No getting around it, the landfill design plays a significant role in how well (or how poorly) the available airspace is utilized. Let’s take a look at how and why compaction density should be included in the design process.
There’s a saying: “If you don’t plan for success, you’re planning for failure.” Without a doubt, this applies to landfill airspace. Universally, landfill managers recognize the importance of compaction, and many of them work hard to achieve it. But for the most part, their hard work is focused on the active side of the equation, which is primarily: Running the appropriate size/type of compactor and using alternative daily cover (ADC) to decrease cover-soil use.
These are good things to do, and certainly a landfill cannot achieve optimum compaction density without them, but they aren’t the only cards on the table. Focusing on the active ingredients of compaction is like working your tail off for 40 years but never investing wisely. Financial soundness, like optimum compaction, requires two things: hard work and good planning.
Before we get into the details of planning, let’s first address how tradition and attitude continue to limit our industry’s ability to plan for good compaction.
The entire process of landfill design—including site planning, phase sequencing, and overall development—falls under the responsibility of an engineer. And most engineers do what they’re trained and paid to do: They focus on the vital issues of environmental protection, safety, and regulatory compliance—all things intended to protect human health and the environment. These are good things.
If you review the assumptions used in the preparation of most landfill designs, you’ll see much in the way of soil mechanics, groundwater characterization, slope stability, structural calculations, and various other technical information.
But when it comes to waste compaction, you’ll find only a brief reference to some industry standard for waste density—usually 1,200 pounds per cubic yard and an estimated cover-soil factor of 20%–25%. No science, no research, just a footnote reference. And it comes with the underlying assumption that compaction is strictly an operational issue and somehow, some way the operators will handle it.
Also lacking is any discussion of how to maximize settlement by optimizing the fill sequencing or surcharging with stockpiles of soil, woodwaste, yardwaste, rubble, or other material. Again, these are considered issues of operation—not design.
I’m not picking on engineers. For the most part, our industry does not expect landfill designers to address these issues, nor are we willing to pay for it. And quite often, when it comes to operational factors (e.g., waste density), engineers are forced to rely on broad industry standards because the operators do not have accurate (if any) data available. And even if the operators have numbers, quite often they are inaccurate and based on the same industry standards.
As an example, many operators profess to use 6 inches of daily cover soil. This is required by the EPA’s Subtitle D regulations and by state regulations and has been communicated to every equipment operator at one time or another. So obviously, cover soil is 6 inches, right? But in the real world, the actual depth of cover soil is seldom calculated. And when it is calculated, by dividing the volume of soil brought in for cover by the surface area over which it is spread, the depth is always much greater than 6 inches. In my experience, the average depth of cover soil at landfills across the country is 16 inches—not 6.
As a result, we end up with this disconnect between landfill designers and operators—between the design and the operation. The designers minimize operational factors because they don’t have reliable data, can’t control the operation or, in some cases, simply don’t know how to deal with them.
And from the operator’s perspective, since the engineer didn’t address such aspects as compaction, settlement, or surcharging, they must not be an issue. This follows the unwritten code that the permitted landfill design is a fixed and immovable thing rather than a working document.
And so, when it comes to maximizing landfill airspace, we focus on the active issues and do the right thing by purchasing a compactor and using ADC. As a result, the other (planning) factors just fall through the cracks.
If we want to maximize compaction and move beyond simply working hard, we must close up those cracks and get compaction and density back into the design equation. Here’s a rundown on some important design factors, along with ideas on how to capitalize on them to increase compaction density.
Type of Design
When it comes to waste compaction, moisture is the single most important factor. Wet trash simply packs tighter and settles faster.
From this standpoint, when landfills are designed and permitted to allow recirculation of leachate through the waste mass, they gain a tremendous advantage over traditional dry-tomb landfills. But because bioreactor landfills cost more to build than traditional landfills, the decision to build one boils down to a cost-benefit analysis.
But what if your landfill was not designed as a bioreactor landfill? What if your goal has been to keep moisture out of the landfill at all costs? Is there any room to adjust? Perhaps there is.
Remember: One of the purposes of daily cover is to minimize infiltration of rainfall and snowmelt. Sounds good on paper, and in some cases, daily cover will actually shed water—or at least hold it until it can evaporate. But at many landfills, you’ll see a dramatic increase in leachate production during the wet season, indicating that infiltration is occurring.
Is this bad? Not necessarily, as long as your leachate- and gas-collection systems are functioning and you aren’t seeing hits in your monitoring program.
