The town of Scituate, Massachusetts is a coastal community located in Plymouth County approximately 20 miles southeast of Boston. The town encompasses approximately 17.6 square miles of land, 14.2 square miles of water, and has a population of 19,000. The town’s wastewater system is operated and maintained by the Scituate Department of Public Works and is comprised of one wastewater treatment facility, nine pump stations, and approximately 52 miles of sewers. The Cedar Point peninsula contains almost a mile of gravity sewer, is centrally located along the town’s coastline, and is home to the historic Old Scituate Lighthouse along with 127 residences.
Figure 1: Aerial photo of the Cedar Point peninsula in Scituate, MA
The project site is located on the Cedar Point peninsula in Scituate on Lighthouse Road, Rebecca Road, and Turner Road. The original gravity sewer collection system in this area was constructed in the early 1970s. As the system aged, it became increasingly vulnerable to infiltration due to the tidally influenced groundwater table and inflow due to severe coastal flooding issues during heavy storms. As a result of the excessive infiltration and inflow (I/I) from this area and other susceptible areas, the town’s sewer system was at capacity and could no longer allow new connections to be made.
Figure 2: Flooding of the Cedar Point peninsula in Scituate, MA
Figure 3: Flooding of the Cedar Point peninsula in Scituate, MA
The town conducted several sewer system rehabilitation projects on Cedar Point over the last decade to attempt to reduce the amount of I/I in the system and regain system capacity. These projects included CCTV pipe inspection, pipe lining, and replacement of select sewer laterals. However, these efforts were not successful, and the town made plans to replace the original gravity sewer system in its entirety. A low-pressure sewer system with individual grinder pumps and generators for each property was originally considered by the town to minimize excavation and dewatering requirements, and provide a more water-tight system. However, the neighborhood did not support this approach, as they preferred a deep gravity sewer system that mirrored the existing system and did not rely on pumps, electricity, backup power or consistent maintenance to function. Consequently, the town engaged a consultant to design and oversee the design and construction of a new, deep, watertight gravity sewer system for Cedar Point and its 127 existing dwellings.
Figure 4: Locus map of the cedar point peninsula in Scituate, MA
The main objectives of the Cedar Point Gravity Sewer Replacement Project included the following:
- Restore Capacity of Sewer System by Eliminating I/I – Design and construct a new gravity sewer system with watertight appurtenances (pipes, manholes, laterals, etc.) to minimize the amount of I/I in the system. Allow the town to regain capacity in its sewer system, thereby allowing modest new development and connections to be made.
- Satisfy Residents' Desire for a Gravity System – Design and construct a new gravity sewer system that mirrors the existing system. Eliminate reliance on pumps, electricity, backup power and consistent maintenance for the system to function in accordance with the desire of the Cedar Point residents.
- Meet TR-16 Design Requirements – Design and construct a new gravity sewer system that adheres to TR-16 design requirements including, but not limited to, pipe sizing, pipe material, pipe depth, buoyancy resistance, reserve capacity, minimum slope, manhole spacing, and manhole size.
- Minimize Disruption to Abutters – Plan the construction schedule such that disruption to abutters is minimized. Halt construction for the summer months/tourist season.
- Minimize Disruption to the Environment – Plan construction to minimize impacts to the highly sensitive environmental resources abutting the project area. Incorporate environmental protection measures into the design of the project, including the requirement of 24-hour sewer bypass pumping.
- Minimize Construction/Betterment Costs – Design and construct a new gravity sewer system that is cost-effective to minimize betterment costs paid by the Cedar Point residents. A portion of the project cost was paid for from town funds obtained through a Community Development grant.
Project Design Approach
In general, the design of the new Cedar Point gravity sewer system focused on the removal and replacement of approximately 4,350 linear feet (1,326 m) of gravity sewer, 30 sewer manholes, and 127 sewer service laterals with new, watertight pipes and structures. To achieve all project objectives with the proposed design, the consultant:
- Reviewed existing record information and prior reports for the area provided by the Town to clearly define existing conditions.
