Using a Piezo-Penetrometer Test to analyze subsurface conditions at the landfill can provide extensive information and allow for a fast and effective diagnosis of the problem. By measuring gas concentrations and temperatures around the area of concern, conclusions can be made about the cause of the fire and the best approach to extinguishing it.
By Reg Renaud and Jonathan Chihoski, P.E.

Subterranean landfill fires can be very problematic in the operation of landfills. Regulatory agencies enforce current laws, regulations and guidelines that require a significant landfill gas (LFG) collection system at facilities and they continue to introduce updates that require more LFG collectors to be installed for a given facility in an ongoing effort to reduce gas emissions.1,2 If LFG collectors are not carefully designed, located and installed, then the frequency of subterranean fires will continue to increase. There are many causes of subterranean fires and they create several potential dangers to the environment and the day-to-day operations of landfills. This paper will discuss these issues and ways to diagnose, prevent and extinguish subterranean fires.

The Cause
The three basic elements of fire are fuel, heat and oxygen. All of these elements can be found at all landfills. The best way of preventing subterranean fires is to control at least one of these elements. Usually, this is done by preventing oxygen or air from entering the trash prism of the landfill. However, this is not always possible, the following are some of the ways air can enter a landfill:
• Air can be trapped in the refuse as it is placed into the landfill (starved air combustion)
• Insufficient or deteriorated soil cover over the refuse allowing excess air to enter or be drawn into the waste mass
• Poorly designed, constructed or located LFG collectors
• Overdraw vacuum on LFG collectors
• Buried permeable haul roads (particularly gravel roads) provide pathways for air intrusion
• Separation of the bench road and the toe of slope due to landfill settlement

Improper placement of waste in the landfill can increase the likelihood of a subterranean landfill fire. Insufficient compaction and placing a large amount of refuse in warm weather can lead to rapid composting and the refuse may begin to smoke. In a similar manner, if the refuse in a landfill is not covered with enough soil to prevent air infiltration, subterranean fires can result. This situation can occur due to the inadequate application of cover materials or as a result of erosion of properly installed materials.

Once the refuse is in place and covered with soil, LFG collectors are typically installed into the landfill to extract LFG. During this process, it is important that the collectors are designed, located and constructed properly. If the vacuum at a particular collector is adjusted too high, it will overdraw the area of influence and air will be drawn into the refuse from the surface. As air enters the refuse, subterranean fires can occur. As a landfill increases in height, a haul road that was used to transport refuse to the working face is often buried with refuse.

These hauls roads are typically constructed using gravel or other coarse materials to provide longer usability and more traction. This permeable pathway can allow air to enter the waste prism. Furthermore, the edges of these haul roads are typically not as compacted as the center of the road where most traffic occurred, and these soft edges can provide another pathway for air to enter the landfill and for LFG to escape into the atmosphere. Additionally, landfill bench roads can separate from the side slope of the landfill as a result of settlement, poor maintenance, and overloading. When this occurs air and storm water can enter the refuse and lead to a fire by providing more oxygen and increasing LFG production. Many landfills install v-ditches at the toe of the slope to help prevent this. These conditions concerning haul roads can lead to subterranean fires in landfills.

Troubleshooting the Problem
Currently, the most common indicators of a subterranean landfill fire that an operator will use are:
1. Smoke emitting from the landfill;
2. Elevated gas temperatures in LFG collectors or headers;
3. Rapid settlement or sinkholes (usually around a collector);
4. Elevated carbon monoxide CO and CO2 levels in the gas stream; and
5. Flyover infrared photos of the landfill (typically before sunrise) showing a heat plume.

Troubleshooting a subterranean fire or rapid decomposition can be easy if smoke is emitting from the landfill or rapid settlement is observed and the cause can probably be focused on a nearby collector that is overdrawing or localized soil cover issues. However, when this is not the case, identifying the cause will be more difficult. If the gas temperature in a particular collector is elevated, (typically 140oF or higher), but no smoke or sinkholes are visible, it can be difficult to determine where the subterranean fire is located. Currently, most operators must adjust their monitoring of the situation until it becomes visibly obvious where the fire is located by smoke or sinkholes. At this stage it may be too late to avoid damage if there is a bottom liner in place at a landfill. Regulatory agencies are very concerned about subterranean fires when there is a bottom liner involved.

