Neal Bolton

By Neal Bolton

Most of us know what a metal detector is. Well, of course—it detects metal. I even have an old metal detector that I received as a gift more than 30 years ago.

In its time it was quite a piece of work. It can detect a quarter at 5 inches and a car at a little over 2 feet. I’ve had lots of fun with it, but it’s really not much of a working tool.

For many of us, when we think of a metal detector, our image is probably of a man searching for coins at the park or beach. Or it may bring to mind an airport’s security screening system. But chances are you haven’t thought about how a metal detector can help your landfill work safer and save money. Neither had I—at least not until after a little research led me to Minelab.

Liner construction is a costly undertaking.

As you might guess from the name, Minelab manufactures metal detectors for locating landmines. This is serious business. The company also makes units for other applications. The one we used, model SD2200v2, is actually designed to locate gold.

White’s Electronics, another longtime manufacturer of metal detectors, also provided us with a unit. This one, a Sierra Madre model, is designed to find large, deeply buried objects. Both of these companies make a good product and have been around for many years.

“That’s interesting, but how,” you may be asking, “can a metal detector help my landfill?” There are several ways. Here’s a rundown of some of the most common.

Avoid Liner Construction Costs
As you know, one of the biggest costs associated with operating a landfill is designing, permitting, and constructing the liner and leachate collection and removal system. Thus it follows that one of the biggest potential cost savings would be to avoid or delay that cost for as long as possible. And sure enough, a look at landfill tipping fees around the country confirms that some of the lowest are found at landfills that aren’t working on a liner. How can they do it?

Well, remember that under the EPA’s Subtitle D regulations, landfill liners are required whenever a landfill makes a lateral expansion. Conversely, no liner is (generally) required as long as a landfill continues to fill on its existing waste footprint. Any landfill that began operating prior to the late 1980s probably had an existing (unlined) footprint of waste. Often, the limits of that footprint were poorly defined.

It was common practice for one of the old-timers to eyeball where he remembered waste to be. Some landfills might even have dug a few test holes with a backhoe just to confirm. In any event, that footprint, with all of its inaccuracies, got surveyed and transferred to a map and became the historic waste footprint. That’s all fine and good, but what if the footprint is incorrect? The fact is numerous landfills that had assumed a certain footprint later found out that the limit of waste extended farther than they thought.

What’s that mean? Well, it means that the landfill might have more (unlined) capacity than it initially thought. It might also mean that the cost of lining a lateral expansion might be postponed for a while.

Let’s suppose that your landfill has a 14-acre existing (unlined) waste footprint. But with a careful evaluation around the perimeter of your landfill (using a metal detector), you find that it’s actually 17 acres. What’s more, you calculate that bigger footprint would allow you to postpone building that $2 million liner for another four years. If money costs you 5% and inflation is 1%, then the net cost to you is 4% per year. The bottom line: Because of the time value of money, that four-year delay will save you $320,000 ($80,000 per year).

Wasn’t it Clint Eastwood who said, “A man’s got to know his limitations?” Well, you probably never guessed that his words might apply to landfill operations, but hey, here it is.

Delineate Old Trenches
Many landfills are situated land that has been used for dumping for many years—maybe decades. But in some cases, the goal is not to expand the existing waste footprint but to locate and eventually remove that old waste and place it in a lined area. Here again, a metal detector can help locate the extent of that old waste, helping you better plan and budget for the removal.

Old trenches are often—but not always—evidenced by obvious settlement. Old landfills can be surprised not only by historic old trenches that were bigger than expected but sometimes by trenches that nobody even remembered. Those kinds of surprises often become big, costly change orders. Again, wouldn’t it have been better to know ahead of time?

Using the right metal detector, you can easily locate waste to a depth of up to 6 feet. Typically, there is enough metal in the wastestream to disrupt the magnetic field set up by the metal detector. But remember, the greater the surface area of the metal present, the greater the depth of detection. In our test run, we were able to get readings on waste as deep as 6 feet.

