Beneath the streets of the City of Pittsburgh, Pennsylvania, there is an intricate network of aging sewers. Built in the early part of the 20th century, many of these conduits are constructed of brick, which has tended to deteriorate and disintegrate over time. The Pittsburgh Water and Sewer Authority (PWSA), the authority that oversees wastewater operations in the city, needed assistance stabilizing several sewers that ranged in size from 46 inches to 10 feet in diameter. These pipes were located between 20 and 40 feet below street level.
Graciano Corporation was engaged as general contractor, with ATS Chester Engineers providing engineering services. The project required careful sequencing to maintain service while work was performed in confined, hazardous underground conditions. Graciano self-performed all of the work, completing the project in May 2010.
Scope of Work
Services Performed
- Cleaning dirt and debris from the sewers targeted for stabilization
- Diverting storm water and raw sewage away from the work areas
- Monitoring the worksite for hazardous gases
- Removing loose bricks
- Installing steel rods in the ceilings and walls of the tunnels for structural stability
- Installing galvanized mesh around the circumference of the tunnels
- Spraying a two-inch coating of shotcrete over the mesh
- Water control injections
- Masonry cleaning and pressure washing
Underground Site Conditions
Working in brick sewer conduits at depths of 20 to 40 feet below grade presents significant logistical and safety challenges. Sewers must remain in partial service during repairs, requiring Graciano crews to divert active storm water and raw sewage away from the areas under work at all times. All personnel working in the confined space environment were trained for confined-space entry, and continuous gas monitoring was maintained throughout the project to detect methane and other hazardous gases that can accumulate in sewer systems.
Structural Reinforcement and Shotcrete Application
The core repair method for the deteriorating brick conduits involved a systematic process of cleaning, mechanical reinforcement, and shotcrete application. After loose bricks were removed and surfaces were cleaned, Graciano installed steel reinforcing rods in the ceilings and walls of each tunnel to provide structural support to the surrounding brick shell. Galvanized wire mesh was then installed around the full circumference of the conduit, tied to the steel rod framework. Finally, a two-inch coating of shotcrete was applied over the mesh, encasing the reinforcement and forming a new structural liner that restores the sewer's load-bearing capacity and seals the brick from further deterioration.
Shotcrete Technology
Shotcrete is particularly well-suited for confined underground applications because it can be applied to overhead and curved surfaces without formwork. Graciano used both wet-mix and dry-mix shotcrete processes depending on the specific conditions of each sewer section. In wet-mix shotcrete, concrete is premixed with water before being pumped through a hose and applied at high velocity. In dry-mix shotcrete, the dry materials are drawn into the application system and water is added at the nozzle. Both methods produce a dense, well-consolidated material that adheres tightly to the reinforcing mesh and surrounding substrate.
About Graciano Corporation
Founded in 1916, Graciano Corporation is one of the oldest and most experienced masonry and concrete restoration firms in the United States. With offices in Pittsburgh, PA, New York, NY, and Florida, Graciano performs approximately 90% of all work in-house with its own skilled journeymen, and bids nationally on public-agency, municipal, and infrastructure restoration projects. The company is a WBENC-certified Woman Business Enterprise with extensive experience working for public owners including water authorities, transit agencies, and civic entities across the eastern United States.