- Conducted a geotechnical investigation to gather information on the underlying soils, groundwater, rock, and rock profiles that would influence the design and construction cost for the proposed work.
- Conducted a topographic survey and wetlands flagging of the project area to construct accurate base plans of the site and quantify the environmental impacts of the project.
- Prepared and assessed design alternatives including alternate gravity sewer alignments, pipe materials, and manhole materials.
- Prepared the final design for the gravity sewer system. This included a Basis of Design Memorandum to identify optimal alternatives for the proposed system as well as design flow and pipe sizing, bypass requirements, and construction recommendations.
- Completed Conservation Commission permitting and coordinated public outreach for the project.
1. Record Review and Definition of Existing Conditions
To accurately define existing conditions within the project area, record information and prior reports for Cedar Point provided by the town were reviewed and analyzed. From this review, it was determined that Cedar Point’s original gravity sewer mains were constructed of vitrified clay (VC) pipe and ranged in size from 6” (15.24 cm) to 10” (25.4 cm) in diameter. Pipe cover depths ranged from 6’ (1.8 m) at the upstream ends of Lighthouse Road and Rebecca Road to 17.5’ (5.3 m) at the intersection of Turner Road and Lighthouse Road. Both Lighthouse Road and Rebecca Road had 8-inch (20.32 cm) gravity sewer mains that discharged into a common manhole in front of #30/#32 Lighthouse Road. Downstream of the connection manhole, the gravity sewer main on Lighthouse Road increased in size to 10” (25.4 cm) to accommodate additional flows from Rebecca Road. Flows from 6 residences on Turner Road joined the Cedar Point gravity sewer system via a 6-inch/8-inch (15.54 cm/20.32 cm) main that discharged into the sewer manhole in front of #10 Lighthouse Road. Flows from all three streets reached a discharge point at a sewer manhole in front of #169 Jericho Road.
Existing gravity sewer services on Cedar Point ranged in size from 4” (10.16 cm) to 6” (15.24 cm) and consisted of both gravity and chimney-type connections. Sewer service material was not consistent across the project area and included reinforced concrete (RCP), VC, cast iron (CI), asbestos cement (AC), and PVC piping. Since the initial construction of the gravity sewer system in the 1970s, many sewer services required repairs or modifications and, as a result, were constructed of multiple pipe materials. In addition, a previous attempt to partially line some of the laterals was of limited benefit as the liner appeared to not be fully adhered/bonded to the wall of the original VC pipe.
2. Geotechnical Investigation
A geotechnical investigation was performed to characterize underlying soils, groundwater, rock, and rock profiles within the project area that would ultimately affect the project design and cost. Six (6) standard penetration test (SPT) borings, including three (3) monitoring wells were advanced across the project site in January of 2020. In general, the test borings on Cedar Point encountered a varying depth (5”-8”/12.7 cm-20.32 cm) surficial layer of asphalt underlain by sand and silty sand with trace amounts of gravel. Bedrock was not encountered in any of the borings. Groundwater was monitored in the three installed monitoring wells for approximately 8 days using piezometers. The study concluded that groundwater levels within the project area are affected by tidal conditions and that the existing gravity sewer was located predominantly below the fluctuating groundwater table. Therefore, significant dewatering efforts were anticipated for the proposed project. A Geotechnical Memorandum was prepared based on the results of the subsurface exploration and was provided as part of the contract documents.
Figure 5: Groundwater monitoring data
3. Topographic Survey and Wetlands Flagging
Following the geotechnical investigation, a topographic survey and wetlands flagging were conducted to construct accurate base plans of the site and quantify the environmental impacts of the project. The topographic survey concluded that in general, the project area consisted of a relatively flat terrain with a range of elevations from 7.4 feet (2.25 m) to 11.75 feet (3.58 m) on Lighthouse Road, and 9.4 feet (2.87 m) to 10.39 feet (3.14 m) on Rebecca Road. Additionally, it showed that the narrow roadways along Cedar Point were congested with gas, electric, water, sewer, and stormwater infrastructure. Wetlands flagging of the site concluded that coastal banks, coastal beaches, and bordering vegetated wetlands would be within 100-feet (30.48 m) of the proposed work and that a Notice of Intent (NOI) would be required.
