ALCOSAN Flow Control Management
Location: Homestead, PA
Owner: Allegheny County Sanitary Authority (ALCOSAN)
General Contractor: Robinson Pipe Cleaning Company
Date Complete: July 2013
Buried in the Monongahela River bed, just offshore of Sandcastle Waterpark, is a 30-inch interceptor line owned by Allegheny County Sanitary Authority (ALCOSAN). The pipe transports storm and waste water from the Homestead area to ALCOSAN's treatment plant near Bellevue some 10 miles away.
Approximately every ten years, the line must be cleaned to allow proper flow. In the past this involved installing a sheet pile coffer box around the pipe, which was sealed to the pipe with grout. The conventional coffer box was then dewatered and the pipe was cut to provide access for the cleaning equipment. Once the pipe was cleaned and repaired, the coffer box was removed. This process, while effective, was time consuming and inefficient.
In early 2013, Brayman was contracted to install precast concrete coffer boxes around the pipe to allow for easier access to the pipe for cleaning operations. The design, developed by Brayman and The Lewis Consulting Group, consists of a permanent base unit with watertight lid and temporary riser units. Brayman Precast was responsible for fabricating all of the precast components.
Excavation was extremely challenging, given the 20-foot depth of the pipe below the water surface and the uncertainty of its exact location. The risk of rupturing the pipe was significant, due to its contents and location in the river. Divers were instrumental in the excavation. The team employed several methods for removal, including clam bucket, air lift and air lance.
The base unit contained a "doghouse" to fit around the pipe. Tremie concrete was poured around the pipe and base unit, essentially joining the two with a water-tight seal. Once the tremie concrete achieved the proper strength, riser sections that projected above the water surface were eventually stacked onto the base unit. The entire assembly was stressed together using four all-thread rods. Neoprene seals in each of the joints created water-tight installation once stressing was complete.
After dewatering of the ports, which was achieved in about 30 minutes, Robinson Pipe Cleaning cut into the pipe with a hydraulic saw and began cleaning operations. Once the pipe was cleaned, a water-tight lid was installed on a flange cast into the base unit, and the riser sections were removed. The base unit remains in place for future cleanings.
This project demonstrates the innovative approach that Brayman brings to all of its projects.