Hydraulic improvement to the high level sewershed collection system in Baltimore

2023 Trenchless Technology Project of the Year New Installation Honorable Mention – Hydraulic Improvements to the High Level Sewershed Collection System in Baltimore

Trenchless Technology Project of the Year New Install Honorable Mention

JMT designed trenchless improvements to eliminate sanitary sewer overflows and basement backups in the High Level Sewershed and improve water quality and sewer system efficiency for the people of Baltimore.

The High Level Sewershed conveys sewage flow from the west side of Baltimore and part of the county to the Back River Wastewater Treatment Plant (WWTP), which serves 1.3 million customers. Although it covers only 10 percent of the city, it is a critical link in the city’s sewer collection system.

// ** Advertisement ** //

A 2009 study conducted by JMT discovered multiple engineered sanitary sewer overflows (SSOs) in the Sewershed, all requiring elimination according to the city’s Consent Decree. Currently, there are three overflows remaining at the north end of the High Level Sewershed. The existing collection system was constructed with minimal slopes, due to flat topography. This caused the undersized pipes to frequently surcharge and activate the SSOs. To eliminate two of the overflows, the city decided to build an underground storage tank near Towanda Park.

Hydraulic improvement to the high level sewershed collection system in Baltimore

After the city chose to convey and store flows at the Back River WWTP, the underground storage tank was removed in favor of increased capacity to the collection system. JMT worked as a consultant for the Baltimore Department of Public Works, examining multiple rehabilitation and replacement scenarios such as a parallel relief sewer, a stacked pipe configuration with new pipe installed above existing pipe, pipe upsizing and a new alignment. After reviewing the site constraints, which included a pipe depth exceeding 25 ft, busy neighborhood streets, a CSX railroad crossing, and other environmental impacts, JMT and the city determined the best option was to develop a new alignment installed by microtunneling.

// ** Advertisement ** //

JMT’s design included 8,000 lf of gravity sewer using a two-pass installation method. The casing pipe was designed with 60-in. steel. However, it was later changed to a 60-in. RCP casing during construction. The carrier pipe inside the casing is 42 in. and fiberglass. JMT also included a 96-in. diversion manhole with a fiberglass insert and stop gates to direct flow into the new sewer, while maintaining access to the existing sewer for flushing and maintenance.

JMT also helped the city install 24 new manholes for access and replace 11,000 lf of sewer using large diameter pipe by open cut. Construction began in November 2019 and was completed in July 2022.

// ** Advertisement ** //
Hydraulic improvement to the high level sewershed collection system in Baltimore

Why This Project Is Outstanding

The location and size made the project challenging. Since the pipeline alignment traverses through a densely populated neighborhood with highly trafficked roadways, significant maintenance of traffic coordination with the Baltimore City Department of Transportation (DOT) was required to safely and effectively direct traffic around launching and receiving shafts that remained open for months at a time. This project also affected homeowners, businesses, public transit, first responders, and school traffic. JMT worked closely with the DOT to implement traffic barriers, signage, lane closures, and signal phasing to keep traffic flowing.

The alignment corridor included numerous large diameter utilities such as a 54-in. PCCP water main, along with hundreds of smaller utility crossings. These utilities were one of the factors that drove the vertical alignment deep, but many of the utilities had to be supported within the shafts. An important fiber optic line providing internet service for a large section of the city was exposed in a shaft. The contractor was required to work around it to complete two tunnel drives, install the carrier pipe and manholes, and reconnect a nearby sewer without interrupting service.

// ** Advertisement ** //

A notable part of the High Level Sewershed Collection System Project was the inclusion of three curved microtunnel drives totaling 1,400 lf. These drives were used in curved roadways and removed short, straight drives. By incorporating curves, we could remove or relocate shafts, thereby reducing the construction footprint and lessening impacts to surrounding communities. Both the city and the contractor saved money by eliminating shafts and manhole installations.

The COVID-19 pandemic also presented project challenges, such as material supply shortages. Through a collaborative approach, our team was able to review the implications of the shortages to both schedule and cost. Alternative materials that still met the design intent were assessed and approved for use, where feasible. This approach recovered several months of delay to the construction schedule.

// ** Advertisement ** //

Project Owner: City of Baltimore Department of Public Works
Engineer: Johnson, Mirmiran & Thompson Inc. (JMT)
Contractor: Northeast Remsco Construction Inc.
Manufacturers/Suppliers: Herrenknecht, VMT, Permalok, Vianini, United Underground Precast, Hobas and PREDL Systems
Value of Trenchless Project (US$): $23,530,000

// ** Advertisement ** //