Restored Resilience: Replacement of Coastal Cedar Point’s Vulnerable Sewer System in Scituate, Massachusetts

• 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.

• 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.

• 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.

1. 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.
2. 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. 
3. 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.
4. Install gravity sewer bypass system along the gravity sewer segment to be replaced including HDPE piping and pumps.
5. Construct concrete manhole bases for fiberglass manholes to be installed along the active gravity sewer segment.
6. 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.
7. 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.
8. 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.
9. 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