MSW Management | Landfills Are Airspace

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Landfill owners and managers don’t spend much time trying to incorporate airspace management into their planning; they know airspace is the plan. Airspace is the marketing plan, the operations plan, and the business plan.

By Neal Bolton

The idea that airspace is the core concept of a landfill must be foundational to everything a landfill owner or manager does, period. And despite what some may say, this doesn’t conflict with the other important objectives of worker safety, environmental protection, or customer service. Proper airspace management is the means of providing those things.
There is no shortage of suggestions for landfills to create a new policy or implement another program. But the only realistic way for those things to happen is through the proper and profitable management of airspace—the primary commodity that landfills have to offer.

Henry Ford once said, “It is not the employer who pays the wages. Employers only handle the money. It is the customer who pays the wages.” And if you own a landfill, your customers are paying the wages and everything else through the purchase of airspace.
In the same way that gold-mining companies focus on extracting every profitable ton of ore from the ground, savvy landfill managers attempt to squeeze as much waste as possible into their facilities.

Maximize That Airspace
When I was in the 5th grade, I got to leave class one morning with my friend, Fritz, to go hide Easter eggs for the kindergarten class. We were given several dozen colored eggs along with instructions to hide them in and around the playground. We hid most of them in plain sight where any 5-year-old could easily find them. But because we were older and more experienced, a few of them were hidden beyond the ability of the kindergarten class to find or reach. I must confess this was not all innocent oversight, and, of course, we were careful to note their location for future reference.

Max Out the Plans
Surprisingly, I’ve found that landfill engineers sometimes operate in a similar manner when it comes to airspace. No, they don’t covertly hide things from their clients. But in the normal course of designing a landfill, they must make judgment calls based on their experience and perspective.

Photo: Sierra International

A balefill harmonizes with the topography.

For example, many landfills are designed with maximum slopes of 3:1 on the internal lined slopes and the external final slopes alike. This decision often comes from a slope stability analysis that shows such a slope to be stable. But it’s often based on what has traditionally been accepted by the permitting agencies and on what the engineers might refer to as “ease of construction.”

Because 3:1 slopes are so common, it’s expected that landfill operators can more easily build a 3:1 slope than one that’s a bit steeper; say 2.8:1. There’s the judgment call: Landfill operators would rather have one less decimal point to deal with than gain thousands of cubic yards of additional airspace.

Similar assumptions are made in regard to property-line buffers, maximum height limitations, perimeter road width, excavation depth, and dozens of other parameters. Each of these is to some extent tied to a judgment call—and each can affect the amount of airspace your landfill contains.

If you suspect there may be undiscovered airspace at your landfill, contact your engineer and review the assumptions he or she used during the design process. You may discover something a lot more valuable than colored eggs.

Maximizing the available airspace refers to more than the raw volume of a landfill. It also means using that airspace in the most efficient way.

Landfill Settlement
Every landfill that contains organic material is at some stage of the decomposition process. As subsequent lifts of trash and soil are placed on top of a landfill, the underlying layers will be pressed down or “consolidated.” As waste decomposes, it releases moisture and heat, which may combine to weaken some of the supporting structure. The bottom line is this: Landfills will settle.

Many factors affect the amount of settlement that a landfill will experience, but it is sufficient to say that in most cases it is significant. Settlement plates can be used to quantify how much and how fast a landfill is settling.

Settlement plates are simply plates (usually metal) that are set on top of a landfill and then surveyed periodically to measure settlement. Some are equipped with a vertical extension so that their elevation can be tracked, even though the plate may be covered with a subsequent lift.

By placing several across the landfill (i.e., in a cross-section) it may be possible to model settlement rates based on the underlying depth of waste.

Many action-minded landfill managers are less concerned with how much the landfill is settling than they are with finding ways to increase settlement as much as possible. Here are some ways of maximizing landfill settlement.

Preplanning
A small investment in preplanning can yield big returns in airspace. For example, many landfills will at one time or another have to stockpile soil. By identifying where and when that soil will eventually be used, it is often possible to use stockpiles to create additional airspace.

