More Than One Liner

Based on the timeline of when people began generating trash and tossing it into a heap, landfill liners are a relatively new development. And depending on your age, you may even remember when landfills were unlined holes in the ground-often gravel pits or wetlands that needed to be filled.

I recall in the late 1970s, working through the night fueling and resetting the dewatering pumps to allow the scrapers to go deeper…and deeper. And then once we reached the point where the pumps couldn’t keep up, we’d stop digging-and start filling the hole with trash!

Shortly thereafter, some jurisdictions began requiring compacted clay liners. My first liner construction projects occurred in California in the early 1980s.

But like most other things in our fast-paced world, landfill liner technology has advanced rapidly. These days, landfills and liners go together like laptops and Wi-Fi-they just…go together.

But the rollout of our first national landfill liner regulation in 1991-Subtitle D-was a big deal. Think about it: In many cases, we went from filling in old gravel pits and wetlands-to composite liners with geomembranes, geotextiles and geocomposites…Geo Whiz! That was a big change.

When it came to designing and building an effective landfill liner system, our industry had a lot to learn…and learn we have.

That initial Subtitle D liner specification consisting of 2 feet of clay and a sheet of 60-mil HDPE now seems as simple and unexciting as an automatic transmission-but there was a time…

I recall one particular training session sometime in 1989 or 1990. I was helping to spread the word about how Subtitle D would impact the landfill industry. I want to say, “I was helping to spread the good news,” but don’t want to lie.

These particular meetings were small gatherings sponsored by the Midwest Assistance Program with the goal of educating rural municipalities. I’d just made a presentation about some of the key ingredients of Subtitle D, including landfill liners and groundwater monitoring, when a county official-a very angry one-stood up and announced that this was just one more stupid government intrusion, and his county’s landfill was not leaking. Interested, I asked him how he knew-did they have monitoring wells…or were they testing the neighbor’s water? He said, “No, none of that. I just know they aren’t leaking!”

I don’t really know if he knew, but I guess it’s like the engineer who said, “It doesn’t matter if you’re right or wrong…as long as you’re sure.”

Fortunately, the designers, builders, and operators of today’s modern landfills are usually sure…and they’re usually right.

Across the board, the world of landfill liners has progressed to a very sophisticated point. Two decades of R&D, and thousands of installations have resulted in some great materials and a new generation of knowledge. As a result, landfill liners consistently perform as intended.

That doesn’t mean we’ve put a box around Subtitle D’s standard liner design and just kept building the same thing over and over. To the contrary, landfill liners are becoming increasingly more complex and more effective. It’s not unusual to see double composite liners that integrate more sophisticated materials. For example, geosynthetic clay liners (GCLs) are widely used in conjunction with geonets and standard geomembranes.

A GCL is a combination of manmade materials and natural clay soil-usually a very high-quality, ultra-low permeability bentonite soil. Some look like a sheet of HDPE (plastic) with a quarter-inch of bentonite clay granules stuck to one side. Other styles look like two layers of a felt blanket …with a quarter-inch of bentonite clay in between. They are flexible and, when installed correctly, provide predictable results. As you see, the common ingredient is bentonite clay.

So, how low is the permeability of that clay? Well, as a reference point, the standard Subtitle D design requires a compacted clay liner (CCL) with a maximum permeability of 1 x 10-7 centimeters per second (cm/s). In case you have a hard time visualizing 1 x 10-7 cm/s, it essentially means that water would pass through it at a rate slightly faster than 1 inch per year. In contrast, the bentonite clay in a typical GCL has a permeability of 1 x 10-10 cm/s or less…which is about 1 one-thousandth of an inch per year. This is very tight material.

We are also seeing an increasing use of geonets as a leak detection layer…or in some cases as part of a leachate collection and recovery system on top of the liner.

Why are we seeing an increasing level of effort going into landfill liners? There are a number of factors.

• Landfill owners are concerned with potential liability…and as better containment options become available, they are willing to spend more money to mitigate any potential risk. To a degree, their customers-or what are commonly referred to during clean-up operations as potentially responsible polluters (PRPs)-are also looking for better protection.
• Regulatory agencies are also applying sometimes subtle-and often direct-pressure to up the ante in terms of liner integrity.
• Cost is also a factor. In many situations where native clay is not readily available, it is more economical to use a GCL.
• The perceived trade-off of 2 feet of compacted clay for three-quarters of an inch of GCL, along with the increased QA/QC required to ensure the integrity of GCLs can push designers toward a second system.
• Double-composite liners also offer the great benefit of an integrated leak-detection system between the upper and lower system. This provides good peace of mind for all stakeholders.

Generally, these more sophisticated liners are applied to the floor of the landfill, with a single-composite system still used on the slopes. The idea here is, of course, that leachate on the side-slope is moving downward quickly-with virtually no head (i.e., pressure).

