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SITUATION
Constructed in 1987, the Wood Road Wastewater Pump Station is an important part of Riverside’s underground system. The station, which discharges to the Trautwein & Allessandro 16-in., ductile iron force main (8,100 ft), ends in a transition manhole that is located near the busy intersection of Allessandro Boulevard and Barton Street in the popular Mission Grove neighborhood. After experiencing two force main failures that resulted in major sanitary sewer overflows and expensive emergency repairs, the City reached out to Carollo Engineers Inc. to evaluate the situation and develop a solution to prevent further failures.
Wary of past issues with rehabilitating force mains, Carollo met with City staff at the Regional Water Quality Control Plant (RWQCP) to evaluate the situation, determine the condition of the pipeline, and consider possible rehabilitation methods.
CHALLENGE
To avoid any further issues with the corroded existing force main, Carollo evaluated three different options for rehabilitation. Close-fit liners, sliplining, and cured-in-place-pipe (CIPP) pressure liner systems were investigated thoroughly and screened to identify the most viable alternative for quickly restoring the long-term reliability of the force main. Based on the pipe’s condition, Carollo recommended a fully structural solution. In addition, sliplining was eliminated due to excessive pipe interior reduction (2 to 4 in.) with a corresponding reduction in design flows. The remaining two options were further studied and, ultimately, CIPP was recommended.
A notice of award for the emergency project was given to SAK Construction (SAK) on June 1, 2017, with the project set to start on June 21, 2017. Headquartered in the St. Louis area with regional offices in Baltimore, Nashville, Sacramento, Las Vegas and Phoenix, SAK is an industry leader in trenchless technology and has extensive experience with challenging CIPP projects. Riverside, following through with Carollo’s recommendation, determined that SAK was the right choice to meet all of the project requirements. SAK collaborated with Carollo to determine the best CIPP lining design to effectively rehabilitate the force main. The project’s requirement to have a “fully structural” liner led to the use of a 16-in., fully-structural fiberglass reinforced liner installed into the same sized existing ductile iron pipe.
The emergency project required that the liner be designed for an operating pressure of about 50 psi. After some investigative work, along with the City of Riverside’s pump station personnel, it was determined that the surge pressure for this station was equivalent to roughly 160 psi. The surge pressure needed to be addressed to ensure that the liner’s maximum pressure capability was not exceeded.
As a result of the significant pressure, Carollo and the City of Riverside decided that modifications would need to be made to the pump station in order to support the CIPP solution. Carollo recommended installing variable frequency drives (VFDs) on the electrical pumps in conjunction with a new 3,500-gallon surge tank to ensure the peak pressure would not overwhelm the new CIPP liner once it was installed. The City ordered and installed the VFDs and the surge tank was added to SAKs contract.
SOLVED
SAK proceeded to set up traffic control and an above ground bypass, consisting of two HDPE parallel lines, to temporarily carry wastewater from the pump station to the gravity line downstream of the transition manual. This was done to isolate the working area, dry out the existing pipeline, and avoid another potential pipeline failure.
With no manholes or valves available to access the existing force main, SAK needed to create eleven 10-ft x 8-ft access pits at different intervals to install the 8,100 lf of 16-in. CIPP liner. This was done in two stages. The first stage had SAK crews excavate down to the pipeline, cutting slots into the top of the pipe to facilitate the cleaning operation and allow the water to drain.
The second stage consisted of removing 4-ft existing pipe sections within the access pit and installing two ductile iron spool pieces on each side of the access pit. The new spool pieces allowed the CIPP liner to be installed from that point down the line. In addition to the CIPP installation, at the City’s request, approximately 180 lf of the pipeline was rerouted with new PVC pipe from the pump station in order to eliminate the force main flow traveling underneath a nearby residential property.
Once all of the liner was installed, at each excavated pit internal end seals were added at the termination points of the liner and access point were closed with PVC pipe.
In addition to thoroughly inspecting the existing pipe, SAK pressure tested each individual liner after it was installed, which resulted in a full system pressure test of 75 psi that was successfully sustained for one hour.