OK, so you’ve decided to spend the money on one of those high-tech, high-powered metal detectors to identify your landfill’s footprint so that you can maximize your old footprint and not be surprised by any hidden ones. But once that task is completed, will the thing just sit on the shelf and gather dust? Not if you’re paying attention. Once you have such a powerful tool, it’s no trick to keep it busy on a myriad of useful tasks.

Locating Old Burn Sites
In the days when landfills were burn dumps, nearly every town had one. Those old burn sites may be closed but they aren’t forgotten—at least they aren’t supposed to be. Because old burn sites can still create lots of environmental problems, much effort has gone into capping or removing them—when they can be found. Finding and marking the limits of historic burn sites is another great application for a metal detector.

PHOTO: Minelab

Using an SD2200v2 with 11-inch coil to search for the limit of waste.

Locate Survey Markers
Often, in the course of surveying, surveyors will set pins or benchmarks around the landfill. Common locations include property lines, aerial target locations, or reference points for buildings, liners, roads, or underground utilities. The location of the benchmarks is often marked with a wooden lath. The benchmarks may last for decades but the lath only for a year or so. And once the lath is gone, the only way to relocate the pins is to rehire the surveyor—or find it yourself with a metal detector.

Locate and Clean a Culvert Inlet
Many landfills use corrugated metal pipe (CMP) culverts to carry runoff water under roads, through berms, and away from sedimentation ponds. Culverts are also used as sleeves to protect methane gas lines at road crossings. It’s fairly common for those culverts to become covered with sediment and blowing litter to the point that they disappear. Once they’re lost, how are they found? Usually with a bulldozer or backhoe. Would it be easier, less costly, and potentially less damaging to locate those culverts with a metal detector? Probably. In our evaluation, we found that we could locate a 24-inch CMP to a depth of up to 4.5 feet.

Locate Underground Utilities
At your landfill, how often do you build a ditch, install a culvert, or reshape a road—once a month? Once a week? Chances are you’re excavating something, somewhere, on a regular basis.

This old trench was discovered when the landfill expanded.

Combining frequent excavation with the network of underground utilities on your landfill (culverts, power lines, telephone lines, gas lines, etc.) can spell disaster. You may be aware that excavation contractors typically call an underground utility locating service before excavating. These guys will come out and locate underground utility company lines for free. If you’re dealing with utility company lines, this is the procedure to follow.

However, if your onsite underground utilities have not been mapped, or if you don’t have time to wait 48 to 72 hours for the locator before you dig, you run the risk of damaging lines or injuring a worker. Of course it’s recommended that you always use a utility locator service if possible, but if for some reason you can’t, or if you are working around non-utility items (i.e., culverts) a metal detector can do the job.

Locate Metal in Grinder Feedstock
Ask any landfill manager to tell you his story about that hunk of metal that went into the grinder—don’t worry; he’ll have one to tell. That’s because the cost of grinding woodwaste and greenwaste skyrockets when big pieces of metal are accidentally dumped in with the wood. No, we’re not talking about nails or wire. It’s that short piece of I-beam or the section of cutting edge that causes the problem. Tossing a large metal item into the grinder can do serious damage and take the machine down for days at a time.

Would you trade the expense of your last grinder repair job for the cost of a few hours for a laborer to metal detect the pile before it goes into the grinder?

The day before grinding, the laborer could go over the pile and mark any “targets” with spray paint. The grapple operator could then carefully sort through those areas as they are encountered. Keep in mind that the depth of detection is inversely related to the size (surface area) of the target. Thus, while you might detect an engine block at 5 to 6 feet, a bulldozer’s corner bit might not register much deeper than 3 to 4 feet. With that in mind, you may want to change the dimensions of the stockpile so that a metal detector can reach through it. You’ll want to experiment with the performance range of whatever type of metal detector you select.

Odds and Ends—Find Lost Cutting Edge
I was working on an excavation job several years ago. We were using 613 paddlewheel scrapers to make sub-grade at a recycling center. Early one morning, one of the operators noticed that he’d popped the bolds and lost the drop-center cutting edge from his machine. It had apparently been picked up and deposited into the fill only moments before—but where?