Can things still go wrong? Well, can you spell M-U-R-P-H-Y? Yes, they can, and sometimes they do. It doesn’t take an engineer to conclude that the primary purpose for a liner is to protect groundwater-and the primary concern is that it won’t. There can be problems. Here’s a rundown on some of the lessons learned.

Design/Installation Problems

Though it has been known to happen, catastrophic landfill liner failure due to a poor design is a rare event, about as likely to occur as California falling into the Pacific during a big earthquake. Some folks expect it-maybe even look forward to it-but I wouldn’t hold my breath. Still, the scarcity of catastrophic failure doesn’t mean that there aren’t problems.

Wind uplift-Wind uplift can occur if the wind gets under the edge of an unsecured liner, but more often occurs as the wind flows over a lined slope and creates a negative pressure zone above the liner.

Why does this occur? We’ll it isn’t rocket science…more like aviation science. In the same way that air flowing over the camber of an airplane wing creates lift, wind flowing over a lined slope creates a negative pressure that can have a ballooning effect on the liner. Without adequate ballast-and it can take a lot-the liner can pull out or tear.

Moisture accumulation-Moisture in the underlying soil can condense on the underside of a geomembrane, potentially lowering the coefficient of friction between the liner and the soil. Of the designer was counting on that friction to hold the liner in place (which, of course, is the case), bad things can happen.

This accumulation of moisture can be more pronounced at the toe of the slope…if it is able to migrate along the underside of the geomembrane. And if the design includes a GCL, the unexpected moisture could hydrate the bentonite, requiring removal and replacement.

Consistency of textured HDPE-One of the quandaries that designers face is determining the correct degree of slope. Steeper slopes provide more landfill airspace but are less stable than flatter slopes. Textured HDPE is intended to increase the coefficient of friction between the HDPE and the soil or GCL…and it does. But it’s important for the designer to compensate for slight variations in texture between material runs. How can a designer compensate for these variations, when the design may be done long before the material is actually produced and tested? They compensate by leaning toward the conservative side of the design scale.

Welding-When most of us think about welding, even in the context of landfills, we’re thinking metal. Trucks, trailers, and tractors-that’s what comes to mind. But some of the most important welding ever done at your landfill is done with plastic. That’s right: When it comes to liner construction, most of us don’t think too much about HDPE welds. But if you were the designer, you would. You’d think about it because welds can be a focal point for stress in the liner.

Following the Northridge earthquake in Southern California in 1994, engineers found some issues with HDPE welds on landfill liners, many of which were related to patches. Apparently the welds were acting as stress points. There were also some issues along the anchor trenches in regard to them being overcompacted, not allowing them to pull out, creating too much tension.

Subgrade variation-HDPE material will mirror the underlying surface, obviously. Generally this isn’t a problem, and it’s not even much of a consideration. But if the underlying surface changes, it could be an issue. Consider what could happen if a sand-filled sump becomes saturated and settles. In that case, the HDPE can become stretched-like a trampoline. This could place unexpected stress on the HDPE.

Placement of drainage layer-After the liner has been designed, placed, and inspected, there remains one more vital step. Like the maiden voyage of a new ship, where the first dip is the most scary, so too is the first pass with a big tractor. Placing the drainage layer-usually sand or gravel-can be risky. The right machine is vital for success, as is the right operator. This work should be done by someone with a light touch and a cool head.

These problems have occurred in the past, and like all fields of design, landfill engineers are constantly learning, modifying, and improving their designs. Today’s airplanes are safer than those of years past, and future planes will be even safer. It’s the same with landfill liners.

That’s why, when it comes to landfill liners, I’m not so pessimistic as to fully buy into Murphy’s Law, which states that, “Anything that can go wrong, will go wrong”…but I suppose it could. And over the years…it probably has.

Geoelectric leak detection testing-Geoelectric leak detection testing is a common next step following placement of HDPE. It is based on the principle that while an electric current cannot readily pass from the underlying soil through HDPE, it can pass through a hole. This allows testing companies use an electric current and monitor subsequent “hits” across the liner to identify holes. This technology provides good piece-of-mind for designers as well as for landfill owners and regulators.

Construction quality assurance (CQA)-Designing a landfill liner is not a cookie-cutter process, and it never will be. There are simply too many variables. Some, like manmade materials and, to some extent, soil, we can quantify to a degree. But others, such as weather conditions during and after construction, or how an equipment operator will perform, are way outside the designer’s control.

That’s why engineers who design landfill liners are so adamant about having good CQA.

The field of landfill liner design is evolving and improving to the point that today, when things go wrong, it’s less likely to be related to a material defect or design problem and more likely related to an operational oversight.