A soil stockpile placed on top of waste will cause the underlying waste to settle. The amount of settlement will, of course, depend on the depth and age of the waste, the weight of the stockpile and how long the stockpile remains.

Before deciding to construct a stockpile on top of your landfill, check with the design engineer to make sure the added weight isn’t a problem. Also affirm that the stockpile’s location isn’t prohibitive in terms of hauling cost and that it won’t be in the way of other activities.

Finally, survey the location of the stockpile and keep track of how much soil it contains. Track it again as it’s removed. The soil, along with any potential airspace it created, could be lost otherwise. If the manager is transferred or the operator retires, the stockpile may slowly settle into the landfill and be forgotten.

Developing a soil inventory is a good way to keep track of soil stockpiles at your landfill. You may want to start by potholing the landfill. At a minimum, the grid spacing should be 100 feet by 100 feet, but it’s even better to go smaller, say, 50 feet by 50 feet so that no area is missed. At each grid point, record the depth of soil, survey the location, and then tie the information into a recent topographic map. Be sure to provide the results of potholing to the engineer who does the volume/density calculations.

If you’re starting from scratch, consider dredging up historical topo maps and having the volumes checked between topos. By comparing the volumes to tonnage received during those periods, you can quantify density. Expect to see quite a bit of variation from one period to another. This could be due to surveying errors, but more often it results from errors in tracking stockpiled material.

Sometimes topographic mapping companies will update the topography of only those areas that have changed since the last map was created. This saves money but may also miss some areas that contained small stockpiles, sliver fills, or areas where soil was stripped or excavated.

Because of these errors, the data points showing each year’s density will likely be somewhat scattered. But there is also likely to be a general upward trend in density due primarily to decomposition, settlement, and consolidation. This historical data can give you a good picture of what’s happening at your landfill.

A Slope Meter provides quick and accurate measurements.

The Time Value of Money
Many landfills periodically expand their footprints, lining new areas every few years. Although these kinds of projects often represent millions of dollars, they may also become a standard budget line item—which means we give it less attention because it’s approved and “in the budget.”

But let’s stop and review a basic economic reality: the time value of money. Because of the interest paid on money, today’s dollars are worth more than future dollars. This applies if you pay interest on borrowed money or earn interest on saved money.

Because of this phenomenon, a landfill benefits by postponing expenditures (e.g., liner construction projects) for as long as possible. Of course, we’re not talking about procrastinating to the point of crisis, because the new liner isn’t finished. But spending money on a liner expansion before it is needed makes poor economic sense—even if it’s budgeted and approved.

Many of the landfill owners and mangers who read this article have plans to build additional liner this year or maybe next. But a close look would reveal that some of them have several years of capacity on their existing waste footprint.

By going back and filling areas that have settled or were simply not filled completely, those landfills could push liner construction out a few more years. How big a deal is this? Let’s see. Postponing an expenditure of $1 million for one year (and assuming an interest rate of 5%) would save $50,000.

Get That Expansion Now
There is an economic incentive to delay liner construction until other areas of the landfill have been filled, but along a similar line, when it comes to getting permit approval for an expansion, stalling may not be advisable. Political tides do change, and if an opportunity to permit a landfill expansion is available it should be seriously considered. The up-front cost of an expansion pales when compared to losing the opportunity altogether.

Tight Surveying
Once the design has been tightened up and you’ve extracted every last bit of airspace from the design, it’s important—no, it’s vital—that the design be translated clearly and effectively to the field. Finding the airspace on paper is a necessary but incomplete step. You must also be able to find and claim that airspace in the landfill.

Just as the finest race car ever made eventually depends on a mechanic who knows how to use a wrench, all landfills could benefit from operators who know how to use a hand level. To truly be considered an accomplished operator, one must know how to build slopes, maintain drainage, and perform basic grade-checking without calling a surveyor.

A very simple way to approximate grade is to mount a slope meter on the dashboard of the dozer or compactor.