4. Assessment of Design Alternatives and Preliminary Design
With the primary goals of reducing I/I, meeting TR-16 design requirements, and minimizing construction costs, the consultant considered various gravity sewer alignments, pipe materials, and manhole materials for installation along Cedar Point. A summary of the alternatives assessed during the preliminary design phase of the project is provided in Table 1, below.
5. Final Design Recommendation
The Final Design for the Cedar Point gravity sewer system identified optimal alternatives for the proposed system and was presented in a Basis of Design Memorandum. The memorandum outlined the following final design recommendations and features:
- Gravity Sewer Alignment – Removing and replacing the original gravity sewer in its existing location was determined to be the preferred alternative for this project so utility conflicts with existing water, gas, and stormwater infrastructure would be avoided. The narrow roads and density and proximity of the existing sewer, water and gas utilities left virtually no lateral corridor for a new pipe. Proposed gravity sewer inverts varied slightly from existing inverts and provided adequate slope to meet TR-16 design requirements.
- Gravity Sewer Main Pipe Material – SDR35 PVC Pipe was determined to be the preferred alternative for this project due to its long design life, watertight joints, compatibility with gasketed tee and wye saddles, and long history of use in gravity sewer systems. All SDR35 PVC pipe was proposed to be installed with external joint wrap and impervious clay dams for further protection against infiltration.
- Gravity Sewer Design Flow and Pipe Sizing – Existing flow estimates were determined based on TR-16 average daily per capita flow rate (70 gpd/265 L/d), and the average number of persons per household for Plymouth County (2.6). Existing flow estimates were used for designing the proposed sewer collection system, as no future buildout within the project area is expected. Existing and proposed sewer capacities were evaluated against peak hourly flows determined by TR-16 multipliers. An allowance for infiltration due to normal aging of piping systems was also applied to flow projections. For the entire sewer catchment, an I/I rate of 500 gpd/in. (745 L/d/cm) diameter/mile of sewer was applied. It was determined that all existing sewer segments were appropriately sized to meet existing demands and possessed adequate reserve capacity. Hydraulic capacity calculations indicated that an 8-inch (20.32 cm) diameter gravity sewer was sufficient for the majority of the service area, using TR-16 guidelines and including a modest allowance for infiltration.
- Gravity Sewer Manholes – Fiberglass manholes were determined to be the preferred alternative for application along Cedar Point due to their monolithic, watertight nature, H-20 loading rating, and ease of installation due to prefabricated options. These manholes were proposed to be equipped with watertight covers to prevent surface water inflow into the gravity sewer system. Due to their light weight, design of the fiberglass manholes included a concrete anchoring pad to prevent buoyancy. Manholes were proposed in locations that adhered to TR-16 recommendations for manhole spacing.
- Gravity Sewer Services – Sewer services for Cedar Point’s 127 residences were proposed to be replaced in their current locations up to three feet (0.91 m) from the house exterior wall. Laterals were proposed to be constructed of SDR35 PVC pipe. Chimneys were proposed to be constructed of C900 PVC pipe and 401-epoxy coated ductile iron tees to ensure structural strength and stability of the service. To further protect against infiltration, all service joints were proposed to be installed with external joint wrap. It was critical that the laterals were replaced beyond the edge of the right of way, not leaving old laterals pipes in service with their suspected poor construction and leaky joints.
- Gravity Sewer Bypass System – Due to the environmental sensitivity of the area, installation of the new gravity sewer system on Cedar Point was proposed to have a full-time monitored sewer bypass system. Sewer laterals were not allowed to temporarily discharge into the trench during replacement of the existing gravity sewer. Manhole-to-manhole temporary bypass main and service connection piping were proposed to isolate a portion of the existing gravity sewer main prior to its replacement. Temporary bypass systems were proposed to be decommissioned following installation and testing of the new gravity sewer main and completion of permanent sewer lateral connections.