Why is that? To some extent, it’s based on experience. Typical manufacturers make material all day long in climate-controlled factories. It’s their business, and many of them have been doing it for years. Similarly, the typical designer has probably worked on scores of liner design projects-and regularly attends conferences to share knowledge. But the average landfill worker may have seen only one or two liner installation projects during the course of a career…period.

Am I pointing a finger at operations folks by suggesting that they sometimes contribute to liner problems? In a word: Yes…in some cases. Okay, I know that’s four words, but I wanted to ease into it gently.

Over the years, I’ve observed that most issues with liner integrity occur after the liner system has been designed and installed. I’ve heard the same thing from many others in the landfill business.

Credit: Neal Bolton A crew works to cover a liner with dirt.

What Can Go Wrong
There are number of things that can cause damage to a landfill liner after installation. Let’s take a look at some of the things that have happened.

Uncontrolled drainage-A bench along the perimeter of a recently lined area served double duty, providing room for both the HDPE anchor trench and a drainage channel. However, the first major rain event eroded upstream soil, filling the drainage channel with sediment. Runoff flowed over the edge, washing way the operations soil, ripping the geotextile, and scouring into the anchor trench.

Machine damage-Landfill liner damage as a result of a piece of heavy equipment is not uncommon. Most landfills have a story or two about the compactor operator that nicked the liner on a slope…or the dozer operator that got careless while placing operations soil.

I am not downplaying the seriousness of these events, but I do recognize that they can happen. I too spent several years operating heavy equipment and know that even the most careful operator can make a mistake. But that’s not the issue. Without exception, I’ve never seen a liner that had been damaged by piece of heavy equipment that couldn’t be repaired with a reasonable amount of effort.

The bigger problem in this category of damage is management-heavy-handed management that creates a culture where operators are afraid to disclose mistakes. I’m not saying there shouldn’t be discipline for repeated carelessness, but managers who flip out over honest mistakes are part of the problem-and perhaps they should spend some time on a machine to gain a broader perspective.

Adjacent activity-Activities that occur adjacent to an exposed liner can cause problems. Other construction projects, haul roads or nearby waste operations can cause rocks or other debris to accumulate on the liner. In most cases, there is a gradation spec that limits the maximum allowable rock size in the operations layer. Usually it’s specified at one-half-inch or three-quarter-inch minus. So if we aren’t careful when working near the liner, it’s possible to roll down much larger rocks.

Landfill operations-Normal landfill operations can also contribute to liner damage – sometimes in surprising ways. One of the more embarrassing ways to damage an in-place liner is to dig a hole through it. This can occur when operators are digging a hole to receive sludge, dead animals, or other waste material that shouldn’t be spread on the active face. I imagine it’s quite a shock to be mucking out old trash …and then see a flap of HDPE or GCL hanging off the bucket! Drillers installing vertical LFG wells have also had this happen, usually a result of a survey or as-built drawing.

Animals-In our culture we don’t think too much about animals being a threat. That comes from living a domesticated lifestyle. But there are places where man-eating crocodiles pose a much greater threat than some maniac in a minivan with a 0.65 blood caffeine level. Similarly, some animals do cause liner damage, though they may not get much notoriety. The most obvious are diggers: badgers, gophers, woodchucks, and other burrowing animals. Bears can also cause damage as they dig for food along a recently filled edge. Did I say bears? Yes, a surprising number of landfills have what you might call serious bear issues.

Other animals may cause liner damage by simply walking across the liner. No, we aren’t talking elephants…but deer. With their dainty but sharp hooves, deer can do some damage on areas where the liner is flat enough for them to travel but still steep enough that they must lean into the slope.

A Good Liner Is…
Designing an effective liner is more than managing permeability and coefficient of friction, as important as they are. It’s more of a holistic approach in bringing together the experience and know-how of regulators and operators, contractors and CQA technicians, because a good liner design starts long before the contractor breaks ground, and it lasts until…well, it just lasts.

And because many of the problems we see today result from acts outside the realm of engineering, it behooves the engineer to think like a contractor or operator. That means designing for and even foreseeing things that could go wrong…before they do.

One important example of this is the management of stormwater. A landfill liner is like a giant bathtub, and when done right is watertight-to retain leachate. But during that time between certification and receiving the first layer of trash, many of those liners retain a lot of stormwater. That lined hole in the ground can accumulate runoff that can, if not properly managed, quickly and quite decisively overload your leachate system.

Developing the right filling sequence for a new liner is a critical preventive measure, as is installation of temporary HDPE flaps or soil berms to help keep leachate and stormwater separated.

Want to design a better landfill liner? Talk to folks just long enough to ask them what problems they’ve had to deal with in regard to landfill liners…and then stop talking. You’ll find, as I have, that there’s a lot about liner design-successful liner design-that they didn’t teach you in engineering school.