- ENGINEERING INFO
- CASE STUDIES
The City of Thornton, Colorado water distribution staff maintains 530 miles of water lines and related appurtenances, such as fire hydrants (over 4,600), valves, and service connections. Many of the water lines in the distribution system have reached the end of their useful life. Several waterlines have already been rehabilitated by sliplining, including 3,640 feet of 20-inch steel transmission main beneath Washington Street that was sliplined with 16-inch Fusible PVC® pipe between 2011 and 2013.
In 2014, a 320-foot segment of 30-inch steel transmission main which passes over Interstate 25 via the Thornton Parkway overpass was targeted for rehabilitation. The aerial, suspended line hydraulically connects both halves of the city, making it a vital asset to maintain in good operating condition. Several trenchless methods were considered by the City, but sliplining (with a slightly smaller pipe) was ultimately selected because it actually reduced the structural load on the aging bridge hangers.
City engineers determined that 24-inch replacement pipe was an acceptable size for the crossing because the transmission main on either side of the I-25 overpass was 24-inch ductile iron pipe. Sliplining with 24-inch pipe would maintain relatively constant hydraulics with minimal head loss.
Once again, the City turned to trenchless installation; this time putting out a bid to slipline 24-inch Fusible PVC® pipe into the 30-inch steel host pipe. Contractors were invited to bid, including BTrenchless and American West. BTrenchless won previous phases of the 20-inch steel rehabilitation project in 2011 and 2013. In 2014, American West was the low bidder for this project.
The City selected Burns & McDonnell to provide the engineering design and construction plans. Mike Lehrburger of Burns & McDonnell reached out to Underground Solutions during design for recommendations and technical assistance regarding various material properties that might impact the project design. For example, because the line was installed in an overpass above I-25, it was critical to identify and properly manage potential expansion and contraction of the fused PVC pipe material.
Underground Solutions offered the following design considerations: 1) fused PVC pipe would expand or contract 0.36 inches per 100 feet per 10 degree change in temperature; 2) grouting the annular space between the replacement pipe and host pipe with a low-density grout could provide surface friction on the pipe to help minimize expansion and contraction; and 3) expansion joints were an additional option to help manage the effects of temperature change.
Mr. Lehrburger ultimately decided to specify expansion/contraction/deflection joints on both sides of the interstate crossing, electing to leave the annular space empty to avoid adding additional weight to the steel bridge hangers. At 71.1 pounds per foot, the new 24-inch DR18 PVC replacement pipe was already adding 22,750 pounds to the structure in addition to the existing 30-inch steel host pipe which would remain in place. In lieu of grout, the team decided to specify casing spacers to prevent the pipe from moving under pressure. Lehrburger commented: “We considered other restrained-joint thermoplastic products, but the City prefers PVC. Fusible PVC® pipe offered the greatest benefit because we were able to maximize the carrier pipe diameter by eliminating bulky external harnesses. Use of bell-and-spigot pipe would have forced a smaller diameter, and negatively impacted hydraulics. The best part about using Fusible PVC® pipe is what’s left behind when the project is complete: a brand new, fully structural PVC water main that everyone in the City is comfortable working with.”
|Pipeline Details and Project Summary|
|Project:||2014 Water Rehabilitation|
|Pipe Size:||24-inch DR18 Blue|
|Pressure Test:||150 psi|
|Owner:||City of Thornton|
|Engineer:||Burns & McDonnell|