on this project, FST required the soil reinforcements to be 100% of the design height where the MSE wall was taller than 30 ft. – meaning 45 ft. strips were needed. However, relocating the excavation support further away from the railroad left only 23 ft. for the reinforcing strips in the central portion of the wall, instead of the typical design length of 32 ft., and far less than the 45 ft. strips mandated by the "100% of design height" requirement.
S. W. Cole Engineering (Manchester, NH), was the project geotechnical consultant. Cole’s analysis indicated that the sheet piles needed to be driven significantly deeper than had been planned, to intersect deep slip circles and satisfy global stability of the hillside above the river. As a permanent structure, the sheet piles had to be designed for a 100-year design ife, leading to extra thick sheeting sections to provide sacrificial thickness and assure adequate section remaining after 100 years. As designed and installed by specialty foundation contractor H. B. Fleming, Inc. (South Portland, ME), this relocated, anchored, deep-seated, extra thick stabilizing structure significantly changed the design parameters for the Reinforced Earth wall.
The excavation support system satisfied global stability requirements, eliminating the "100% of design height" strip length requirement and reducing the lateral earth pressure at the bottom of the MSE wall. With reduced lateral earth pressure, overturning and sliding could be satisfied using the 23 ft. base width available in front of the sheeting. Since the sheeting rose less than half the Reinforced Earth wall height, RECo designers used normal 70% design length (32 ft.) reinforcing strips above the sheeting, creating what is called a trapezoidal section (Fig. 1).
The additional internal load imposed on the MSE structure, including a potential 100-year design flood, was handled by increasing the number of reinforcing strips at the bottom of the wall, while the free-draining crushed stone backfill minimized the development of an unbalanced head during drawdown after a flood. Other design details specified by NHDOT included an impervious membrane and perforated collector pipe above the reinforced backfill, and a "rail support slab" atop and overhanging the wall to support a bridge railing.
Construction of this 495 ft. long, nearly 16,000 sq. ft. retaining wall required working from the base of a deep hillside excavation, with steep access grades to be negotiated by equipment and personnel. Sandwiched between the river in front of the wall and the sheet pile wall behind it (Fig. 2), working space was limited. But the contractor used the knowledge gained building Reinforced Earth walls on previous projects, coupled with dependable facing panel deliveries from precaster Dailey Precast LLC (Shaftsbury, VT) and the easily placed and compacted backfill, to produce a well-constructed, good looking wall. When asked what he considered special about this wall they had built, R.S. Audley’s Vice President and Project Manager, Scott Stevens, proudly answered, "the immense height, length and curvature of the wall, along with the innovative cofferdam [excavation support wall] – that should cover it."
The coarsely-textured ashlar stone surfaces of the cruciform-shape facing panels (Fig. 3) give the Broad Street Parkway retaining wall a pleasing appearance, consistent with New Hampshire’s nickname, "The Granite State." And completion of the project gives Nashua’s citizens a new route into their city. Yet as people travel the Broad Street Parkway, high above the Nashua River, few will realize the complex engineering or the challenging planning and construction that went into the Reinforced Earth wall supporting them on their journey.