Grading and Excavation Contractor

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Finding the obstacles below the surface is not so difficult today.

By Paul Hull

I have never seen an explosion caused by the bucket of a backhoe or excavator striking a buried gas line, but I have seen pictures, and I would not like to be the machine operator or anybody nearby.

Hitting an electric line could be worse, and there is usually more trouble than an injury to the careless or misinformed person who struck the buried utility. Whole districts of a community may be cut off from their sources of electricity, telephone, gas, and water when a buried utility is broken. There is also the possibility that somebody (you?) will have to pay for all the damage and inconvenience.

Photo: RIDGID

Photo: GSSI

The responsibility for damages caused to underground utilities is something that should be clearly agreed upon and recorded before you start work. The responsibility may be with the property owner, the contractor, subcontractors, utilities, or public agencies, but remember that nobody wants to be responsible. If your business regularly involves projects where you need to locate and work near underground utilities, it would be wise to have a contract drawn up by your legal advisors that protects you from alleged carelessness or damage. You need to minimize your risk, especially if you should meet unknown or incorrectly identified and located utilities. If the work you are doing is being done as a subcontractor, be extremely careful and wise about the terms of any subcontractor agreement.

Don’t always blame your operator when the blade hits the pipe or cable. He or she should know beforehand what is in the ground where the excavation will be done. Whose responsibility was that? Any operator should be encouraged to ask before digging. Has somebody checked on the ground below? Should I do it? What were the results of the check? The “One Call” systems (they have different names in different states but are basically the same concept) that have been in operation for some years now have been extremely useful, and we see them used every day before contractors do their work. Unfortunately, not every buried underground pipe, wire, or cable comes under the One-Call umbrella. In some communities, it is the property owner who is responsible for knowing where everything is and nothing is registered publicly. You should check this especially if you are doing residential, school, or hospital work. Common sense, however, tells you to make an inspection below ground before plunging a blade, bucket, or auger into the soil. In some places there is no record of what is buried, but you can find out.

Photo: Sensors & Software

The City of Woodbury, MN, in a most welcome, wise, clear statement, has this to say about locating underground utilities: “Under state law, if you are planning to do any work on your property that involves excavation more than six inches deep, you need to contact the Gopher State One Call system 48 hours before you plan to dig. Gopher State One Call will notify any known parties who have underground utilities in the area that a locate has been ordered. The operators are then required by law to mark their utilities with paint or flags within 48 hours. There is no fee for this service.” The city adds that, when property owners or tenants have any type of privately owned underground facility (such as invisible fences, underground sprinklers, pool heaters, etc.), they are responsible for locating those facilities. Or they should hire someone to locate them. One point struck us forcefully in those words of advice. They are talking about excavation more than six inches deep. That’s not much, is it? That’s a gardening-level dig.

It wouldn’t hurt for operators of equipment to know what the colors shown by locates mean. There’s a whole rainbow of information painted on the ground. Red indicates electric utilities; orange is for CATV and communications; yellow is gas, oil, and steam; blue is for water; green is for sewer; pink is for temporary survey markings; and white marks areas of proposed excavation. It occurred to me when writing this that those colors would not be too difficult for schoolchildren to learn, would they? How early in life should we know what is hidden under the surface and what could be dangerous? In our business of grading and excavation, the communication between all parties concerned must be perfect. Before the job starts, everybody should know (and know that everybody else knows) what is down below. Some owners of the facilities where you will have your work have contracts with expert locate technicians to find any potential underground hazards before you start to dig. “Digging” can be manual with a shovel, or with your regular machines like skid-steers, loaders, excavators, and dozers. Yes, even a shovel thrust in by one person can find and ruin some buried pipe or cable. The bigger problem that locators face is the surprising lack of records of underground construction—not just the work done 50 years ago but recent activity, too. Today, they could find gas, electric power, water and sewer, and cable television installations, all packed together tightly as space in available easements becomes more and more limited, and there is a tendency to put as much equipment underground as possible (rather than as unsightly overhead cables). The problems of confusion and congestion are greater in communities that have seen significant growth, in big cities, and in older cities where there has been redevelopment. Clearly, the longer the city has been in existence, the more buried utilities there are likely to be. (Some accidents, however, have occurred in small communities where the contractor just knew there couldn’t be any utilities under the surface. The lucky ones are alive to find out where they were wrong.)

