NC State University Campus Project
Moffat Pipe, Inc. and HammerHead teamed up to perform a tricky pipe-bursting project at North Carolina University, where in-depth planning and accurate execution were required to avoid multiple obstacles.
For returning North Carolina State University (NCSU) students, the only evidence that a 255 ft length of sewer pipe had been replaced beneath their feet two days earlier was a brand new manhole near Tomkins Hall. The golf-course-quality turf had been carefully replaced, as has the pavement walkways. Moffat Pipe had completed the works just in time for the fall semester by utilising pipe bursting to replace a 6 inch terra cotta pipe with 8 inch high density polyethylene pipe. With the excitement among students of fall registration and festivities, it’s safe to say the new manhole went unnoticed.
Preserving historic landscaping
In addition to a tight deadline, Moffat had to negotiate some extraordinarily difficult restrictions. One was that tree roots of the historic landscaping enveloped the pipe along most of its length. Among the trees is a venerable Chinese Evergreen Oak whose trunk stands only 4 ft off the pipe’s path. The tree is one of only two on campus, both bearing placards that give their individual identification numbers and their scientific name, ‘quercus myrsinifolia’.
Students are warned not to harm so much as a twig of this tree, which meant caution would have to be taken by Moffat to pull an 8 inch replacement pipe behind a 7 inch HammerHead pneumatic hammer right through the network of roots. The 11.5 inch diameter bursting head chosen for the project fragmented the existing 6 inch pipe down its length, pressing the remnants to the sides of the hole. Craig Moffat, one of Moffat Pipe’s owners, said at one point, as they were going through the root system of the trees, the hammer progress was excruciatingly slow.
“I was a little nervous it was not going to make it,” said Mr Moffat. “But the burst was completed without hurting either the oak or any other tree – including one growing directly on top of the pipe’s path.”
Choosing the right technique
The 80-year-old clay sewer had several known leaks, suffered from root intrusion and was one size below today’s specification for 8 inch diameter pipe in this application. Simply sleeving the 6 inch pipe was not an option due the condition of the clay pipe and the need to maintain flow capacity.
Hydrostructures designed the project, and Division Manager of Engineering Brandon Sykes said pipe bursting was the only technique that could be used for this project, given the historic value of the location. Open trenching would have killed the trees and scarred the famous aesthetics of the courtyard. Mr Sykes said pipe bursting was not only least intrusive but also least costly method – only pipe bursting could increase the sewer pipe diameter to code without more excavation.
Team of specialists
The City of Raleigh entrusted the delicate, high-profile job to Moffat Pipe based on the company’s performances as a specialist in completing other municipal installation and replacement contracts. Moffat Pipe often uses HammerHead trenchless equipment, owning static pipe bursting equipment for 4–6 inch lateral replacement jobs. Moffat consulted with HammerHead to rent equipment suited to the job’s special requirements.
Brian Cowles, Southeastern US Sales Manager for HammerHead, recommended pneumatic pipe bursting rather than static technique for this job, setting Moffat Pipe up with its 7 inch reversible hammer and an 8 inch DIPS burst head. To complete the bursting system, Cowles matched the hammer and head to a HydroGuide HG12 winch.
In a pneumatic pipe bursting setup, a winch is generally used to keep a steady, guiding pull on the burst head, as well as to counteract the increasing drag as replacement pipe is pulled through behind it. Although the HG12 is capable of maintaining up to 12 tonnes of pull, less than 5 tonnes was needed at any time during this project, and for the most part it ran steadily at 4 tonnes. The hammer itself was powered by
375 cubic feet per minute of compressed air set up to 110 pounds per square inch on this job, Cowles said.
Bursting alongside other utilities
Mr Moffat described the red clay surrounding the pipe as “a little hard but not too bad”. He explained that even though the pipe had sat in the ground for decades, the clay originally used to fill in the pipe trench still accommodated expansion better than regular soil. Moffait Pipe would have the room for a good burst, yet it was not going to be a simple run.
The vacuum truck and air knife survey Hydrostructures conducted prior to designing the project verified the known complications. The knife uses high pressure air to form an 8 inch diameter bore with little risk to any other utilities. The soil is vacuumed into the truck as it is removed. This potholing showed that the current sewer pipe lay directly on top of a storm sewer and also passed above a steam tunnel. Nearer Tomkins Hall it lay immediately beneath a
12 inch watermain. The main was well within the compression zone of the pipe burst and might be adversely affected by expansion during the burst.
Planning flexibility
Unable to guarantee the integrity of the aged watermain pipe, the project team altered the bursting plan slightly. Moffat said initial plans called for a single excavation, and an 8.5 ft deep, 4×30 ft entry pit, whereby the winch would be positioned upstream of the manhole at Tomkins Hall. However, in deference to the watermain, an additional manhole was installed at the edge of the brick pavement approximately 50 ft from Tomkins Hall.
Moffat said the earth around the storm pipe and steam tunnel was carefully excavated before the burst to alleviate soil compression against those structures when the burst head pushed material aside in its progress. Just as Brian Cowles planned, the head, hammer and replacement pipe edged past the utilities during the expansion without any impact on old conduits nearby.
Where the existing pipe had been positioned above the storm sewer, the original installers had maintained its grade by cutting into the storm sewer’s top. Moffat’s crew repaired and sealed the storm sewer before burying the pipe there.
The one-hour burst seemed anticlimactic compared to the week spent preparing for the burst, dealing with the utility concerns, building a new manhole and then cleaning up afterward. Southernscapes co-ordinated with the university’s own ground maintenance crew to replace grass sod, since a proprietary hybrid Bermuda seed was required to match the lawn. Any pavers and bricks that had been removed were replaced.
NCSU’s ground maintenance office said by mid-September there was little sign the project had taken place. Despite some delay and a slight change in plans to work out the demanding requirements of this application, it was a prime example of pneumatic pipe-bursting technique used to its best advantage.
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