How do you install or rehab water, sewer, power, communications, gas or other lines under highway intersections, buildings, rivers, railroad crossings, and other barriers?
The answer is often to use “trenchless technologies,” which, like unseen electric moles, excavate earth, replace and repair existing water and sewer systems, install new pipelines and cables, and remove the excavated material. Trenchless technology will be used more and more in the future — as will traditional “cut and cover” procedures, to meet skyrocketing needs for installing new, or replacing old, underground infrastructure.
No longer is “Dig We Must” a motto of all underground work. In contrast to usual techniques using equipment like backhoes or excavators to dig trenches which then need to be refilled after a conduit is laid, the trenchless approach involves minimal digging or disruption of facilities, or traffic flow, on the surface. “No Dig” is one of its maxims.
“Going Trenchless” avoids disrupting urban or rural economic activity on the surface and also can solve environmental problems. When contractors meet an environmentally-sensitive area such as a wetland, for instance, they can simply tunnel under it with remote drills and other equipment. This remote technology also is frequently used to bore cable or other lines beneath rivers.
Trenchless technologies were pioneered in England, Germany and Japan, where much older infrastructures needed to be repaired. They have slowly emerged into greater recognition as more people have learned how to use them, and as costs have decreased. In the United States, where infrastructure systems have long passed their design life of 50-plus years, utility contractors use these technologies more and more in special situations.
“Though trenchless techniques are a relatively small percentage of total excavation work, they are certainly not insignificant,” John Hemphill, executive director of the North American Society for Trenchless Technology (NASTT) in Arlington, VA, told Construction Equipment Guide (CEG). “As more and more officials become knowledgeable in their use, trenchless techniques are being increasingly utilized. Market share is relatively small on a national basis; however, there have been some fairly large trenchless projects. Houston, TX, for instance, utilized microtunneling trenchless procedures in replacing and expanding their sewage system over a period of several years.” Currently, there is a significant multi-year pipe bursting rehabilitation project being undertaken in Jacksonville, FL.
Rapidly increasing infrastructure needs are one big reason why growth seems assured over the long run. Water infrastructure needs, for instance, are skyrocketing to the breaking point.
“In 1996, the Environmental Protection Agency [EPA] estimated that approximately $200 billion was needed over the next 20 years for infrastructure rehabilitation in the drinking water or waste water areas,” said Eban Wyman, vice president, government relations, the National Utility Contractors Association (NUCA) in Arlington, VA. “Now EPA estimates that from $650 billion to $675 billion will be needed over this period when you consider new construction as well as rehab. Other estimates reach as high as $1 trillion. If Congress funds this work at adequate levels, it means that use of trenchless technologies will increase. If present legislation passes, there would be a 300-percent increase in funding with a lot of work for everybody, including both trenchless and open cut. EPA would administer $40 billion over the next five years in state revolving funds for drinking water and wastewater needs, which would be a huge step in the right direction for cleaning up this mess.”
Costs, of course, are an important consideration in growth in use of trenchless technologies. The trenchless option may sometimes appear to be relatively more expensive, but this has to be balanced against the savings from not having to disrupt surface operations. These savings can be significant in urban areas.
“More and more municipalities are taking into consideration the costs of disruption,” Hemphill told CEG. “When these costs are factored in, it is often clear that trenchless is the best alternative. Having said that, decisions are often made solely on the construction costs of the job, without considering the entire cost to society and that can be a disadvantage to the trenchless industry and a disservice to the public. We believe the public is best served when all costs are considered. We are not saying the trenchless always wins. The proper choice will depend on the conditions and circumstances of each job; sometimes trenchless techniques are cheaper and sometimes they aren’t.”
Another positive factor is that users often like the relatively new technologies (see Joystick Generation).
How do you remotely dig an underground hole in a trenchless procedure?
One of the most significant techniques for new construction is horizontal directional drilling (HDD). An operator on the surface sends a drill head on a slant to the required depth, and then guides it as it drills out a hole horizontally. The operator also monitors the location of the drill head along the way, until the drill comes out at the other end (such as the other side of a river or highway crossing).
