Johnson, Mirmiran & Thompson Inc. (JMT) designed trenchless improvements to enhance the structural integrity of the M29 Combined Sewer Outfall (CSO) culvert on the Monongahela River in Pittsburgh, Pennsylvania and provide water quality improvements.

The M29 Outfall was originally built to culvert a stream and provide land for a steel plant operation along the Monongahela River in the 1800s. The original culvert was a 10-ft by 14-ft rectangular brick structure, over 30 ft deep and 120 ft long. It was extended another 330 ft in 1892 with a 15-ft brick arch structure, for a total length of 450 ft to the current river bank location. Over the years, a state highway and local railroad were constructed on top of the M29 structure, preventing access from above to the 30-ft deep culvert. Today, the M29 Outfall serves as the second largest CSO in the city of Pittsburgh and is a major portion of the combined sewer interceptor network.

In 2016, the M29 Outfall was inspected and found to be in severe disrepair, requiring structural lining to prevent the system from collapsing and causing a catastrophic failure to the sewer system. JMT, as a consultant to Pittsburgh Water and Sewer Authority, evaluated several replacement, repair, and trenchless rehabilitation options for the culvert and ultimately selected geopolymer structural lining or shotcrete lining with steel reinforcement to repair the culvert due to accessibility and the size of the structure. Additionally, a physical scale model of the culvert and outfall, as well as a computational hydraulic model were created by others to understand the hydraulics of the system and determine any restrictions that could be removed during construction.

RELATED: City of North Olmsted, Ohio, Makes Maintaining the Manholes a Priority

The final rehabilitation design included filling major voids in the brick walls from inside the structure, lining the brick culvert walls with shotcrete, reforming and establishing a new concrete invert, applying an epoxy corrosion inhibitor on all surfaces, and replacement of the existing stone endwall with a new cast-in-place concrete endwall. The endwall also included a 15-ft metal flap gate to control river intrusion into the combined sewer system, relieving a major hydraulic bottleneck upstream. Additionally, repairs were required on the upstream influent diversion chamber and weir to assist with the hydraulics of the combined sewer system and turbulent flow conditions. Construction began in the Summer 2021 and was completed in Spring 2022.

Project of the Year Rehab Honorable Mention

Why Project Is Outstanding:

The M29 Outfall Improvements project location and size created several limiting factors for construction which make it unique for trenchless rehabilitation applications. The primary access to the project site was by barge or boat from the Monongahela River into the culvert. The existing invert of the culvert sat three feet below the normal pool of the river which required the installation of a large sheet metal cofferdam with a continuous, 24/7 pumping operation to dewater the structure. Secondary access to the structure was 450 ft upstream in the diversion chamber through a single manhole opening 30 ft below an active state highway travel lane.

Trenchless construction activities required continuous air monitoring within the CSO, as well as continuous weather monitoring for the occasional summer pop-up thunderstorm. Since the facility is a combined sewer overflow that serves a large portion of Pittsburgh and is along the Monongahela River, all construction was planned to occur during dry weather operations. During wet weather events, flow in the outfall reaches the crown of the culvert and the cofferdam is overtopped by the rising river levels. Safety and scheduling for all construction activities were a constant consideration during both the design and implementation of the project.

RELATED: 2022 Trenchless Technology Project of the Year Rehab Winner – Houston 60-in. Sanitary Sewer Spin Cast Lining

Additionally, traffic detours and flagging were in place to allow for secondary access within the PennDOT roadway. The existing railroad at the top of the riverbank transported loads of steel across the culvert on weekday nights. Contractor site access to the riverbank was coordinated each shift to allow for crossing the tracks to unload materials. Coordination of the access constraints led to a very detailed construction sequence to facilitate the construction of the shotcrete lining of the culvert.

Prior to the lining, the contractor removed more than 400 tons of sediment, gravel and brick from the CSO (within the 15-ft culvert) with a mini excavator. Lastly, construction of the new concrete endwall required shoring and demolition of the existing endwall within 12 ft of the active railroad. The design incorporated Cooper E-80 railroad loading for not only the new endwall and shoring, but also for the shotcrete lining system 30 ft below the tracks to ensure it met the design lifetime requirements.
Inclusive of weather delays, construction was substantially complete in six months except for the fabrication and installation of the metal flap gate.

Project Owner: Pittsburgh Water and Sewer Authority (PWSA) and Allegheny County Sanitary Authority (ALCOSAN)
Engineer: Johnson, Mirmiran & Thompson Inc.
Consultants: University of Iowa and Anser Advisory
Contractor: Allison Park Contractors
Products: Frank Bryan Inc., King Shotcrete Solutions, Sauereisen and Golden Harvest
Value of Trenchless Project (US$): $3,665,000