Taking that idea a bit further, how would it work to flatten the slope on the landfill’s top surface during the interim lifts? Infiltration could increase more or less, depending on your soil type and climate. If the thought of increasing infiltration in a landfill violates some internal code of honor with you (as it did with me), consider the following points:
- Flatter slopes will reduce the risk of truck rollover
- Increasing infiltration means less runoff—potentially reducing the cost of downstream drainage facilities
- Flatter slopes may reduce erosion by reducing and slowing runoff
- Increased infiltration will accelerate decomposition and will increase gas production
- And of course, increased infiltration will cause more initial settlement
Are there risks associated with increased infiltration? Of course there are. So the decision to increase infiltration for the sake of settlement should be made cautiously, with careful consideration of the potential risks and benefits.
Several years ago I was evaluating a landfill operation at a site where moisture was added to the active face for “dust control.” My internal alarm went off, and I told them, “You can’t add water to the active face.” They responded, “Yes we can. The state allows it, and we aren’t seeing an appreciable increase in leachate production. On top of that, our groundwater-monitoring results have been clean for years.”
To prove my point, I conducted a water balance on the landfill but was surprised when the results showed virtually no increase in leachate production. Also on the plus side, this landfill was receiving a tremendous benefit in overall landfill density. Its overall waste density was over 1,800 pounds per cubic yard.
Because of the tremendous potential to increase usable airspace, managing moisture should be a basic consideration of every landfill design.
Moisture Management
How much denser are wet landfills? In wet climates or where the wastestream contains lots of foodwaste, agricultural waste, or sludge, landfills may achieve compaction rates ranging up to 50% higher than the often-quoted industry standard of 1,200 pounds per cubic yard.
If the landfill is not prepared for it, excessive moisture can cause problems related to leachate, gas, and differential settlement. But when properly understood and controlled, moisture can be a good thing. The landfill design should be developed with an understanding of how the existing or added moisture will impact waste density. When the effects of moisture are not clearly understood, there can be difficulties. Here are two examples where moisture content affected landfill performance.
A company with lots of experience operating landfills bid on the operation of a large desert landfill. The desert landfill received approximately two-thirds less rainfall than the company’s other landfills and the wastestream also contained a high percentage of yardwaste (there was no greenwaste diversion program at the facility). As a result, the operator had to work much harder (by running four compactors to handle 2,000 tons per day) to stay above the contract’s penalty range of 1,100 pounds per cubic yard.
Another company, also with lots of experience operating traditional landfills, bid on the operation of a landfill in an agricultural area where lots of lettuce, broccoli, and other high-moisture crops were grown. The wastestream contained a high percentage of wet trash from local packing plants. As a result, the operator achieved a higher-than-expected waste density—and a significant performance bonus.
In both examples moisture content of the waste played a major role in the operator’s ability to compact waste.
So how does this fit into the design equation? First by understanding how the wastestream affects density, and, second, by using that knowledge to make accurate projections of how quickly airspace will be used.
Depth of Fill
The landfill depth will greatly impact the overall waste density. A small bulldozer may impose ground pressure of 4 to 8 psi, and a large dozer may exceed 20 psi. A compactor will typically be even higher.
But it may surprise you to find the weight of the waste/soil mass may impose even higher pressures. The waste/soil matrix in a typical landfill might weigh 50 pounds per cubic foot. This works out to approximately 0.4 psi per foot of depth. Thus, while at the bottom of a landfill 50 feet deep, the load may impose 20 psi, a landfill 200 feet deep may impose a load of 80 psi. Loads of this magnitude will create significant settlement over time as decomposition kicks in and the moisture content within the waste mass becomes more uniform. By considering this in the design process, engineers can use these massive loads to maximize overall landfill density.
Sequencing of Phases
A general rule of thumb for landfill design is to first develop the shallow portions of the landfill. The idea here is to minimize up-front capital costs by starting in the areas requiring the least amount of excavation. From a cost perspective, starting shallow makes sense. But the ultimate density of a shallow area will be much lower than what will be reached in a deep fill. In other words, while it may cost more to develop a deeper area, the density of the waste mass also increases exponentially with depth. Bottom line: Deep fills result in a lower (capital) cost per cubic yard. By looking at those options in terms of net present value, designers can select the best option.
Regardless of when the deeper areas are developed, in order to take advantage of the compressive forces, consider bringing those portions of the landfill up close to the maximum depth but stopping just one lift short of the final grade. Moving the operation to another area for a while allows time for gravity to do its work on the deep areas.
The settlement process could be further enhanced by stockpiling soil, greenwaste, compost, rubble, or other material on top of those deep areas for the sole purpose of increasing the load on the underlying waste.