6. Permitting and Public Outreach
Replacement of the Cedar Point gravity sewer required extensive permitting and public outreach prior to beginning construction. Preparation of an NOI was completed by the consultant and an Order of Conditions was received from the Scituate Conservation Commission. As part of the work, the contractor was required to obtain street opening and trench permits from the town DPW as well as coverage under the NDPES Construction Remediation General Permit. Public outreach included coordination with the Cedar Point Homeowner’s Association prior to construction and presentations during town Select Board Meetings.
Project Construction Approach
Construction of the proposed gravity sewer system took place between September of 2020 and May of 2021. Construction halted for the summer months and final restoration and pavement of the site was completed in the fall of 2021. To achieve all project objectives, the following general approach was taken during construction of the Cedar Point gravity sewer system:
- Install all environmental protection measures required by the Order of Conditions, including filter sock and catch basin silt sacks, to prevent disruption to the surrounding sensitive environment.
- Replace all 127 existing sewer services between the house foundation and the gravity sewer main, including joint wrap, cleanouts, temporary connections to the existing main, and surface restoration.
- Prior to mainline excavation, install dewatering system along the gravity sewer segment to be replaced including deep wellpoints, PVC piping, frac tanks, pumps, and dewatering bags.
- Install gravity sewer bypass system along the gravity sewer segment to be replaced including HDPE piping and pumps.
- Construct concrete manhole bases for fiberglass manholes to be installed along the active gravity sewer segment.
- Beginning at the downstream end and working upstream: Install trench support, excavate, and remove existing VC gravity sewer, wood shoring, and precast concrete manholes. Install new PVC gravity sewer with joint wrap and fiberglass manholes at the depths and locations indicated on the drawings. Connect new PVC gravity sewer services to mainline as work progresses. Complete temporary surface restoration. Remove temporary bypass and dewatering systems as segments are finished and re-install along active segments.
- Test installed portions of gravity sewer via low pressure air testing, mandrel testing, and CCTV inspection. Perform all work necessary to correct deficiencies discovered as a result of testing and/or inspections.
- Hold weekly meetings with the contractor, the consultant, the town, and the representatives of the Cedar Point homeowner’s association to review project progress and address any abutter concerns. These meetings were a key factor in establishing and maintaining open communication during construction.
- Complete final surface and pavement restoration (full-width, full-depth reclamation) in the fall to avoid disruption to abutters during the summer season.
Figure 6: Gravity sewer service replacement
Figure 7: Gravity sewer service replacement
Figure 8: Dewatering system
Figure 9: Gravity sewer support of excavation
Figure 10: Gravity sewer manhole installation
Figure 11: Final surface restoration near lighthouse
Installation of the new gravity sewer system on Cedar Point was met with various challenges that were overcome by diligent construction oversight, project planning, and communication. Production was slowed due to the deep excavation, trench support, and existing utility support required for installation of the gravity sewer and manholes. Additionally, removal of the unexpected existing wood shoring system left in place from the original construction took significant effort and manpower. In conjunction with gravity sewer installation, the contractor was required to maintain traffic and detours along one-way roads, maintain the sewer bypass system, and control groundwater levels with the dewatering system during tidal fluctuations. Due to these requirements, gravity sewer installation was limited to 20-35 feet (6.1-10.7 m) per day. Finally, the final design, bidding, and construction of the project all took place during the COVID-19 pandemic, which required significant coordination and planning to overcome supply chain issues and to adhere to Massachusetts COVID-19 guidelines and procedures.
