Underground Construction – Part 1

Sept. 1, 2000
Gx Bug Web

Let’s think positively right from the start. Whether to use a trenching or trenchless technique is not a question of right or wrong, good or bad. It’s not a question of which equipment is better. It’s usually a question of terrain. How much would it cost to repair and replace this stretch of pavement or that section of highway if an open cut is made across it? On the other hand, how much would it cost to take 30 mi. of telephone fiber-optic cable across the country with no underground obstacles, using a directional boring system? Many underground construction projects require mostly open-cut or mostly trenchless techniques and equipment. For the short stretches that require the type of equipment you do not have, renting the suitable machinery and tools might be a good option. Before you start, know what you intend to do and how you will accomplish your goal.

When trenchless technologies became accepted as more than sophisticated alternatives to traditional methods, many contractors were still cautious. “Directional boring is an operation for skilled workers, and they are hard to find,” observes Erin Kuehn of Kuehn Trenching, a company in Glendive, MT, that has done many miles of open-cut trenching in the western states. “We would have to justify the initial investment in trenchless equipment and find enough projects to keep it busy. And we would have to make sure that the equipment is operated at its best levels.” For contractors who have regular applications in metropolitan areas, such as installing or repairing pipe and other underground utilities, ownership of trenchless equipment makes sense. Going under a street, a river, or a railroad is usually less expensive with horizontal directional boring equipment than with open-cut methods. If your work is mostly in rural communities, it is unlikely there will be underground construction projects where trenchless equipment is required except for short stretches. Most of your trenching will be in open country or alongside roads, and open cuts do that more quickly and economically. Local conditions and public attitude will also determine if an open trench across Main Street is more practical than a trenchless approach. Specialists in horizontal directional boring, even those who come from hundreds of miles away, are seen more often in smaller communities these days as local regulations stiffen and the disruption of streets becomes more costly.

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“For directional boring, it is definitely matching the initial cost of the equipment with the amount of business you can generate with it,” adds Ty Kuehn, the other owner of Kuehn Trenching. “Many trenching contractors like us probably work within a 100-mile radius of our home base. If that circle includes some larger cities, there are probably good opportunities for regular directional boring, enough to keep the equipment busy. We are based in eastern Montana and have no cities with populations over 20,000 within 100 miles. There is a company in Williston, North Dakota, about 150 miles away, that has directional boring equipment.”

Open trenching is still, therefore, a practical method for much of the telecommunications installation required nationwide. “Because so much of the work is in areas where there are few surface and underground obstacles, plows, trenchers, and saws are the equipment most often used to install the duct into which fiber cable is placed,” explains Bryan Brady, an international marketing manager for Ditch Witch of Perry, OK. “These three types of equipment are designed specifically to install utility lines; they are much more productive than backhoes or excavators for this type of construction.” Many of the machines doing this work are Ditch Witch HT185 and RT185 units. The HT185 is a track-mounted model that can be equipped either as a vibratory plow or a saw. The RT185 is a four-wheel-drive version of the same basic model. “These 138-kilowatt machines are the only underground construction models in this size category that are available as either track- or wheel-mounted units,” Brady points out. “They also are the only machines in their class that can be equipped as a chain-type trencher, a saw for cutting through pavement and other difficult conditions, or a vibratory plow, depending on which interchangeable attachment is used.”

Obstacles and Courses

For both open-cut and trenchless projects, the precision achieved by the operators is vitally important. A straight line from start to finish would be ideal but is rarely possible. Before they start to cut or bore, contractors must know what is already lying in the soil – some of the obstacles may be extremely hazardous, such as gas or electric lines, in addition to being expensive to replace. You cannot see the directional boring tool underground, but there are ways to ensure precise control for an invisible object.

The operators need to know what is already in the ground that they intend to drill, and they need to know exactly where their drills are going. The Polaris EM Guidance System from NorthStar Drilling in Houston, TX, provides accurate, reliable data that are transmitted via an electromagnetic signal through the ground and received at the surface by an antenna attached to a signal processor. This means there is no need to use wire to transmit data from the downhole steering tool, which could be a significant benefit to the user. “If a guidance system with wire is used, it is typical during trenchless boring for the wire to be spliced and soldered at each connection in order to follow the bore,” describes Jeff Dugan of NorthStar. “It has been known to take up to 40% of the project time locating and repairing splices and, in some cases, redrilling the borehole.” The Polaris EM system can also prevent those costly pullbacks of the drill associated with systems using wire line.