An alternative approach to identifying the cause and location of the subterranean fire is to perform a Piezo-Penetrometer Test (PPT) and then use a thermistor to measure the temperatures as the PPT cone is pushed into the landfill.3 The PPT can provide the following troubleshooting abilities:
• Map out possible void spaces in the refuse;
• Locate layers of vacuum that may be pulling in air;
• Locate buried haul roads;
• Focus temperature readings in the affected area; and
• Recover in-situ gas samples and test for CO and elevated CO2 levels.

A grid pattern of PPT soundings would be performed in the area of concern. The PPT would also profile the vacuum influence of nearby collectors and as well as identify areas with low tip resistance indicating possible voids that may eventually become sinkholes. When accurate surface elevation data is provided, the PPT can indicate the temperature near the bottom liner without the risk of damage due to the sensitive depth control of the PPT. The PPT can also be used to locate possible pathways for air caused by buried haul roads or from leaking bulkheads for horizontal collectors. In-situ gas samples can be recovered during the investigation using a down-hole gas sampler to check CO and CO2 levels.

subFigure-1
Figure 1: A PPT log from a landfill investigation.

Figure 1, shows a PPT sounding that depicts several parameters found at this landfill. The first column indicates the friction values as the PPT cone is advanced into the waste prism. The second column shows the tip resistance as the cone is advanced through the landfill. The third column shows the pore pressure experienced by the cone. Figure 1 indicates that a dense cover soil (up to 600 tons per square foot) is about three feet thick and then waste is present to a depth of 17 feet below ground surface (bgs). At 17 feet bgs the PPT showed a dense layer that indicates a buried haul road. It was also observed that LFG pressure increased with depth, but then decreased to zero as it reached the haul road, indicating that the gas was most likely transported through the mixed gravel layer to the perimeter of the landfill and to the atmosphere.

subFigure-2
Figure 2: A PPT log from a landfill investigation.

Figure 2, indicates that soil, not refuse, was placed on the haul road at eight feet bgs. This is typical for a sounding performed near a slope where soil is used to cover the side slope of a landfill. Even with three feet of cover soil, the gravel that is in place can allow LFG to migrate out of the landfill. Additionally, Figures 1 and 2 did not show that vacuum was present, indicating that there were no collectors in the area, or at least that they had no effect in these areas. If vacuum was being applied in an area where air intrusion was likely, this could result in a subterranean fire.
If an area is suspected of having a subterranean fire, then a grid pattern PPT investigation can be performed, augmented with temperature probe readings. Once the PPT grid is complete, the data can be used to create a 3D profile of the area of concern as shown in Figure 3. This information can then be used to develop a focused mitigation plan to extinguish the subterranean fire.

subFigure-3
Figure 3: A 3D profile of a landfill identifying dense layers, gas pressure and liquid.

Solutions
Drilled-in Collectors Versus Push-in Collectors
If the landfill is smoking around a particular collector, it is most likely due to overdrawing and the vacuum control valve on the collector should be restricted to reduce or cut off flow. Once this is done, smoke may start emitting from the location where the air was entering the landfill that caused the subterranean fire. It may be necessary to add more soil in this area and make certain it is compacted sufficiently. Oxygen starvation is one way to retard combustion, but it may not correct the underlying cause of the fire. If the operator increased the vacuum influence of a particular collector, it was probably because there was a gas emission problem in the area.

A major cause of air intrusion from overdraw is due to drilled-in collectors, these collectors are prone to leaking when a vacuum is set too high. Push-in steel collectors (typically two inches in diameter) with oilfield mill slot screens have demonstrated their increased performance by not allowing surface air to infiltrate the collection system even at elevated vacuum levels, unless they are installed near a side slope or there is insufficient cover in the area of influence. Push-in collectors are typically only about a third of the cost of traditional drilled-in collectors, installation is simplified, and no cuttings are produced.