This too becomes a question of cost-benefit. Pausing just shy of the final grade could affect your closure funding or require the early construction of additional liner, etc. Those negatives must be compared to the benefits of gaining some “free” airspace through settlement. In a moment we’ll look at an example.
Interim Fill Sequencing
Interim fill sequencing refers to the short-term planning required to keep the landfill on track from month to month and season to season where the main considerations include drainage, access, litter control, soil management, development of winter fill areas (if required), and various other operational issues.
Again, these things are considered operational issues, but in terms of airspace, they include much more than compaction and ADC. For example, shifting the access roads periodically to gradually cover the surface of the landfill will provide some increased measure of settlement. On the downside, it may also require placement of additional cover soil to provide a driving surface, create more dust, and result in the surface of the landfill being rutted and prone to ponding. Again, this is a cost-benefit issue.
At this point, you’ve probably seen a similarity among these design issues in that each must be based on a cost-benefit analysis.
Let’s step back and review the factors engineers currently use when developing a landfill design. Most of the decisions made by engineers during the design process are based on solid numerical values, such as soil permeability, coefficient of friction, or historical rainfall data. But when it comes to the operational issues we’ve been discussing, there is very little reliable data.
Here’s where the ball gets tossed back to the operations side of the field:
- How much will it cost to construct a soil stockpile on an interim fill surface and how much benefit (e.g., settlement) could you expect over a stated period of time?
- How much settlement could you expect, and how long would it take to achieve it (the benefit) if you postponed closure on the deep area for a while—and how much would it cost to develop additional liner to allow you to do it?
These are just two examples of the many questions you must answer in order for your landfill to move beyond compaction and ADC and start realizing maximum overall density.
Because of the variation among landfills, it’s unlikely we’ll come up with an accurate equation that applies to all facilities. The best numbers for your site are the ones you develop. We’re talking empirical results here.
Start by identifying the questions that are most applicable to your landfill. Is your landfill deep? Then look at ways to use that depth to your advantage. Set up settlement plates in areas that will not be disturbed for a few years, even in areas that have recently been closed. Survey them regularly and track the rate and quantity of settlement.
Of course, while this type of information provides sound data, it takes years to develop it. Knowing that a blurry target is better than no target at all, you may be able to use current data until something better becomes available. If—as is the case at many landfills—you develop annual topographic maps, you can overlay them and measure how much certain areas have settled from year to year. Yes, I know about the potential error associated with topographic maps, but if the only target we have is blurry…
We’ve done this many times and typically find that settlement follows specific patterns:
- It occurs fastest immediately following placement of waste and slows over time
- It is greatest in deep areas
- It can be enhanced by surcharging with stockpiled material
- It is enhanced in areas that receive more infiltration due to poor surface drainage
After comparing several consecutive topographic maps, you’ll be able to start making predictions regarding the amount of settlement to expect over specific periods of time based on the above-listed factors.
Along that same line, you could also erect settlement plates and survey them prior to placing temporary soil stockpiles. Track the depth of soil placed at the plate. When the stockpiles are removed, resurvey the plates and calculate how much settlement was achieved at that point. Correlate that value to the depth of underlying waste and how long the stockpile was in place. Again, this type of data will be very useful when making decisions regarding placement of stockpiles on waste.
This chart shows the expected settlement per year based on the depth of fill. We’ve found lots of variation in terms of the amount of settlement, but the general trends are consistent: Settlement is typically greatest during the first year and then decreases in subsequent years.
Here’s how this information can be applied. Suppose you’ve filled an area to an average depth of 160 feet and have, on top of this area, a 5-acre deck. It is one lift shy of the final grade, but you are considering postponing the placement of the final lift of trash…if it makes economic sense.
Let’s assume that for various reasons, postponing closure and moving into another area costs $60,000 per year. And let’s also assume airspace is worth $3 per cubic yard.
With this information, does it make sense to pause—and if so, for how long?
Based on our example, in the first year the 5-acre area will settle an average of 4 feet, creating an additional 32,267 cubic yards worth $96,800 (see table). In the second and third years, it also makes sense to stay off the 5-acre deck and let it settle. But by the fourth year, the rate of settlement has slowed and it appears best to place the final lift of trash and close that area. But wait! By the fourth year, the area has settled nearly 12 feet, so it will require two additional lifts to reach the final grade. Thus you’ve benefited from not only more time, but also the added weight of another 12 feet of trash working for you to create additional free airspace.
When properly understood and utilized, landfill settlement is a tremendous tool.
Want your design engineer to provide more details in regard to maximizing waste density at your landfill? Provide him or her with some reliable numbers to work with.
You want increased density? Your engineer needs numbers. Testing and recordkeeping is the best (and only) way to provide them. Get with it.