At one project, a fiber-optic line had to be lowered far below the ground when a new highway was built because bridge pilings went down to about 80 ft., below the existing fiber-optic level. The owners of the fiber-optic line decided to put it down to 118 ft. A Polaris EM system was used with a powerful American Augers DD140 rig. The site was next to a railroad and under power lines, so interference could have been a problem for the guidance system, but there were no troubles. “I was impressed with the system,” remarks contractor Shannon Yarborough. “There was no wire to splice or break, and the accuracy was equal to or greater than wire-line systems.”

The Rotasteer 3 from Steve Vick International of Bath, England, uses a palm-size computer as its data processor, with a separate sensor board that is light and portable. The system detects the depth and plan location of the underground drill head or down-the-hole hammer by using magnets rather than radio sondes. It gives accurate readings up to depths of 20 ft. “We have two powerful, permanent magnets positioned just behind the drill head,” explains Anne Cotton, marketing manager. “As the pilot bore rotates, a changing magnetic field is created. This is detected on the surface by the Rotasteer sensor board, which houses microprocessors and magnetometers. The data are transmitted to the handheld computer.” The system gives the operator information about the location, depth, and angle of the guide bit via the screen. With this method there are no underground electronics to be damaged and no batteries to be replaced down the hole. Rotasteer can also record the insertion of every drill rod.

For successful and safe directional drilling, you do not simply make a hole in the ground and go; you must know if your bore path will pass too closely to an existing underground utility. The Atlas Bore Planner 2.1 from Vermeer Manufacturing of Pella, IA, can tell you that, and it can tell you if the proposed bore path will exceed the minimum bend radius of the drill rod. This software allows the operator to follow a graphic visualization of the job. It displays the proposed bore path and takes into account previously identified roads, rivers, and utility obstacles. “When the user selects such parameters as the diameter and length of the drill rod, the depths of the desired bore path, the desired minimum cover over the underground construction, and the diameter and allowable bend radius of the product pipe, this software will plot a proposed path,” says Vermeer’s Jodi Bosscher. “There are two formats provided. One screen shows a top and side view, while the other shows a report that lists the numerical depth, pitch, and heading of each rod used.” The system links electronically to such internationally used instruments as those from Radiodetection, McLaughlin Mole Map, and DCI Datalogger. It uses a desktop, laptop, or handheld computer.

To quickly and accurately locate buried power, telephone, CATV, water, and gas lines, Subsite Electronics (a division of Ditch Witch) offers the 75R (R for receiver) and 75T (T for transmitter). The manufacturer says the technology is sophisticated but the operation is simple for this lightweight system. The receiver weighs 4.3 lb. and is designed for one-hand operation, with color-coded controls. The transmitter weighs 12.5 lb. and, in the active mode, transmits via direct-line connection, induction clamp, or induced broadcast signals. By offering simultaneous 8- and 29-kHz transmission, this instrument makes an alternate signal available if the first one chosen is hard to detect. Unlike analog readings, the readings of the 75R will not vary with changes in temperature or time. Both the receiver and transmitter run on readily available alkaline batteries. Also from Subsite Electronics is the Subsite 750 Tracker. When used with any of the Subsite directional boring beacons, this instrument will provide all vital tracking information, including depth, location, roll angle, pitch, beacon temperature, and status of battery power. It offers three primary methods of operation for the drilling contractor (remote guidance, walkover beacon tracking, and line-locate mode), so this tracker should work well for both deep and shallow bores.

For normal use of Tracto-Technik’s (Lennestadt, Germany) Grundodrill system, there is a walkover detection system. In the case of a river or a canal, there’s a nonmetallic boat from which the operators can monitor the exact position, depth, roll, and pitch of the bore head. At one job, under a canal, the crew chose a mud-motor system for the bore, and a cable was passed from the bore head to the bore rig through the drill stems to supply all the information required. Such a detection system (via cable) has none of the depth restrictions of a walkover system and is less susceptible to interference from such items as overhead electrical cables. The Verifier Digital Locator from McLaughlin Manufacturing of Greenville, SC, offers five accurate methods of locating buried metallic utilities. It can also work with a remote probe transmitter to verify the location of nonmetallic pipe and conduit. The Verifier is able to locate to a depth of almost16 ft. and is easy to operate because it requires the use of only two buttons. Multiple coils in the instrument measure the magnetic field and identify the exact location and depth. The display also indicates the current and battery strength. A high-gain filter and a current value index help the operator distinguish among multiple lines buried beneath the proposed boring path.