Photo: McLaughlin

Examples of Locating Techniques
The practical, real-life problems of excavation include the fact that buried utilities are not all at the same depth and each may contain a different amount of metal. If they anticipate unusual problems in the project, it may be worthwhile for contractors to hire experienced (insured?) locate technicians rather than rely on the skills of their own employees. The locating instruments of today, however, are becoming simpler to use and the do-it-yourself attitude to buried obstacle locating is (understandably) gaining favor, especially if it is a frequent occurrence. Locating tools fall under the category of “instruments,” and that still seems to put some people off immediately, just as computers and lasers used to frighten many contractors away from more organized bidding, machine guidance, and project management. As far as we can determine from conversations and reading, the makers of locating instruments have been doing everything to make them understandable, practical, and easy to use. Improvements in technology and operation seem to appear constantly. Prices are now in the affordable range, too.

With any luck there won’t be unexploded military ordnance at your job site. That’s been one of the targets for magnetic locators from Schonstedt Instrument Co. for many years; their instruments have also been popular with surveyors. In 1985, Schonstedt produced the first instrument to combine a magnetic locator with a dual-frequency conductive/inductive locator. That was the Mac-51Bx, and two decades later, hundreds of them are still sold each year. A step forward after that was the 2001 TraceMaster, a conductive/inductive locator with a patented radio link that lets the operator have on-the-go control at the receiving end. For those who do several locates each day, the TraceMaster, with its light weight, good battery life, and compact configuration has proved popular and efficient. Just two years ago Schonstedt introduced another step forward, the XTpc. This is a single-frequency locator with a pistol grip; it is carried in a holster! It extends to 26 inches when you need it. It works with a transmitter not much bigger than a king-size Snickers bar (an easy handful but not edible). There is also an XT locator.

“The XT and XTpc are simple to operate,” says user Frank Boswell, who is head of Plant Operation and General Maintenance for the Dunkard Valley Joint Municipal Authority in Greensboro, PA. He was challenged by a poorly mapped, large utility network that included cast-iron pipes more than 60 years old—a fairly typical problem that many of you could meet in municipal, commercial, or residential work. “The Schonstedt rep gave us a lesson, I read the manual, and now two of us are using the locators. We keep them in the truck all the time. And, yes, they save us money. We dig less, do less damage and make better marks for our contractors.” That user found a water line crossing an electric line he wasn’t aware of and two cast-iron pipes that were reported to have been removed years ago. That gentleman put his finger on the reason for hesitation by contractors about purchasing their own locating instruments. It saved him money. The decision to buy your own (as opposed to hiring somebody else to do the work with his own instruments) depends on the frequency of your locating needs. It’s the same reasoning you use when you purchase other equipment. Will I use it often enough to justify its price? Is renting for a temporary project my best procedure? Is outsourcing for the locating a more economical and practical option?

Photo: RIDGID

Photo: Subsurface Instruments

You can get accurate, fast line location and depth calculations of utilities down to 30 feet with McLaughlin’s Verifier G2 Digital Locator. That depth ability seems significant, because you don’t know where they decided to put everything years ago and the records, if they exist, may be inaccurate. The G2 uses “Smart” transmitter technology to simplify locating and ensure proper connection to the line to be identified. Its “drop the box” inductive locating technology allows contractors to “Verify” one-call marks quickly, with minimum effort before exposing the lines non-destructively. Verifier’s lightweight receiver displays line location and depth on a large backlit LCD screen. Choose the Null Mode for quick line tracing, or Peak Mode for accurate depth measurement. The G2’s unique “transmitting” coil clamp eliminates the need for multiple-size coil clamps and allows this induction clamp to be used on previously inaccessible “U” guards on telephone poles. The Verifier G2 is weatherproof and comes with a three-year warranty. McLaughlin says that the key benefits of the receiver with this locating device are its easy-to-understand controls and display and the fact that four active frequencies are used for better locating accuracy. Getting more technical, the transmitter is 3 W and has dual-frequency broadcast, with load matching. There is a smart plug that knows how to set itself to give connect, external coil, or indirect mode.