The design and construction of the new, deep gravity sewer system fulfilled the desires of the Cedar Point residents and was successful in achieving all objectives outlined for the project. Existing gravity sewer mains, laterals, and manholes that were observed to be in poor condition during their removal were replaced with TR-16 compliant watertight pipes and structures. Since its implementation, the town has observed an approximate 80% reduction in flow as measured by the nearest downstream flowmeter (25-40 gpm to 4-8 gpm and 15,000 gpd to 20,000 gpd)/(1.57-2.52 L/s to 0.25-0.50 L/s and 56,781 L/d to 75,708 L/d). There has been visibly less flow in the newly installed manholes and a lesser direct reaction to tide cycles when compared to existing conditions. Consequently, the town’s downstream sewer pump station is running less frequently, and additional capacity has been regained in the system.
Figure 12: Existing sewer service found in broken condition
Figure 13: Existing sewer service found with damaged liner
Through careful and innovative project planning, the project team was able to minimize construction costs as well as disruption to abutters and the environment. Overall, construction change orders did not exceed 3% of the bid cost. Though challenging, disruption to abutters was minimized by performing construction throughout the winter and outside of the summer months. Disruption to the surrounding sensitive environment was prevented by adhering to Conservation Commission requirements, installing environmental protection measures, and providing continuous sewer bypass during the project. Due to its success, the Cedar Point gravity sewer replacement project has served as the new standard for coastal sewer replacement in the town of Scituate.
Figure 14: Downstream flowmeter readings – pre-construction
Figure 15: Downstream flowmeter readings – post-construction
Originally installed in the 1970s, Cedar Point’s gravity sewer collection system underwent several failed rehabilitation projects prior to being fully replaced during 2020 and 2021. The new, watertight gravity sewer system was carefully and methodically designed and constructed to eliminate I/I and meet TR-16 design requirements. Special considerations were also made to minimize construction costs as well as disruption to abutters and the environment. The system was placed into service in May of 2021. Since its installation, the town has observed an approximate 80% reduction in flow as measured by the nearest downstream flowmeter. The additional capacity that has been regained in the system because of the project will allow the town to continue economic and residential development and accept new service connections in the future.
Environmental Partners Group, LLC
Town of Scituate, Massachusetts
Albanese D&S, Inc.
Cedar Point Association
Waller, J. (2018, March 2). 33 photos from the nor’easter that show the sheer power of Mother Nature. https://www.boston.com/news/weather/2018/03/02/photos-boston-noreaster-storm-march-2-2018/. Accessed August 16, 2022
About the Authors
Francesca Barilla, EIT, Senior Project Engineer, Environmental Partners Group, LLC - Francesca Barilla is a senior project engineer at Environmental Partners with over 5 years of experience in emergency management, drinking water, wastewater, stormwater, site civil, and landfill design projects. Her project experience includes infrastructure design and planning, construction oversight, permit drafting, preparation of bid documents and specifications, construction cost estimation, and construction contract administration.
Paul Millett, PE, Senior Principal, Environmental Partners Group, LLC - Paul Millet is a senior principal and a regional manager at Environmental Partners with over 35 years of experience in drinking water, wastewater, stormwater, owner’s project management, emergency management services, and construction management services. Specializing in supporting complicated utility and building projects including sewer force mains and treatment plants, he provides construction management and engineering services for communities in need of everything from on-call services to critical pipeline failure assistance.
Kevin Cafferty, Director of Public Works, Town of Scituate, MA - Kevin Cafferty is the director of public works for the Town of Scituate. He manages the water, sewer, engineering, trees & grounds, highway, and transfer station divisions. He has been with the town for over 13 years. Prior to working on the municipal side, Kevin worked for private contractors specializing in utility and marine projects. This expertise has been instrumental in overseeing the town’s departments as Scituate is a coastal community with a unique set of challenges. Kevin holds a degree in civil engineering from the University of Massachusetts Lowell.
William Branton, Sewer Supervisor, Town of Scituate, MA - William Branton is the superintendent for the Town of Scituate’s sewer division with over five years of experience in managing and maintaining municipal wastewater collection and treatment systems. William holds a grade 7 combined operator’s license for Massachusetts wastewater systems. He oversees all regulatory functions as well as operations of the town’s wastewater treatment plant and collection system which is comprised of 52 miles of sewer mains and nine pumping stations across town.