In directional drilling, the transmitter, emitting an electromagnetic signal to the surface, is located down hole near the drill head. A person walking overhead receives the signals, indicating depth, location and other information, on a hand-held receiver and gives appropriate directions via a two-way radio to an operator at the rig controlling the drilling. (A technique which uses a direct wire from the drill to the surface also may be used on larger jobs including those going under rivers.)
After the hole is drilled, the rig pulls the product (such as a polyethylene pipe) back through the opening that it has created, thereby completing the installation. Most utility installation using HDD is small diameter of 2 to 10 in. (5 to 25.4 cm). However, pipe diameters between 2- to 48-in. (5 to 122 cm) spanning distances up to 6,000 ft. (1,829 m) have been successfully completed using horizontal directional drilling.
A second widely used technique is microtunneling, or jacking-boring. This typically involves, first, digging a relatively small vertical trench where a contractor places tunnel-boring equipment. Then, by remote control, he or she steers the equipment, which employs a thrusting and drilling action.
As the drilling moves forward, a jacking device pushes along the pipe, or other product, behind the drill until it reaches another vertical pit. Excavated material can be removed by a mechanical auger or as a slurry. Microtunneling, which uses fairly large equipment, is typically used with larger-diameter 30-or 60-in. (76.2 or 152.4 cm) pipe and is the preferred option for sewers where very accurate grades must be maintained.
Pipebursting, or pipe splitting, is another trenchless technology used to replace old pipe. This technique, which goes manhole-to-manhole, involves pulling or pushing a piece of equipment that cracks or splits the existing pipe, while simultaneously installing a replacement pipe behind it along the original alignment. The replacement can be larger than the existing pipe, which has been fragmented during the bursting process and left in the ground.
In rehabilitating old pipe, the simplest technique is grouting. The equipment goes to the point where there is a problem with water coming into or escaping the pipe and injects a grout into the damaged area. The grout feeds into the surrounding soil, helping to stabilize it and correct the infiltration and exfiltration problem. While grouting does not repair the damage, it can be an extremely effective method of reducing inflow.
Sliplining rehabilitates by inserting a smaller new pipe into an existing pipe. Because the new pipe is a smaller diameter, some capacity is lost, but the new liner will almost always have a lower friction factor and the result will be essentially the same capacity.
This rehabilitation approach differs from sliplining in that contractors invert or pull in a flexible fabric “sock” which is impregnated with a thermosetting resin, into the old pipe. Once the sock is in place, it is inflated using water or air, expanding it, and then conforms to the host pipe. The sock is then heat-cured to form a rigid structural liner inside the existing pipe.
Manhole Repair and Coatings
Manholes also can be repaired using trolled, injected, spray applied, plastic liner, cementitious, or CIPP systems to create a water tight and/or structural repair.
Which trenchless techniques are growing the fastest?
“A few years ago it would have been horizontal directional drilling, without hesitation,” Hemphill said. “Right now I would say perhaps pipebursting, but tomorrow it could be something else. Certainly, sewer and water rehab is a big area where a lot of work is going on. The amount and type of trenchless work will vary, not only over time but from location to location. I live in Alexandria, VA. There’s a large development on the north end of town. Our treatment plant is on the south end of town. Trenchless, in this case, microtunneling, was the clear winner for installing the 30-in. pipe to connect this development with the wastewater treatment plant.”
The trenchless industry currently is investigating the use of the Global Positioning Satellite (GPS) system to improve accuracy of locating and logging underground infrastructure. The biggest area of refinement and improvement, however, is in improvements to closed circuit TV (CCTV), and its linking with high power computers to monitor and record the condition of utilities.
“New and improved CCTV/computer systems can help us identify and prioritize repairs,” said Hemphill. “Used with a computer, CCTVs can be integrated down the manhole, or though the sewer or water lines. The computer gives the system the ability to measure, record, and store information along the way for prioritizing work. This includes basic information like the size of the utility and specific information on its condition at a specific location. The challenge is to develop uniform systems for interpreting and prioritizing this information.”