There are more high-tech surveying options available today than ever before. But, fortunately, sorting through them to find the one that best meets your needs may well be the hardest part because most are fairly user-friendly.

GPS
Starting at the top, a tractor-mounted GPS provides the most hands-off system available. When properly set up, your compactor(s) or dozer(s) actually become the landfill’s most active surveying instrument(s), sending and receiving information on topography, waste density, and other useful information.

Before long, a GPS will be as standard on landfill machines as a remote control device is for your television. If you don’t believe it, take a stroll through a local car dealer’s lot and see how many cars are now equipped with GPS navigation.

My family recently visited a beach in Monterey, CA. Curious about the small four-wheel all-terrain vehicle driving back and forth along the beach, I finally flagged the driver down and asked what he was doing. “Oh,” he replied, “I’m using the GPS receiver on this four-wheeler to map the beach. We’re trying to see if and where the beach is eroding.”
Not surprisingly, the same thing can be done at landfills to provide topographic mapping for seasonal planning and density calculations.

Photo: Terra Compactor Wheels

Wheels can help achieve high density.

Standard Surveying
Standard surveying is still a very integral part of providing control for landfill projects, although the multiperson crews are rapidly being replaced with one person and a GPS instrument.

Many landfills are also using laser-based systems for providing onboard grade control for the operators. These systems work best when used to construct such large plane surfaces as those found on as interim or final fill lifts.

Other types of laser systems can provide accurate mapping using only one person on the instrument. All measurements are performed by bouncing a laser beam out to a point on the ground and back to the instrument—no rod-man required.

Regardless of the surveying method you choose, it must be accurate and available. When it comes to maximizing a landfill’s airspace, the perimeter slopes and top deck must be fully constructed to the design grades.

Finally, the manager and operators must understand how to interpret the plans. Do the final grades include cover soil, or are they “top of trash” grades? Are the outside slopes shown on the plans “build to,” or should they be overbuilt to compensate for settlement?
As the manager’s understanding of the landfill design increases, so will the chances of fully utilizing all available airspace.

Fill It Wisely: Trash Only
Imagine reading about a large retail store that was hiring workers to help give away, for free, 25% of its inventory—not as a one-time event but as a normal part of doing business. It would be difficult to imagine such a thing without also wondering how long it would take them to go broke. If a store manager did this, we’d call him nuts.
Now stop imagining and come back to reality. Many landfills hire workers (operators) to help give away (fill with soil) 25% of their inventory (airspace) ... and we don’t think anything of it. Instead, label it “normal.”

Of course 25% is just an average. At individual landfills the percentage can vary from around 10% to over 50%, but excessive soil use—for daily and intermediate cover—remains the number-one contributor to lost landfill airspace.

This explains the widespread use of—and range of products available for—alternative daily cover (ADC). ADC can be used effectively at any landfill and offers the same benefits regardless of whether the facility receives 10 tons per day or 10,000.

ADC products come in various forms, including spray-on materials, films, and tarps. Large landfills might utilize several different types. ADC also includes a variety of waste-derived material such as processed woodwaste, greenwaste, construction-and-demolition (C&D) debris, auto-shredder waste, chipped tires, or petroleum-contaminated soil.

If you haven’t yet become an ADC user, talk to some other landfill operators in your area and see if you can find a way to fit it into your operation.

Soil may be the biggest factor in lost airspace, but it’s not the only one. Significant parts of the wastestream can also waste airspace. Included here is anything that could be extracted from the wastestream and put somewhere other than the landfill. Of course we’re talking about recycling, but here our focus is on those big-ticket items that represent a major percentage of the wastestream. This list commonly includes woodwaste, greenwaste, concrete or asphalt rubble, C&D, and, in many cases, cardboard.

When considering any type of diversion program, one should start by quantifying the wastestream. Find out how much of each material currently goes into the landfill and then look for ways to extract it and use it somewhere else.