Any object that contains some iron or steel (a ferrous target) can be detected by the Ml-1 and ML-1M from SubSurface Instruments. These are known as passive locators, and they detect and report magnetic fields that emanate from ferrous metals. Some people call them Gradiometers, because the process measures the magnetic gradient or the difference in the magnetic field strength between two sensors in the unit. There are many obstacles, common to our work in excavation and grading, that can be found by passive magnetic locators: steel and iron water and gas pipes, valve boxes, utility boxes, tanks, drums, and survey corner markers. The ML-1 gives the user an audio output and the ML-1M gives both audio and visual output. They have a lifetime warranty, too.

Here’s a company with which you are already pleasantly familiar: Ridge Tool Co., makers of hundreds of different RIDGID tools, and some attractive calendars over the years. It introduced a line of SeekTech products that are designed to give you fast, accurate locating. The RIDGID SeekTech SR-20 uses a combination of multi-directional antennae, active trace, and low-frequency bandwidths. It’s a fairly new utility line locator but already praised by contractors, utilities, and public works departments. The display of the instrument (which is light enough to hold in your hand as you walk across the terrain to be worked) gives arrows that guide you to target the signal, and it shows the underground line depth continuously. “By combining the audible signal with the visual information on the mapping display, the user can easily confirm the locate through the distortion that may be present in congested areas,” comments Scott Aiello, director of marketing, diagnostic equipment, for Ridge Tool Co.

The SR-20 is the first of a line of related instruments from Ridge Tool. It weighs 3.9 pounds; light weight is an obvious plus in this task. It uses four C-size batteries for power and folds for compact storage and portability. The high-resolution display has an automatic backlight function, and (no surprise here) it has the usual RIDGID lifetime warranty. We mentioned the passive mode above. That’s a definite advantage for users. It allows you to trace any type of signal emitted by a metallic line, even if the line is not energized. The passive mode can also tell you that there is too much distortion in a certain area to locate a line accurately. To choose the available active and passive modes on the SR-20, you simply push a button.

Photo: RIDGID

The DML2000 locator from Dunham & Morrow Inc. weighs only 1.6 pounds. It runs from four alkaline AA batteries and provides up to 100 hours of operation. An interesting and helpful aspect of those batteries is that the “low battery” LED indicator begins flashing when the remaining life of the batteries is 20 to 25 hours, so you don’t go out into the boonies and suddenly find you need new batteries. This locator—it looks rather like a wand or stick with a little box on top—is built to be used in the most rugged conditions. It has a lifetime warranty. There are only two controls and one switch, so your users can quickly learn how to run it. When power is applied and there are no iron or steel objects in the immediate vicinity of the locator, you hear a steady 20-Hz tone. As you walk along, you slowly sweep the locator/wand back and forth. The audio signal will remain constant until you begin to get near an object. Then the pitch of the audio tone increases, and it peaks when the tip of the locator is directly over a target. This product from Dunham & Morrow seems to be one of the simplest to use and most appropriate for some contractors and their crews. Can the weather interfere? The sensor housing is waterproof and the electronics of the DML2000 are water-resistant, so you can use it in most weathers. You can expect correct operation when the ambient temperature ranges from –20ºF to 120ºF. That suits most of our states most of the time. To what depths will this work? For iron valves, a 55-gallon drum, and cast-iron pipe (4 inches), you can find the target down at 10 feet. You can detect a well casing at up to 18 feet and the handles of a septic tank to 5 feet. You can locate a 1.5-inch PK nail at 12 inches.

According to Radiodetection, another world leader in this locating art, active frequency if the best for pinpointing cables, pipes, and drains, even if passive location gives a good sweep of buried cables and pipe. Late last year the company introduced the RD2000 range, with three application-specific locators combined with a new general-purpose transmitter. This combination was developed after numerous requests for more than one active frequency. The RD2000 range uses three frequencies to pinpoint your underground obstacles. The three options offered are for particular challenges. The RD2000S (Sonde) is great for locating pipes, including cast-iron, clay, concrete, and plastic. The RD2000CPS (which stands for Cathodic Protection System) works best for locating metal pipes. If you have multiple utilities down there, choose the RD2000+. A fact of which Radiodetection reminds us is that a common cause of cable strikes (and the injuries and damage that can follow) is the shallow depth of the buried utilities. The company offers an option called StrikeAlert that warns the user especially of shallow cables and utility lines; that will work in both the Power and Transmitter modes.

