Larchmont Area Sanitary Sewer Improvements

The comprehensive approach addresses immediate needs and establishes a sustainable, resilient framework for the area’s sanitary sewer infrastructure

Our Client’s Challenge

HRSD has served southeast Virginia since 1940, using advanced water treatment methods to treat wastewater before safely returning it to the region’s delicate water ecosystems. With nearly 2 million people and critical bodies of water like the Atlantic Ocean and Chesapeake Bay in its 5,000-square-mile service area, HRSD is dedicated to protecting public health and the environment.

HRSD is upgrading sewer infrastructure in accordance with the Larchmont Sanitary Sewer Master Plan published in early 2020. The master plan is an essential part of HRSD’s Rehabilitation Action Plan, which identifies improvements to reduce the frequency and severity of sanitary sewer overflows (SSOs). The upgraded infrastructure will protect public health and improve resilience to tidal flooding and sea level rise by reducing the risk of sewage overflows, pipe leakages, and interruptions to service resulting from aging infrastructure.

Our Solution

Gannett Fleming is providing resilient design and construction solutions to replace the existing pump stations and associated infrastructure with two new sanitary sewer pump stations, three rehabilitated pump stations, and two pipeline projects of 6- to 12-inch gravity sewer and force main replacements, including rerouting backyard sewers. The pump stations range in capacity from 300 to 2,200 gallons per minute.

To support cost control and constructability in design, HRSD engaged a construction manager at-risk (CMAR) team, which proved beneficial during market escalation, inflation, and supply chain issues. The CMAR team recommended cost-saving measures like rehabilitating instead of replacing certain pump stations and eliminating expensive sewer realignments. Collaboration under the CMAR relationship optimizes construction methods and minimizes community disruption. Innovative techniques, such as “silent” pile driving technology, reduce noise and other impacts during excavations.

One of our key challenges in the planning phase was determining the realignment routing to reverse flow for nearly 1,700 linear feet of gravity sewers. This process required addressing construction depth, high groundwater levels, and maintaining traffic flow in narrow residential roadways. Additionally, we had to keep the existing sanitary system operational while preserving 150-year-old magnolia trees lining the streets. Despite these hurdles, our team developed cost-effective, constructible pipeline alignments that minimized impacts on residents.

We also faced the complex task of reversing sewer laterals for over 20 residents, enabling the abandonment of difficult-to-maintain alleyway sewers. Without lateral location records, we coordinated with residents and used CCTV inspections and dye testing to identify and confirm lateral locations accurately.

For each pump station requiring rehabilitation, the team employed lidar scanning technology to generate a highly accurate point cloud of the interior. This point cloud is incorporated into each station’s 3D design model, ensuring highly accurate and precise design and enhancing the construction process’s efficiency. Gannett Fleming generated virtual reality models for operability reviews and equipment removal animations for maintenance of operations.

For the structural design, we incorporated lightweight geofoam fill in some pump station foundations to eliminate the need for pile foundation systems. Using geofoam fill will reduce project costs and disruption to the community.

The facilities were designed with flexibility for future improvements or expansions to avoid design rework. We recommended that HRSD retain existing properties adjacent to the new pump stations after demolition for wetlands development, maintenance of views, and additional tree cover for the community’s benefit.

Before construction, we addressed the community’s concerns about odor, maintenance, and construction impacts. We implemented project phasing, bypass pumping, and extensive community outreach to minimize impacts during construction. The project team prepared documents and presented them at public meetings, developing trust with the community and providing a forum for discussion and resolution of issues.

Construction for the sewer system improvements project must be completed by May 2027 to comply with a consent decree issued by the U.S. Environmental Protection Agency.

Key Features

Rehabilitation or replacement of pump stations, gravity piping, and force main throughout Larchmont.
Comprehensive public outreach and stakeholder engagement.
Lidar scanning provided precise surveys of pump stations to be rehabilitated.
Immersive technologies such as virtual reality and 3D animations optimized operability.
Structural and geotechnical considerations, such as geofoam fill and hydraulic-driven pile installation systems, reduce cost and disruption to the surrounding neighborhood.

Outcomes

The updated infrastructure is expected to significantly reduce the risk of SSOs and improve overall resilience to environmental stressors.
Value engineering reduced costs by focusing on rehabilitating rather than replacing certain assets.
Potential wetlands development and additional tree cover at demolition sites will benefit the community and local ecosystems long-term.