26 января 2015
The right Choice for HDD
This article also appeared in the July 2014 issue of Trenchless Technology.
Like several of its counterparts in Florida, Pasco County is a name frequently repeated in the news when it comes to growth. Founded in 1897 and located just north of Hillsborough County and Tampa, the relatively young Pasco County is approaching a population of 500,000 — spurred in part by a 35 percent increase between 2000 and 2010.
Less known to the public but noteworthy nonetheless, Pasco County Utilities has made headlines in the waterworks industry as a pioneer in the application of horizontal directional drilling (HDD). HDD is a technique in which pipe joints are assembled above ground and then pulled through a hole drilled under natural and manmade obstacles. HDD allows for environmentally friendly pipe installations under wetlands, rivers, creeks, highways, railroad tracks, buildings and other obstacles.
Pasco County Utilities recently tied a record for the longest pullback — or length of pipeline pulled underground — on an installation with AMERICAN Ductile Iron Pipe 36-inch Flex-Ring® pipe. That installation, which went under wetlands, was 1,640 feet in length.
“It was necessary to not disturb the wetlands because they are part of the county’s established wildlife corridor that links major environment preservation lands/properties within the county,” said Bruce Kennedy, assistant county administrator of Utilities for Pasco County. “The wetlands are home to bald cypress, water hickory, maples, oaks, cabbage palms and bay trees with a dense understory of vines, ferns and herbaceous plants. They provide habitat for river otters, deer, squirrels and a variety of birds and snakes.”
The record-tying HDD installation in Pasco County was one of three executed under more than 4,000 feet of wetlands during construction of the North Central Reclaimed Water Transmission Main. The other two HDD installations also used AMERICAN Flex-Ring 36-inch ductile iron pipe and saw pullbacks of 1,140 feet and 1,340 feet.
Pasco County is a longtime user of AMERICAN ductile iron pipe and has used it several times on previous HDD pipeline construction jobs. The utility gained experience by initially doing shorter HDD jobs with smaller diameter pipe before taking on larger HDD installations involving 36-inch-diameter pipe. One of those was in 2006 in which the pullback was 940 feet.
That was a record pullback for 36-inch-diameter pipe until the recent installation on the North Central Reclaimed Water Transmission Main, which tied the record of another HDD installation in Meridian, Mississippi. Both of those installations measured 1,640 feet and both were done with AMERICAN Flex-Ring pipe. In all, Pasco County installed more than 38,000 feet of AMERICAN 36-inch ductile iron pipe as part of the North Central Reclaimed Water Transmission Main.
Pasco County Utilities prefers the strength of ductile iron pipe, and “AMERICAN’s Flex-Ring restrained joint is ideal for HDD jobs, because it provides a boltless connection, good joint deflection and ease of assembly,” Kennedy said.
Garney Construction Company was the general contractor for the North Central Reclaimed Water Transmission Main and subcontracted the HDD installations to Cornerstone Businesses, Inc., of Zephyrhills, Florida. While Cornerstone had experience pulling smaller diameter flexible restrained joint ductile iron pipe in an HDD application, this project and size of pipe was a first for the company. Cornerstone used a Vermeer 330 x 500 drill rig, which was suitable for handling the predicted installation load of approximately 110,000 pounds, which was much less than the allowable pulling load of 310,000 pounds for the 36-inch Flex-Ring joint.
According to Kennedy, AMERICAN’s level of service for the HDD jobs was excellent. “An AMERICAN representative was present for the drilling operations and was very instrumental in providing guidance to the driller. AMERICAN provided the HDD contractor whatever training, guidance and technical support was needed to successfully execute the HDD pulls,” Kennedy said.
The pressure that rapid growth puts on a utility’s infrastructure is well-documented, and in Pasco County that also applies to the reclaimed water system. That’s because Pasco County, which covers 750 square miles, uses all of its treated effluent through spray irrigation or rapid infiltration basins without any surface water discharge or deep-well alternatives for handling excess wet-weather reclaimed water supplies. All flows must be handled by the reuse system, which consists of two major wastewater treatment plants and interconnecting reclaimed water transmission mains that move reclaimed water between a heavily residential area in one area of the county and an extensive system of rapid infiltration basins in another area.
The North Central Reclaimed Water Transmission Main is a major improvement to that system, which will get another major asset in February 2015 with completion of a 500-million-gallon reclaimed water storage reservoir.