Ground-Penetrating Radar
What if the hidden, underground obstacles are not metallic? One of the leaders in ground-penetrating radar (GPR) is Mala Geoscience. This company has a radar machine (called the Easy Locator) where the technology will complement standard electromagnetic techniques in the field to give you a total solution for your locating. The Easy Locator can detect all types of material, metallic and non-metallic, including such materials as plastics, ceramics, concrete, and asphalt composite. At industrial and urban job sites, this product minimizes the risks of damage and can save time and money with no project delays. Mala says the Easy Locator is simple to operate, with its clear user interface and only a few commands required to start scanning the ground. It has three antennae, designed to manage the locating at different depths. The depths are called Shallow, Mid, and Deep. Shallow handles locating with depth penetration to 9 feet; Mid takes you down to 12 feet; and Deep will locate as far down as 15 feet. The Easy Locator is on wheels (adjustable for height), and the user has the capability of pulling the machine backwards to improve access for rough terrain or loose soils such as sand.

Geophysical Survey Systems Inc. (GSSI) gives a succinct definition of GPR. “It is the best technology to non-destructively investigate the earth and concrete structures.” GSSI offers several models for a broad range of applications, including the detection and mapping of buried utilities, inspection of concrete, inspection of highways and bridges, railroad applications, forensics, and other projects that are more academic than those associated with grading and excavation. For roads, for example, the GPR unit can give accurate pavement thickness, practical evaluation of the base and sub-base, and a good assessment of the general condition of a roadway. On bridges, a GPR system can determine cover thickness of asphalt, concrete, and rebar. It can also point out areas of deterioration. Yes, you can even find gem deposits!

A development from GSSI that grabbed our attention and applause is the SIR-3000. It has a “breathe” function that can locate trapped victims in mudslides, avalanches, or building collapses. During a search after the disaster, this instrument displays real-time information about the location of trapped victims. It helps search-and-rescue personnel act more quickly and effectively to rescue survivors. “After many recent catastrophes around the world, we realized that a breath-detecting device was imperative,” observes Dennis Johnson, president of GSSI. “We are pleased to be able to add this vital functionality to our existing systems in the field, at no added cost to our customers. It gives first responders and search-and-rescue workers the ability to detect persons buried under the debris.” The SIR of the product name stands for Subsurface Interface Radar, described as a non-destructive system for exploring the subsurface of the ground and inspecting our infrastructure.

GPR, say makers and users, is especially useful for locating and differentiating between metallic and non-metallic conduits and pipes. (The signal response of a metal object or obstacle is much stronger than that of plastic, for example.) Sensors & Software Inc. offers several GPR units, and the company also recommends that you use your Noggin! The Noggin and Nogginplus are GPR systems offered as complete systems in a single package. The former acquires easily read graphic image files, and the latter acquires raw digital data for editing and processing by the user. You can use the Noggins in several configurations: with a SmartCart, where you walk behind the cart and observe the results; with a SmartHandle, rather like a metal detector you’d use on the beach; with the Rock Noggin for working on walls; or with the Hand Tow, which you pull behind you while watching the results on the display hanging from your neck in front of you. Any unit is lightweight. You can move around with it (on wheels, or not), and it is easy to take from job site to job site. GPR used this way, says Sensors & Software, can detect a plastic pipe installed between two metal pipes. GPR can also determine the depth, slope, orientation, and even the diameter of a buried pipe, even if it’s not metallic. You could, for example, locate a concrete storm drain and calculate its diameter and the rise and fall of its alignment. Operating these GPR systems is not difficult, but it seems wise to have some free instruction from your manufacturer or dealer before using them.

There is no doubt that we should check for the presence of underground obstacles every time, before we start excavating. That depth of six inches mentioned by one city is disturbing, isn’t it? For the addition of a communications line through my neighbor’s yard, however, some of the utilities were hardly more than six inches down, near enough to the surface to be caught by an innocent grader. Of greater concern to many contractors are the confusion of utilities in urban areas and the number of unrecorded installations, from projects of long ago and just yesterday. The tools to find those hidden obstacles are available. The most common decision being mulled seems to be whether a contractor should own and operate his own locating devices or hire somebody else to do it. Keep an eye on the ease of operation and affordability of locating instruments and the escalating costs of outsourcing such jobs. “We’ve always done it this way” may not be the most cost-effective method today and tomorrow.

Paul Hull writes on construction and environmental topics for several international magazines.

GEC - July/August 2006

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