Michael Cruz, PE, Bridge Design Group Leader, Green International Affiliates, Inc. and Tiffany Card, PE, Bridge Group Project Manager, Green International Affiliates, Inc.
The Crescent Street Over Millers River Bridge, which is under the jurisdiction of the Massachusetts Department of Transportation (MassDOT), is nestled within feet of the L.S. Starrett Company, the leading employer in Athol. Starrett has been at the heart of business in Athol since its founding in 1881 and over time, the Crescent Street Over Millers River Bridge became deeply woven into its operations and therefore vital to the Company and Town.
While Starrett’s business grew, the bridge aged. Wear and tear from decades of use combined with the upstream dam’s spillway creating harsh environments due to water spray and ice flows caused the bridge to deteriorate to the point where a full replacement was needed. Given the importance of this bridge, it was crucial to approach its replacement with exceptional care and communication within the team itself and with stakeholders.
The new Crescent Street over Millers River Bridge, nestled within
the L.S. Starrett Company today
Abundance of Site Constraints
The incredible number of physical constraints present at the site drove the need to incorporate unique and innovative design and construction techniques and made the project very challenging and highly complex. These physical constraints included Starrett-owned buildings, wall structures, utilities, tunnels, the dam, and Millers River. Existing building foundations that date back to the 1800s directly abut the bridge abutments on the west side and support a Starrett building spanning over the river. There is less than 3 feet of clear space between the edge of the bridge and the edge of the building. On the east side, the bridge wingwalls tie directly into the retaining walls for the upstream dam structure. At the northeast corner there is an existing splash block to mitigate the flow over the dam spillway mid height of the bridge wall. At the southeast corner, the bridge walls connect to the dam walls which support an electrically powered metal head gate. The head gate structure controls water flow through Starrett’s 6 foot-diameter steel penstock which sits directly behind the existing south abutment, within 1 foot at the closest point. There was also an existing sewer manhole wedged between the penstock and partially built into the south abutment concrete wall that connects to the sewer line carried by the bridge.
Installation of the new beams and abutment construction
within site constraints
The south and north roadway approaches contained multiple structures and water, sewer, and storm drain utilities. Additionally, overhead wires run along the west side of the bridge with utility poles on either side within the sidewalk, with service lines for the Starrett buildings. A utility tunnel crosses the south approach roadway between Starrett buildings carrying a critical steam line, buried less than 3 feet below the existing pavement. Within the north approach roadway is an underground pedestrian tunnel enabling Starrett employees to move between buildings. This tunnel has less than 3 feet of cover at the shallowest point.
Responding to the Project Challenges
The design of the new and widened bridge was specialized to fit the site constraints. For example, the western portion of the north abutment wall was designed as a cantilever to avoid adding load to the existing building foundation. The bridge carried both a water main and a sewer line. The sewer line was a gravity flow pipe that connected to manholes and siphons on either side of the bridge. Due to the density of other utilities on either side of the bridge, and the need to maintain gravity flow between the siphons, the inverts could not be changed. Green designed a unique bump out in the abutment wall to accommodate the sewer pipe, and suspended pipe hanger supports were detailed between the beams. The density of the utility structures behind the ends of the bridge led to the use of controlled density fill for backfill since gravel compaction would be difficult and introduce more vibration. Lastly, all four corners of the bridge required customization to terminate the bridge railing so that it provided a safe condition while also tying into Starrett’s structures and accommodating pedestrians. Green’s design incorporated a Modified Texas Rail, a deviation from MassDOT’s standard rail detail, to prevent water from the dam spillway from spraying onto the bridge and creating a safety hazard for pedestrians.
The design and installation of water control measures to allow the abutments to be replaced in dry conditions was another challenge. Green’s design included time of year restrictions for all work within the river. The design team coordinated with the Starrett facilities manager as to when they typically saw the highest flow periods each year and discussed how much water the company could divert during these events through their two penstocks. Through this cooperative planning effort reasonable windows were established for the Contractor to perform work within the channel, and still provide some level of backup plan during an unseasonably high rain event. The Millers River channel bottom is entirely exposed bedrock in this area from years of high flow velocities and erosion. Since standard control of water measures such as sandbags and driven steel sheeting could not be installed under these conditions, the design anchored the water barrier into the bedrock and created a seal. This approach was effective and there were no issues creating and maintaining dry conditions during the demolition and pouring of new concrete.
Water control measures were installed on bedrock adjacent to the dam
Also for the construction phase, Green’s Team developed a plan for monitoring and protecting the critical infrastructure abutting the project site through the use of survey and vibration monitoring. A plan for sensor placement was included in the design package with thresholds and reporting requirements for the Contractor. Any time a demolition or construction activity produced vibrations exceeding the established thresholds, the Contractor was required to stop work immediately and develop a plan for reducing vibrations before the activity could continue. This strategy was successful, as none of the items monitored during construction experienced any damage or deficiencies.
At the beginning of construction, the Contractor proactively coordinated with Starrett to pursue a full closure of the bridge, by identifying alternative solutions for pedestrian crossings and utility service staging. They were able to develop a plan that met all stakeholder’s needs and, due to Starrett’s strong relationship with the community, the Town approved the new approach. This allowed the Contractor to complete in-water work, including installation of control of water measures, bridge demolition, and construction of both abutments and wingwalls, all in one 2-month period. They were able to take full advantage of a relatively dry year, coupled with Starrett’s willingness to bypass water through their penstocks during heavy rain events. This greatly mitigated risk associated with the schedule for staged in-water work and potential damage to the work area during flood events.
The abutting buildings were built over 100 years ago and there was minimal record information available. This led to several unforeseen site conditions that required the entire team to respond quickly and develop collaborative engineering solutions to make sure the bridge could still be completed in a single construction season. For example, during the excavation and installation of the temporary support of excavation at the existing north abutment wall, the Contractor found a buried retaining wall not included on any of the plans. This wall abutted the existing abutment wall and ran parallel to the northwest sidewalk and Starrett building. The buried wall foundation sat directly on the existing Starrett building foundation. The wall supported the roadway and fill material so the loads would not be applied to the building basement walls. To construct the new bridge, the existing abutment wall and part of this retaining wall needed to be removed so those loads were not carried by the building foundation walls. All parties recognized the importance of having all the new structural elements be independent of the section of the buried wall that would remain, as well as the Starrett building foundation. The solution was to design and re-build a section of the buried wall extending from the back of the new abutment wall so that it would not transfer any loading to the existing structures. All loads on the new extension of wall tie back to the new bridge structure. This also required a quick re-design of the new north abutment wall to account for the additional loads.
By working effectively together, the project team, comprised of Green, MassDOT, Starrett, the Town, and the Contractor, tackled all of these challenges to deliver the replacement bridge ahead of schedule. Despite the small footprint, the bridge was widened to accommodate two sidewalks to meet the pedestrian needs of the Town and the business. All this was done in one construction season so as to avoid prolonged impacts to Starrett and the local community. The new bridge surpasses the function of the previous bridge. The new superstructure is also designed to combat the constant water spray and icing issues which will help this new bridge stand the test of time and continue to be a reliable structure for the Town and Starrett for decades to come.
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