Much of the discussion about our aging infrastructure concerns the decay of the dense web of underground stormwater and wastewater pipelines that interlace the nation. Unfortunately, the intended life of many of these pipelines has elapsed. Decades of exposure to corrosion, rust, and erosion are now causing many of them to weaken and fail, causing problems such as cave-ins, flooding, and consequential damage that creates emergency situations.
Depending on the type and location of a pipeline, expensive emergency open-trench reconstruction projects usually create nasty and seemingly interminable consequences, such as disruption of commerce, traffic detours, dangerous health conditions, and extensive rebuilding of facilities. Using trenchless pipe rehabilitation produces minimal disruption to surface areas and can be a less expensive and faster approach, not to mention improve the existing pipe's flow characteristics.
To minimize surface disruption and avoid emergency situations such as pipeline cave-ins, regular pipeline inspections should be conducted using video cameras or even walking personnel through larger pipelines to determine when and where repairs should be made. This should be done on an annual basis when pipelines have been installed 10 years or longer.
However, once a pipeline or portions of a stormwater or wastewater pipeline require refurbishing, use of a trenchless technology known as slip lining, whereby heavy-duty, high-density polyethylene (HDPE) pipe is inserted through an original pipe system, can provide many years of extended service.
One of the most important benefits of trenchless pipe restoration, including the use of slip linings, is that it is minimally invasive. The surface of areas where pipes are to be restored does not have to be excavated, which means much less interference with traffic and businesses near the project site. Additionally, when compared with replacing with an entirely new pipe, slip lining is effective and lower in cost. While the slip lining must be of a smaller inside diameter than the host pipe, many times the flow capacity remains much the same because the new liner offers improved hydraulic characteristics.
In cases where a portion of the original pipeline has caved in, the collapsed area might have to be shored up with a protective shield. But for the most part, trenchless contractors simply dig a pit at a convenient position along the pipeline and push the new liner through.
Saving the day for shoppers
Slip lining is an excellent solution when disruption of business or traffic is a key consideration. Such was the case at The Point Shopping Center in Harrisburg, Pa., when an 800-foot-long stormwater drainpipe started to collapse beneath the center's 25-acre, 1,200-car parking lot. Owned and operated by Cedar Realty Trust, Inc., a fully integrated real estate investment trust that owns more than 90 shopping centers nationwide, a large sinkhole appeared in the lot during the summer of 2011.
"When I investigated the site, I found that the cave-in was not due to a geotechnical problem, it was the result of a failed drainage pipe caving in 30 feet under the parking lot," said Robert Mastandrea, Cedar Realty corporate director of special operations.
Although he had no direct experience with slip lining at the time, Mastandrea had heard of it, and thought it might be a good solution for the situation at The Point. He explored the local market for a contractor with slip lining capability, and decided to contact Aaron Enterprises.
"Aaron engineers visited the site, evaluated the situation, and then recommended the slip liner approach," Mastandrea explained. "The firm was also equipped to shore up the cave-in, and could handle the re-paving requirements, so we had a single bid opportunity."
Instead of having to cut an 800-foot open trench that would disrupt a large portion of the parking lot, which sometimes accommodates several thousand cars per day, Aaron dug a relatively small working pit (approximately 12 feet by 40 feet) at the far end of the lot, from which they slipped the liner into the drainage pipeline. The original 42-inch-diameter drainpipe was lined with a 32-inch HDPE insert.
"This approach saved us a lot of problems with business disruptions of the retail stores," Mastandrea said. "That was very fortunate, since I am sure that the sight of a large open trench and heavy excavation equipment would have sent customers away from the center."
The quarry quandary
Another stormwater pipeline collapse occurred a year ago at a major quarry located in Virginia. The pipeline, which is approximately 800 feet long, is part of a stormwater diversion system that protected the quarry and surrounding area from flooding that could result from heavy weather.
Under normal circumstances, the diversion pipeline drained into a nearby river. However, when a hurricane struck the region, the aging 72-inch pipeline, buried under 75 feet of earth, partially collapsed and threatened to flood the quarry as well as a nearby asphalt plant. Engineers from Aaron Enterprises were called in to survey the situation, locate the section where the collapse had occurred, and recommend a solution that would prevent future flooding and collapses.
Using a tunneling shield and a new steel casing, Aaron Enterprises encapsulated and removed the collapsed pipeline. They had to tunnel back 150 feet and remove large boulders, tree stumps, and the collapsed pipe from the discharge end. An 800-foot length of 63-inch-diameter HDPE liner weighing 208,000 pounds was then fused together and pulled through the existing and newly encased portion of the corrugated pipe. The annular space between the host pipe and the new HDPE liner was then pressure grouted to restore the overall structural integrity to the pipeline.
To avoid other catastrophic problems, the quarry owner asked Aaron Enterprises's team to conduct an evaluation of similar piping on other properties. This is the recommended approach for anyone wanting to ensure the quality and safety of their pipeline infrastructure. The evaluation allows engineers to establish priorities and recommend repairs that may be needed in the future to avoid emergency repair situations.
Vincent T. Rice is president of Aaron Enterprises Inc. (www.aaronenterprises.com), York, Pa., an integrated provider of trenchless excavation technologies including directional drilling, auger boring, pipe jacking, and micro tunneling as well as slip lining of existing pipelines.