Dual Force Mains for Enhanced Reliability
Force mains play a critical role in a raw sewage system. It is crucial to be proactive in force main repairs and improvements as force main failures lead to sewage spills and emergency repairs are extremely costly. Repairing only the failed sections leaves vulnerable the remaining length of the force main, which has equally aged. A capital improvement project (CIP) to replace the aging force mains are often warranted.
The West County Wastewater District (District) recently experienced multiple force main failures. After performing emergency repairs, the District awarded LEE + RO with a capital project for three (3) critical force main improvements, and requested that LEE + RO consider a secondary discharge option using a dual force main for enhanced system reliability.
The emergency-repaired, 40-year-old, 1,550 LF, 8-inch ductile iron force main at the Lakeside Lift Station carries pumped flow to a manhole at 500 gpm and 120 ft. of total dynamic head. The first 300 LF of the force main is through brush-covered hillside terrain sloping up to a shopping mall. The remaining 1,250 LF of the alignment follows the right-of-way (ROW) around the mall and then through a common utility trench in the shopping mall parking lot.
LEE + RO specified horizontal directional drilling (HDD) for the installation within the brush-covered hillside terrain. Compared with open trench methods, HDD avoids environmental permits and significantly reduces the construction duration. High-density polyethylene (HDPE) was selected for both sections because of its superior corrosion-resistant properties. The dual force main system is operated via motor-operated valves (MOVs), powered and controlled from the lift station. With dual force mains, future maintenance will be simpler to perform since bypass pumping will not be required.
Challenges ranged from the project terrain, through the pipeline alignment, to environmental concerns, utility conflicts, and easement rights issues. Most challenging was the compressed design schedule set by the District for delivering the bid documents – a 30-working day turnaround from the District’s Notice to Proceed.
The force main at Hilltop Green Lift Station recently suffered a catastrophic failure and a part of the existing force main was subsequently replaced. However, the most upstream portion, consisting of 870 LF of 40-year-old 10-inch diameter ductile iron pipe, was not and it is now the weakest link. As Hilltop Green Lift Station is not adjacent to public rights-of-way (ROW), several routing options were developed. The District selected an alignment that runs through a parcel owned by the Community Center. Construction consists of a combination of short runs utilizing horizontal directional drilling (HDD) in addition to open trench sections. The District also expressed interest in dual force mains where each force main will discharge to a separate manhole for enhanced reliability.
LEE + RO specified HDD on key segments of the replacement force main within the current easement. For example, the control building for the lift station sits immediately downstream of the lift station within the District’s existing easement. Utilizing an HDD method of construction avoids the need for acquisition of costly additional easements and extended construction durations by means of a deep pipeline profile that only the HDD method can achieve. In addition, with the alignment running through a wooded hillside, HDD requires neither environmental permits nor incurs additional project costs for removal of trees, which would be required for open trench methods.
High density polyethylene (HDPE) was the selected force main material of construction for both the HDD sections and the open trench sections due to its superior corrosion resistance properties. The dual force main system operates via motor-operated valves (MOVs), both powered and controlled from the lift station. With dual force mains, future maintenance will be cheaper and simpler to perform since bypass pumping will not be required.
Challenges with this project included developing the force main alignment alternatives because the lift station is surrounded by individual parcels and not adjacent to a public ROW. The construction start was delayed significantly due to prolonged easement negotiations with the Homeowners’ Association (HOA) that represents the neighboring homes and the Community Center. Similar to the Lakeside Lift Station Force Main Project, the design schedule set by the District for delivering the bid documents was compressed to a 30-working day turnaround from the notice-to-proceed date.
The La Honda Lift Station force main is a 60 year old 6-inch diameter cast iron pipe. LEE + RO designed approximately 150 LF of dual force main using HDPE pipe. MOVs allow the selection of the duty and standby force main. HDD construction methods were not used due to short 150 LF pipe length. Because HDPE is flexible, tree removal was avoided while utilizing traditional open trench methods of construction. The project required extensive coordination with the District and the County for easement acquisition and with the District and a neighboring parcel owner for a temporary construction easement.
Challenges with this project were mostly the environment, terrain, and the lack of available mapping information for the existing surrounding parcels. LEE + RO sought to protect an adjacent stream through carefully thought-out construction activities and methods. As with the other two projects described above, the most challenging aspect was the compressed 30-working day design schedule set by the District for delivering the bid documents.