Neal Bolton

By Neal Bolton

It’s a given: If you’re in the landfill business, you are also in the business of moving dirt. And although some use excavators or loaders and trucks to excavate and move soil, the most common method of moving dirt is with a scraper.

A scraper is a self-contained dirt-moving machine. All by itself a scraper can load, transport, and spread (or stockpile) soil. In a pinch a scraper can also do a tolerable job of grading its own haul road.

Scrapers are used to place gravel for a new road … or pick it up again when the road becomes obsolete. They can strip and stockpile topsoil … or spread clay for a new liner. Paddlewheel scrapers can delicately—almost surgically—pick up a small windrow of soil, while push-pull scrapers can get in there and hog out the landfill’s next fill area like nobody’s business.

Scrapers certainly are versatile machines. But they are also quite specialized: When you get right down to it, scrapers are made for moving dirt. The question for many landfills is: Are the scrapers moving dirt … or just making dust?

Having scrapers and using them efficiently isn’t rocket science. It’s a good thing too, since dirt stiffs like you and I didn’t transfer to the landfill from NASA.

There are just a few basic concepts that one must understand in order to use scrapers efficiently. Once you get a handle on them, you can almost sit back and let the machine do the rest of the work. Please keep in mind that we’re talking about how to work efficiently, how to maximize production. Of course, while efficiency is important—almost critical—if it comes down to a choice between working safely and working efficiently, it’s always safety first. With that said, let’s get on with the job of moving dirt.

Moving dirt efficiently requires only two ingredients: full payloads and fast cycle times.

Payload
A scraper’s payload—the amount of dirt it hauls with each load—plays a major role in the machine’s overall productivity. “Duh,” you say. “This sure isn’t rocket science.” But wait, it’s not quite that simple. Even though it might appear that big, fat loads are always best, the fact is that sometimes they aren’t.

Let’s switch for a minute and talk about groceries. No, I’m not hungry—this is an analogy.

What would happen if, in the interest of maximizing your payload, you tried to carry a carload of groceries (12 shopping bags full) into the kitchen in one load? First off, it would take awhile to get them all arranged in your arms, in your hands, balanced on your head, and so on. If you could do it, it would be one heck of a load, but it would take extra time to achieve that maximum payload. Then there’s your cycle time. With all that weight, you’d not be bounding up the steps or bopping down the hall. The trip would take longer than normal because you’d be moving slowly. Finally, consider the risk factor. There’s a good chance that you’d lose part of your load on the way. The bottom line is that maximum loads aren’t always the most productive loads. Sometimes it’s better to carry lighter loads and make more trips—sometimes.

In order to compare payload to productivity, you might consider developing a “load growth curve” for your landfill. To develop this curve, the scraper is loaded for various loading times and the payload and cycle time for each load are measured. From this information, you can determine the most productive payload. As an example, let’s develop a load growth curve for a 20-yd.3 scraper.

1. Allow the scraper to load for just 30 seconds. (It won’t be full, but don’t worry. It’s part of the analysis.) Once it’s loaded, send it across a scale and weigh the load (or measure it) to determine how much soil it’s carrying. Then, from a set starting point, let the scraper run its regular route from the borrow area to the unloading area and back. Using a stopwatch, record its cycle time (be sure to add in the 30 seconds it took to load).
2. Repeat step 1, but change the loading time to 35 seconds, then 40 seconds, then 45 … on up to however long it takes to get a full load (let’s say 70 seconds).
3. Compile the data into a chart and identify the load time that yields the best overall productivity.

The results of our analysis are shown in the table.

A quick look at the table shows that the maximum productivity is achieved when the loading time is limited to 50 seconds even though the scraper’s payload is only 18 yd.3 This is because apparently, based on our example, getting that final couple of yards in the scraper takes too long and increases the cycle time too much. It’s just not worth the effort.

Think about it: Have you ever seen a scraper struggling to get those last few yards that would make a heaping full load? The operator has to spin the tires, pump the bowl up and down, and in essence “horse” that last bit of dirt into the scraper’s bowl. It simply isn’t productive.

I know that most of the landfill folks who read this article might not develop a load growth curve for their scraper(s). That’s OK. Forget the curve but remember the concept: When hauling dirt, the goal is to move dirt from A to B as quickly as possible, regardless of how big the payload is. The overall concept of a load growth curve can be boiled down to this simple rule of thumb: When the scraper begins to fight and scratch for those last few yards of soil, it’s time to close the apron and go with whatever load you have.

Cycle Time
This is the other half of the efficiency equation. Once you’ve obtained that “optimum payload,” the goal is to haul, dump, and return as quickly as possible. Here are a couple of tips to speed up the scraper’s cycle time:

Look for Shortcuts. First of all, don’t wander. You wouldn’t go from Los Angeles to Dallas by way of Seattle. The fastest route is usually the straightest route. And so it is with scrapers. Take a look at the scraper’s haul roads and see if there isn’t a better, shorter, faster layout. Remember, those few minutes that make up the scraper’s cycle time will, over the years, eventually wear out that machine. Finding a shortcut for your scraper is like finding money.

Maintain Good Haul Roads. Keep the haul road(s) in good shape. Bumps, sharp turns, and blind spots are unsafe, time-consuming, and hard on the operator and the machine. A haul road that seems a little bumpy when driven in a pickup can be downright torturous when you’re in a scraper.

Another factor related to haul-road quality is pumping. Pumping describes what happens when a heavy machine (e.g., a loaded scraper) drives across a soft surface (e.g., a landfill). As the tires pass, the ground is pushed down and then springs back into place as the machine passes. It might appear that pumping helps the machine travel gently across the landfill—almost like floating on a cloud. But in reality those tires feel as though they’re constantly climbing out of a hole. In fact they are!

According to Caterpillar Performance Handbook, for every 1 in. of tire penetration (i.e., pumping), the machine “feels” a 1.5% slope. Extrapolated to a situation in which the tires are pumping 10 in. (not uncommon on a typical landfill haul road) and the machine feels as if it’s climbing a 15% slope even though it’s on flat ground!

Maybe this is what our grandparents were talking about when they told us how, when they were kids, they had to walk 5 mi. back and forth to school, uphill both ways.

Moving soil efficiently with your scrapers isn’t easy, but it isn’t impossible. Keep it simple by focusing on payload and cycle time … and go haul some dirt.

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 - November/December 2002