Instruments can be as useful in open-cut trenching as in directional drilling. Vermeer has formed a strategic alliance with Topcon Laser Systems of Pleasanton, CA, a leader in high-precision surveying instruments, to address the problems of contractors when they must meet strict depth and slope specifications. Topcon’s 3D MC system is already well known in the construction industry for its work with dozers, motor graders, asphalt pavers, concrete pavers, and milling machines. It is a local positioning system that gives trenching contractors good control of both grade and course with a single system. The system is composed of a GRT-2000 total tracking station laser transmitter with a radius of about 1,000 ft., a laptop computer cabled to the transmitter, and an LS-2000 digital receiver mounted on the trencher. The trencher operator clearly sees the data received, and a bright LED indicator gives steering indication. This Topcon system can go around corners. As long as the total tracking station transmitter has a line of sight to the trencher, the program will guide the operator through the course, no matter how curved or offset it is. The system can also feed data to the laptop (for storage and verification) regarding what the trencher actually achieved; this could help in disputes about meeting specifications for depth and direction.

Some manufacturers make trailers specifically for directional boring equipment, and there are trailers for trenchers and plows that vary in design quality. As in decisions regarding the size of all construction equipment, choose models that best suit your particular range of projects.

Safety Definitely First

The other site condition that must be determined before any work begins is safety. Running equipment safely should be second nature to contractors, who know that people (workers or bystanders) are not welcome to linger close to any rotating machinery. Trenchers, plows, and saws cut whatever is presented to them, so make sure they are given only the specified ground; keep inquisitive persons away at a safe distance and mark the site clearly. The general public does not resent orange cones and colored tapes if it knows they are there for its protection.

Trenches can bring their own dangers. Trenches more than a couple of feet wide and more than 4 ft. deep (such as those often required for water- and sewer-pipe replacement) are more dangerous than they look, and the force of falling earth is much more powerful than you might imagine. Every year thousands of accidents occur worldwide because some contractors and public works departments believe they can dig a long, deep hole and the soil around it will not move or collapse. Thousands of people die or suffer permanent disabilities from preventable trench accidents, and expensive machines are buried and broken. Sadly, many people understand the risks but do not think accidents can happen to them at that site, so they take a chance. The simplest method of protecting workers and equipment from cave-ins seems to be shoring or shielding. There is also evidence that the use of shoring is less expensive than the traditional sloping or benching of trenches, and it allows installation work to progress more efficiently. With shoring, there is usually less excavation required (as much as two-thirds less), which constitutes a real savings of time, labor, and machinery.

Manufacturers of trenching equipment stress the importance of good preparation for all operations, and some public authorities are requiring that one worker be designated the observer at the site (someone to stay aboveground at all times and keep an eye on what is happening in the trench). One of the best sets of instructions and information is published on the Internet by the Province of Manitoba (www.gov.mb.ca). A good point made is that injuries and fatalities occur because employers and workers forget that when they remove earth from the ground, they are creating a situation where extreme pressures may be generated at the face of an excavation. Perhaps the most dangerous aspect is that trenches seldom look dangerous. One popular rule of thumb is that any trench as deep as a worker’s height should have some kind of shoring or shielding.

For those who imagine that trench shoring or shields are a waste of time and money, manufacturer Efficiency Production has some interesting facts. A crew laying two 8-ft.-long pipes, using two 20-ft.-long trench shields, would install 224 lin. ft. of pipe in an eight-hour day. By comparison, the same crew using bank sloping or benching would install only 96 ft. of pipe in eight hours. Since the time it takes to lay pipe is relatively constant (15 minutes in this example), the main factor that determines productivity at the site is the amount of excavation required per cut. By using trench shields, the trench excavation can be reduced from 255 yd.3 to 79 yd.3 per cut. This is a reduction of almost 70% in trench excavation required cut and an increase of more than 130% in the daily pipe productivity. And the trench won’t cave in.

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Griswold Machine and Engineering (GME) of Union City, MI, is another manufacturer of shields, shoring, and slide rail systems. The latter come in many sizes and strengths, including heights of between 4 and 16 ft., and the modular component system permits many configurations to suit the site. Slide rail systems protect not only workers in trenches and pits, but also equipment that must work down there for such projects as foundations. At the other end of the scale for GME are the Lite-Shield modular shoring systems, which come in two-, three- and four-sided configurations. Assembly at the job site is fast, with a pickup truck big enough for transportation. The components stack flat in the truck bed to minimize storage and transport problems, and workers can easily handle the lightweight aluminum panels. To assemble, a panel connector is attached to a panel and a strut is fastened to the connector with an easy-on locking pin. Then the worker connects the strut to the opposite panel and connector, and a strut is attached to the opposite end of the panels. If needed, additional panels can be stacked to add height. Such shielding can easily be fit in a trench that has pipe, without disturbing the pipe, so that repairs, maintenance, or new installation can be completed.

Having planned the course of the underground construction, you are ready to go.