Large diameter sewer diversion is a massive undertaking with each project having its own unique set of challenges. Essentially, there are only two outcomes of a bypass: either it went well, or horribly wrong.
The differentiator between success and failure of a bypass is in the planning. The following is a recap of a successful large diameter sewer bypass located in Saskatoon, Saskatchewan, Canada, during the winter. This sewer bypass was installed to allow for the over-the-hole installation of a 2,100-mm, cured-in-place pipe (CIPP) liner in Saskatoon’s interceptor trunk just upstream of the City’s wastewater treatment plant.
To reduce cost and negate liability, the City of Saskatoon put tremendous thought into the design of this project. The timing of the project was a critical factor because during the summer months the flows were much higher due to the seasonal surcharges caused by inflow and infiltration (I/I). Therefore, by choosing to perform the project in the dead of winter the City was able to reduce the size of the bypass, and ultimately, save bypass costs.
The winter months offered the city a respite; however, this became the biggest concern during advanced bypass planning. How do you keep the pipes and pumps from freezing? Every detail had to be planned and scrutinized to mitigate any chance of failure.
Designing an Effective Bypass
Just to bid an effective sewer bypass, items such as flows, suction access, discharge access and elevations all must be taken into consideration; if an item is missed, then the entire project has the chance of becoming a costly failure.
Dan Willems, the engineer for the City of Saskatoon, provided thorough bypass specifications at bid time, which exceeded most municipal standards. We have found this to be uncommon in the marketplace, because some municipalities and engineers regard the bypass portion of a project as the contractor’s “means and methods” requiring the contractor to be responsible for gathering all the required information such as flow, site access and elevations to build a complete bypass bid. Since the City provided detailed bypass specifications, nothing was left to chance.
To meet the bypass design requirements, a total of six 450-mm Quiet Flow Diesel Auto-Priming pumps were used; three of which were required to maintain the peak specified flow of 1,600 l/s (36 MGD), with two additional pumps tied in as backup providing 66 percent bypass redundancy. As an extra measure, an additional 450-mm pump was placed just out of the bypass system in the event of a total catastrophic pump failure.
All five of the pumps were tied into a large manifold 12 m in length with five 450-mm ports with knife gate valves to independently isolate each pump. And on the discharge side, three 450-mm ports with knife gate valves for the three 450-mm high-density polyethylene (HDPE) discharge lines, which discharged 560 m into the headworks of the Saskatoon Wastewater Treatment Plant.
Working in the Winter
It’s one thing to work in the northern part of the United States in the dead of winter, but the northern part of Saskatchewan takes it one step further. Just two weeks prior to the start of the bypass construction, Saskatoon saw a record low temperature of -38 degrees C.
Luckily, this cold climate was a factor in the bid of the bypass, because not only was the winterizing of the equipment taken into account, but also outfitting personnel with the proper PPE and cold weather gear. Fortunately, -30 degrees C was the coldest temperature experienced during the construction and operation of the bypass.
One of the technical challenges of this bypass design was accounting for the depth of the 2,100-mm sewer at the suction point. In order to achieve the required 1,600 l/s (36 MGD) personnel either had to surcharge the sewer, and/or dig a suction pit excavation. It was determined that due to the manholes upstream being at lower elevations, the suction pit was the only option.
Hamm Construction Ltd., a local Saskatoon civil contractor, was hired by Insituform to perform 21- by 15-m and 4-m deep excavation. It was really quite impressive, not only did they dig the excavation, but they cut five suction access ports in to the top of the 2,100-mm sewer to greatly reduce the suction lift.
Once the pumps were set into place, the pumps were “hoarded.” All the pumps, and even the manifold, were enclosed into a temporary structure with insulated fabric and indirect-fired heaters were plumbed in to provide heat. Apparently, it worked so well the technicians performing pump watch were able to work in T-shirts.
Before the sewer plug was installed, pumping equipment was checked and rechecked because once the bypass was in operation, there was no shutting down due to the risk of lines freezing. Therefore, everything had to be 100 per cent operational. The bypass system was air tested instead of hydraulically to avoid freezing the three unprotected 450-mm HDPE discharge lines. After successfully passing the air test a pneumatic 1,500-mm to 2,400-mm sewer plug was installed after the plant’s influent lift station pumped down the 2,100-mm line surcharge.
Operating the bypass started out routine with Sunbelt Rentals performing around the clock pump watch with one technician onsite in three eight-hour shifts. After a few days, this was short lived because the pumps were being clogged with an abnormally high amount of debris. This was mitigated by a very labor intensive process of manually removing the debris from the pumps impellers.
In response, Sunbelt sent another technician to assist in the around the clock debris removal. The pump watch was then adjusted to two technicians on-site around the clock in twelve hour shifts. It was later determined that the cause of the debris was due to the 84-in. line being surcharged for an extended period of time and the line decanted.
Insituform was experiencing similar troubles with the heavy cleaning of the sewer line. The Sunbelt technicians had the debris removal down to a science and could even tell you what day of the week it was by the type of debris that was being removed from the pumps. The plug was pulled after 26 days of successful bypass operation.
Despite all of these challenges, teamwork and a great working relationship with the owner the City of Saskatoon and the prime contractor Insituform Technologies Ltd., and Hamm Construction Ltd. led to a successful execution of one of the largest sewer bypasses in Western Canada.