Unable to find a replacement cutting edge for a couple of days, and unwilling to remove all of the fill that we’d placed, we were forced to simply park the machine until a new cutting edge arrived. If we’d had a metal detector in our arsenal of tools, we could have located the cutting edge, bolted it on, and been back to work much sooner. Oh, and that lost cutting edge? Nope, we never found it. It’s still buried somewhere in the fill.

Tomorrow’s Technology—Today
For many years, the Department of Defense (DOD) had and used GPS technology. Many of us knew about it, but because of the cost and lack of commercial availability, we couldn’t use it. Of course that’s all changed and today many landfills are using GPS for general surveying and even real-time tracking of machines like compactors and dozers. Similarly, metal detecting technology has advanced (in part through DOD applications) to the point that it can be cost-effectively applied today to help us work smarter.

How It Works
A metal detector works by projecting an electro-magnetic field outward from the coil into the ground. When the electro-magnetic field encounters a metal object, the field changes and the metal detector perceives the change as a “target.” Also, the larger the surface area (from the coil’s perspective), the larger the target appears. The mass of an object has less of an impact on a detector’s range. Thus, a metal pie plate appears as a much larger target than a steel bearing, though both may have the same mass.

Similarly, the pie plate could be detected at a greater depth.

Actual Performance
How deep will it go? Well, that’s like asking what size fish you’ll catch in the ocean—it depends.

To answer that question, we started by conducting an air test. Using different combinations of detectors and coils, we measured detection depth versus surface area. We used short pieces of galvanized steel studs to create targets ranging from approximately 0.4 to 4 square feet. Figure 1 shows the results of that test. The results of the air test show that as expected larger coils have greater range.

Figure 1. Air Test (Depth Versus Surface Area)

As part of our evaluation, we also took the two metal detectors to a landfill where we tested their ability to detect trash at varying depths. This was also an air test. We found that contrary to the air test where we measured depth versus surface area, when it comes to finding trash, the Minelab (with 11-inch coil) was comparable to the White’s (with 15-inch coil). The results are shown in Table 1.

Keep in mind that detection depth (in air) does not necessarily indicate how a unit will perform through soil. Variations in the mineralization and moisture content of the soil will impact how deep a certain detector can go.

Finally, when you consider a metal detector’s ability to locate trash, consider this fact: Many metal detectors, like the ones used in this evaluation, are designed to locate relatively small pieces of metal—coins, jewelry, gold nuggets—often weighing just a fraction of an ounce. Consider, in contrast, that waste often contains 8% to 10% metal (by weight). And with most landfills achieving density of at least 1,200 pounds (of waste) per compacted cubic yard, every cubic yard of waste could contain approximately 100 pounds of metal.

Yes, it’s true that because of recycling, the wastestream at many landfills may have less metal. However, the waste in those old trenches has more than enough metal to allow detection by a modern metal detector. There is another factor. Over time, ferrous metal will rust. That rusty “halo” can actually improve a detector’s ability to detect because it makes the object appear larger. Compared to trying to find a gold nugget or a lost ring, finding trash is relatively easy.

Keep in mind that a metal detector is not a magic wand able to solve all of your problems. We found limitations. For example, because of their sensitivity, we discovered that metal detectors would find small bits of metal. In the woodwaste pile, that can be a bit frustrating when you want to find big pieces of metal and the detector is screaming at every nail and staple in every pallet. However, with practice we were able to find ways to deal with it by changing the angle of the coil and adjusting (on the Minelab unit) the “iron” discrimination knob.

Table 2 presents a comparison of the two units we used.

So, if you get a metal detector, can you also find buried treasure? Some folks do, but like anything else, it might be a good idea to keep your day job until you learn the ropes.

Neal Bolton is a consultant specializing in landfill operations and management. He is principal of Blue Ridge Services in Atascadero, CA, and author of The Handbook of Landfill Operations.

MSW - May/June 2005