Neal Bolton

By Neal Bolton

Slope stability is one of those things that we don’t spend much time thinking about. Sort of like earthquakes and flashfloods, stability doesn’t become an issue until it becomes, well, a problem.

There are many types of stability problems we could discuss. For now, we’ll be talking about stability of excavation areas. Maybe at your landfill it’s referred to as the borrow area, the excavation, the cut, or maybe just "the pit." We might vary in our terminology, but when it comes to the potential risk, we’re all in more or less the same place. When ol’ terra firma starts moving around, it’s bad news.

Many people think of slope failures in association with earthquakes. And certainly, stability during seismic events is an important consideration.

But don’t be fooled. Landslides, slope failures, and cave-ins can happen at any landfill at any time–earthquake or not. Most landslides occur as a result of slopes that are too steep and/or poor drainage practices. But as with any other risk, there are ways to minimize the chance of it happening at your landfill. Here are some ideas.

Understand the Problems

In one of his movies, Clint Eastwood once said, "A man’s got to know his limitations." That’s also a good idea when it comes to slope stability. Part of making wise decisions in regard to any risk is to understand what’s going on. What causes an excavated slope to become unstable? How can you identify problems in the making?

First off, the landfill designer should have provided some guidelines in terms of slope stability. Often this comes as part of the slope stability analysis done to determine stability in the event of an earthquake. If you don’t know how steep is too steep at your landfill, find out. Every landfill is unique. And because slope stability depends to a great extent on soil type, many landfills will have a variety of criteria: one for each soil type.

Very often, designers find themselves in a double bind. On one hand, they keep slopes flat to ensure stability. But on the other hand, steeper slopes create more airspace. It’s a Catch-22. And because it really does boil down to a judgment call, it’s important that you trust the designer’s judgment. Yes, it’s science. And yes, there are accepted standards of practice. But remember, designers are people too and, sad to say, some can be swayed by arguments that flat slopes waste airspace and don’t producing enough cover soil. They’ll let the wishful thinking of the client dictate the design. Others, though, are conservative and bullheaded. If you have a choice, choose the latter.

But even if the cut slopes at your landfill are conservatively flat, they can still fail, especially if you don’t properly manage surface water. When soil gets wet, its strength can diminish drastically.

Eliminate the Water

Obviously, using ditches to quickly drain away water from the slope is vital. So too is eliminating all ponded water both above and below the slope. Remember, even a dry slope can fail if the supporting soil at the toe becomes saturated, or if water seeps in from above.

When it comes to preventing water-caused slope failure, it’s good to be a little paranoid. Details are important. The odd tire track or rut that seemed quite harmless during dry weather can become a fatal flaw in the wet season by directing water to a sensitive area. Similarly, the burrows of such animals as squirrels, gophers, or badgers can pipe water right into the heart of a slope.

Eliminate the Risk

One of the best ways to avoid problems with slope failures is to avoid working in risky conditions. In an ideal world, that means keeping the slopes as flat as possible. If the design says a 2:1 is stable, work at a 2.5:1 or flatter. Often, intermediate slopes (in contrast to final slopes) do not have to be cut to an exact grade, so leave them flat. Of course, when you’re excavating to subgrade for a liner or cutting the backslope for a roadway or ditch, you’ll have to build to the design slope. But otherwise lay those slopes back.

Pay Attention

I’ve heard people say they’re so broke, they can’t even afford to pay attention. Well, when it comes to slope stability, you can’t afford not to.

Sure, sometimes there are early signs of failure–indicators that point to a potential problem before the slope actually fails. I remember my uncle telling stories of how whenever men were working in a deep trench, there would always be a spotter up on top, watching for early signs of a cave-in. The spotter would watch for growing cracks or wet, seepy areas on the wall of the trench. They’d even sprinkle small amounts of fine dust into existing cracks and watch for it to puff back out of the crack, indicating subsurface movement. Of course this was back in the days before contractors followed safe excavation practices of shoring.

Trying to outguess a slope failure is risky business and not recommended. The price of failure is too great. It’s much safer and wiser to learn to spot potential problems early on–before there is a chance of somebody getting hurt. Whenever you begin a new excavation, look for old landslides in the area. Are there patches of vegetation that could indicate the presence of a spring or a small perched aquifer? As you are excavating, watch for changes in soil type. A layer of clay, silt, or sand could provide a slip plane.

We’d laugh at the thought of a farmer blindly planting his field and then waiting until harvest time to see what he’d get. Along that same line, it makes sense to learn something about slope stability at your landfill before a problem occurs. Look around at other construction projects, at other landfills, at cut slopes along local roads and highways. Do you see any problems? Are there any slides or failures? It might sound a little callous, but it’s always best to learn from somebody else’s mistake.

Evacuate

If all else fails (including the slope), get your people away from the danger zone. Yes, in some cases, that means pedal to the metal and get the heck out of there. More often it means pulling your crew out of a questionable area until the conditions can be evaluated and made safe.

Ask any operator who has been in a tractor and felt that sick feeling of the machine moving on what was supposed to be solid ground. They’ll tell you it’s one thing to move dirt, and quite another when the dirt moves you!

What can you do to avoid the dangers of slope failures? Get educated. Be conservative. Pay attention. And if you’re ever in doubt, call off the crew, call in an expert, and make sure the situation is safe.

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 - September/October 2002