- ENGINEERING INFO
- CASE STUDIES
As underground utilities age, they often become an immediate liability for the utility owner. Sanitation districts across the country wrestle with the challenges of maintaining and operating their systems, while also considering budgetary and regulatory constraints. Though aging force mains are assessed and budgeted through a capital improvement process, line operators routinely face major arterials that need immediate repair and are commonly addressed through an emergency repair process. As such, when the aging Pearl City Force Main reached a critical stage in 2010, the City and County of Honolulu took immediate action.
The Pearl City Force Main is a large diameter, dual force main system that experienced four breaks over a four-year period. The breaks all occurred within a 250 LF section of the 11,500 LF pipeline system, which then consisted of 30-inch, 36-inch, and 42-inch ductile iron pipe. At the location of the breaks, both pipes were installed in corrosive soil conditions and were subject to changing groundwater levels influenced by sea level changes. If this problem went unaddressed, it would have posed a serious public health and environmental concern to the City and County of Honolulu. Therefore, Honolulu declared the project to be an emergency repair and contracted The Limtiaco Consulting Group (TLCG) to engineer a solution.
Several construction alternatives were evaluated: sliplining; cured-in-place-pipe (steam, water, and ultraviolet curing methods); horizontal directional drilling; tight-fit liners; and shallow-bury, open-cut pipe replacement. Sliplining rehabilitation was selected as the preferred method to meet the challenging site constraints and aggressive schedule tied to the emergency declaration. In order to accelerate the project, a construction contract was awarded with pre-final design drawings. The strategy was to make engineering adjustments during construction as unknown conditions were encountered, such as existing bends, fittings, and thrust blocks.
Pipe Material Selection
The alternative analysis performed by TLCG indicated sliplining with plastic pipe was the most cost-effective solution, saving just under $1 million compared to conventional open-cut/shallow-bury installation. Although there had been very few sliplining projects locally, the minimal impact to the surrounding area and expeditious process made it an appealing alternative. The cost of maintenance after installation would also be considerably lower.
The pipe material would have to accommodate the maximum system surge pressures, in addition to the daily flow conditions, which required a 125 psi minimum pipe pressure class. Utilizing comparable pipe pressure ratings, an assessment comparing HDPE and Fusible PVC® pipe was performed. The study indicated that, on average, the use of Fusible PVC® pipe would provide a 20% greater flow area than HDPE. The investigation also revealed that Fusible PVC® pipe has higher tensile strength thus allowing for longer pull lengths, making the final decision even easier.
|Pipeline Details and Project Summary|
|Project:||Pearl City, HI|
|Slipline Length:||1,200 LF of 24-inch and 36-inch each|
|Pipe Size:||36-inch DR32.5 Fusible PVC® Green 24-inch DR25 Fusible C-905®|
|Owner:||City and County of Honolulu|
|Engineer:||The Limtiaco Consulting Group|
|Contractor:||Hawaiian Dredging Construction Company|
|UGSI Contact:||Rob Craw (925) 577-7566 firstname.lastname@example.org|
Hawaiian Dredging Construction Company (HDCC) mobilized to the site in the fall of 2011 and cut the project into four insertion events based on pipe alignment. Once the pipe was sliplined into place, the newly installed Fusible PVC® pipe was reconnected to the system utilizing standard ductile iron mechanical joint connections. The system was then pressure tested and put back into service.
Chad Uyehara, project manager for HDCC, stated, "The project went very well, and UGSI was extremely supportive with their field crews. We look forward to working with them again."
TLCG project manager Kyle Kaneshiro said, "The project was an overall success."