Pack It In
Compaction remains a major factor in getting the most from a landfill’s available airspace. Yet waste compaction is like an exercise program: Everyone has an opinion; most would agree on the benefits; yet the why and how can be hard to understand.
One of the most common problems landfills encounter has to do not with the mechanics of compaction, but on the assumptions with which we begin.

Here are two assumptions typically used during a landfill’s design stage.

An engineer may assume, based on historical data or operator input, that the landfill achieves a waste density of 1,200 pounds per cubic yard and a 3:1 cover ratio. Based on this assumption, the engineer calculates overall airspace and soil quantities. In essence, a landfill will be filled based on this configuration.

When looked at from the basic perspective of tons of waste per cubic yard of airspace, this works out to an effective density of 900 pounds per cubic yard. Effective density is calculated by dividing total pounds of waste by total cubic yards of airspace consumed. In this example, every cubic yard of waste weighs 1,200 pounds and is accompanied by another one-third cubic yard of soil. Thus, every 1.33 cubic yards of airspace contains 1,200 pounds of waste. Dividing 1,200 by 1.33 cubic yards equals 900 pounds per cubic yard.

And so, year after year, tonnage received and volume consumed are measured. The engineer divides total pounds of waste received by total cubic yards of airspace used and comes up with an average effective density. And as long as this annual effective density appears to be tracking with the initial assumption, all appears to be well and no questions are asked.

Landfills are dynamic, however, and overall waste density tends to increase with each successive lift. Similarly, because many landfills do not accurately track the amount of soil used as cover, it is common to see soil use increase over time. Here’s a common scenario:

This graphic shows how the incremental settlement of underlying lifts, combined with an increasing use of soil (years 1–4) can mirror the expected airspace use (Expected) based on the initial design assumptions. Landfills where this is occurring will eventually hit the wall when they run out of soil or finally begin tracking soil volumes and realize what’s happening.

Excessive soil use continues to be the major culprit in regard to wasted landfill airspace. The point here is that a simple evaluation of airspace consumption may not reveal underlying problems. It is vital that landfill operators track soil use and include that information in all periodic airspace volume calculations.

Good Old Compaction
When it comes to the actual process of compacting waste, most managers and engineers are aware of the importance of making an adequate number of machine (compactor) passes. An ever-increasing number of landfill operators are utilizing GPS to track the compactor’s effort. This includes counting passes or monitoring elevation change. GPS-based units can also direct the compactor to work toward specific design grades and record a constantly updated topographic map of the landfill’s active area.
Having the right size and type of compactor is also a critical part of achieving the best possible compaction. In general, heavier compactors provide greater density, as do wheels with lots of big teeth. When it comes to maximizing airspace, everything eventually comes down to using a machine to compact waste.

For most facilities, the machine most often used to compact waste is a steel-wheeled landfill compactor. But there are other choices. Some landfills achieve compaction through the use of balers. Baled waste landfills or “balefills” can also achieve very high waste densities.

At the St. Lucie County Solid Waste Authority in Ft. Pierce, FL, Assistant Solid Waste Director Ron Roberts speaks highly of the baler in use there for six years. “Our bales range from 3,600 to 4,500 pounds each,” says Roberts. “And with the bale system, we use 80% less cover soil.”

With baled waste densities of 1,500 to 1,900 pounds per cubic yard, this facility ranks among the best of traditional landfills when it comes to efficient use of airspace.Balers can yield high waste densities and also provide the benefits of reduced litter and a deterrent to birds.

There are many factors that can affect a landfill’s airspace. The more you know about how it’s derived and utilized, the better chance you have to use it wisely.

But remember, it’s pointless to go through all of this engineering effort if the folks who will be implementing the plan don’t know what they’re doing or why they’re doing it. Train your crew to be airspace-conscious. Communicate to them the importance of using airspace wisely and then provide them with the tools they need to do it.

Proper airspace management has been, and will continue to be, the primary factor in defining success.

Neal Bolton is a consultant specializing in landfill operations and management.

MSW - May/June 2007