The City of Baltimore released the Sanitary Contract 875 — Rehabilitation of Southwest Diversion Pressure Gravity Sewer — Phase III in early June 2013 with a bid date of Aug. 28, 2013. Designed by engineering firm RK&K, the scope of the project included the trenchless rehab of a 78-in. pre-stressed concrete cylinder pipe (PCCP) utilizing two types of rehab technologies. The first method involved the use of approximately 2,550 lf of CIPP, and the second included 2,140 lf of carbon fiber reinforced polymer (CFRP). This project also required the rehabilitation of a bar screen structure at the upstream section of the 78-in. pipe in which two interceptors converge (one 54-in. and one 60-in.) to form the 78-in. pipe. Lastly, and perhaps most critically, was the provision and maintenance of a dry work environment while the rehab work was performed.
Several pre-bid meetings were held, and after an intensive process that took into consideration detailed information on the bypass including flows and distance to be bypassed, site access details and redundancy requirements, Spiniello was awarded the project as prime contractor and Sunbelt Rentals secured the bypass component. Mobilization began in mid-March 2014.
In order to accommodate the full flow of the 78-in. sewer, the bypass design required two specially designed suction structures installed by Spiniello to accommodate all sixteen 18-in. Quiet Flow pumps on two upstream lines — a 66-in. and a 56-in. sewer. Sunbelt designed the main bypass for a capacity of 156 mgd (108,333 gal/min), and 14.5 mgd (10,000 gal/min) for the 21-in. force main lateral, a section that came in about mid-project in which four 10-in. Quiet Flow pumps were installed. Measuring 5,000 ft in total, this bypass was required by Spiniello to keep the sewer line dry while 2,550 lf of CIPP liner was installed, along with 2,140 lf of carbon fiber reinforced polymer (CFRP).
To successfully execute a project of this magnitude, careful logistical planning had to be done to schedule crews, locate and source equipment, and identify laydown yards. Based on their extensive bypass experience, team members at the Maryland Pump & Power Services branch headed up the project. In addition, national setup foreman Rick Henshaw served as lead setup foreman, and Scot O’Bryan, national bypass manager, oversaw the bypass as project manager. Once operational, the project was manned 24/7 by certified pump mechanics.
Due to the size of the project, equipment had to be sourced from various locations. More than 100 trucks were required to deliver the pipe and pumps alone. In order to convey the required 156 mgd from the main 78-in. sewer and the 14.5 mgd from the 27-in. maidens choice force main, the team needed eight HDPE discharge lines for a total of 45,000 lf of 24-in. pipe. To assemble this immense bypass, the team utilized a mini excavator; two 10,000-lb shooting boom forklifts; three 24-in. fusion machines; two 18-in. fusion machines and one 1,900-lb tracked skid steer with forks.
As with any large project, identifying laydown areas for support materials was critical. Thanks to strong communication between all involved, a careful balance was struck to ensure the bypass setup crews did not get overwhelmed with offloading and run out of storage space, and that they also had enough equipment to maintain production.
The team faced challenges during the course setup, the most critical being the restricted site access. Accommodating all sixteen 18-in. pumps for the suction location in such a limited space made this bypass especially difficult. Thanks to the long hours of preliminary design by O’Bryan and Henshaw, the pumps, piping and discharge manifold fit precisely into the small staging area. Another unique challenge was the eight 24-in. discharge lines running through 5,000 ft of businesses, backyards and alleyways. To minimize the impact, three major road cuts were required with one on a main road.
Prior to the main 78-in. line being plugged to initiate bypass operations, all pumping equipment was checked and rechecked because once the bypass was in operation, it could not be shut down. To assess the integrity of the system, a hydraulic test was performed to test up to 40 psi, 1.5 times the operating pressure on the HDPE discharge lines and manifolds. After a successful 24-hour water test, the bypass went live, the primary pumps were placed in manual, and the backup pumps were placed in auto. Once Spiniello installed the bulkhead to stop the flow, the fine-tuning of the bypass system began by setting automatic transducer control levels and exercising each pump to determine the best configuration. Bypass operation ran ‘round-the-clock with two Sunbelt technicians onsite in two 12-hour shifts.
On Oct. 15, 2014, the Sunbelt watch crew reported high levels in the sewer due to heavy rain. During that time, it was estimated that about 3 in. of rain accumulated in the area, causing heavy flows in the sewer. During the high flow event, the bypass went from an average of two pumps running on the 54-in. sewer and three on the 66-in. sewer, to all but three pumps running in the entire system. Essentially, 13 out of the 16 pumps were running to maintain the high flow. This was the true test of the bypass integrity, and after one full day of high flow, the sewer slowly returned to average flow. Following that incident, the pumps were re-tuned, and subsequently, even with higher-than-normal flows, the pumps never ran that hard again.
The most critical portion of this project was maintaining a dry work environment while the rehab work was completed. Because the project was performed during the fall and winter months, industrial climate control solutions were utilized to support the rehabilitation process. Spiniello’s subcontractor Structural Preservations required precise dehumidification and heat for the surface preparation and drying of the pipe while the carbon fiber pipe was installed. Sunbelt provided the ideal conditions with temporary 150-kW electric heaters powered by 175-kW generators.
After 120 days of bypass operation, Spiniello completed lining activities and pulled the plug, concluding a successful execution of the bypass. Despite the challenges posed by restrictive access and heavy rains, this project was a success thanks to strong communication and coordination between Sunbelt Rentals and Spiniello and marked the effective completion of one of the largest sewer bypasses in the region.
Ladd Gould is national strategic account manager at Sunbelt Rentals.