In a related manner, a gas collector that is overdrawing is pulling in air and can also dry out the refuse to a point where it is more likely to burn or begin rapid composting. Use of air starvation techniques may stop the fire, but the gas emission problem will also return and if the same collector is opened again to respond to this condition the fire can also return. This scenario is very common and the best way to solve this problem is to install another collector closer to the surface emission area. However, it is not always possible to locate the actual source of the LFG production especially if the surface emission is on a slope. In this scenario, the PPT can be used to locate the gas layer that is responsible for the gas migrations to the slope and a push-in collector can be installed with screen sections installed at the optimal locations to have a direct effect on the elevated gas layer. This approach can solve the overdraw issue causing the fire and still provide gas emission control.

Extinguishing Subterranean Fires
If the PPT profile indicates that a large area has been impacted by a subterranean fire, a more aggressive approach may be required to extinguish the fire. Currently, nitrogen or CO2 injection can be used to cool the refuse and displace the air that may be feeding the fire. Another alternative to this method is to inject steam into the landfill.

Steam Injection
Steam injection has many advantages over nitrogen or CO2 injection, but the most obvious one is that it will not only put the fires out, but it will change the localized conditions that caused the fires. By controlling the moisture in the refuse, there is less chance of the fire returning, nitrogen and CO2 will only dry out the refuse more and dilute the gas stream (see Figure 4).

subFigure-4
Figure 4: A steam injection system in operation.

It has also been demonstrated that steam can increase LFG production while maintaining and even improving the quality of the gas. By using the PPT rig to push-in the steam injectors and collectors, steam injection can save approximately $2,000 to $3,000 per acre when compared to traditional nitrogen or CO2 use. Furthermore, after the subterranean fire is under control, continued use of the steam injection system can increase LFG production by enhancing decomposition, also resulting in increased airspace recovery rates and volumes.3 Another added benefit of the steam injection system is that steam injectors and LFG collectors can be used interchangeably in the future, allowing for easy system modifications, if needed.

Prevention
Subterranean fires are typically a result of the system adjustments by the operator of a landfill in order to stay in compliance. It is generally agreed that LFG collectors are a major cause of fires in landfills. Common gas related issues such as gas emissions or subsurface migration require the landfill operator to install LFG collectors to stay in compliance with regulations. The typical procedure is to drill a large borehole (two feet in diameter or larger) through the full depth of the landfill at a location that is usually more convenient than it is scientific. The borehole is then filled with gravel or sand around a pipe (four to six inches in diameter). This method can cause liquid and moisture within the waste to drain down the large gravel column to the bottom of the landfill, as well as provide a pathway for air to infiltrate into the landfill. It is very difficult to seal the area around the top of these large boreholes and, as a result, the area of vacuum influence is restricted due to short-circuiting to the surface. These large boreholes are extremely oversized and costly considering the amount of LFG they will ever be expected to extract. These large collectors are usually connected to the header by a two-inch diameter lateral pipe. In contrast to this method, a properly installed two-inch diameter push-in collector pipe can extract all the LFG a landfill will produce in a given area at a fraction of the cost. Additionally, push-in collectors do not produce cuttings and they are much less prone to short-circuiting from the surface.

If surface emissions of LFG occur in the general area of a collector, the typical initial response by an operator is to increase the vacuum influence on that collector to see if that will control the gas emissions. Sometimes this will work if the collector is not too close to a slope, but more commonly the surface emissions occur on a slope. If increasing the vacuum influence does control the gas emission, this is indicative of a connection between the surface air, the gas layer and the collector. Now instead of gas emitting from the slope, air is entering the landfill and if this continues over time, the refuse will dry out, a significant amount of oxygen will be introduced into the waste prism, and a subterranean fire can occur.

Frequently, increasing the vacuum influence on the nearby collector does not affect the gas emissions due to the lack of a vacuum connection to the gas layer that is migrating to the slope. When the operator increases the vacuum influence, the vacuum will find its own pathway to the slope, dry out the refuse, pull in air and can start a fire. Quickly drilling another large diameter borehole for a collector, without properly understanding subsurface conditions, has the potential to increase the possibility of a fire by introducing more air into the landfill. To prevent this from occurring a PPT profile should be completed in the area of concern to understand what is causing the surface emissions and then take appropriate action to correct the problem.

Conclusions
Subterranean landfill fires can occur as a result of air intrusion caused by gas collection system adjustments made to address surface gas emissions, as well as poorly functioning landfill cover and road elements. The system vacuum should not be increased too drastically without knowing the underlying cause of the gas emissions. Appropriate cover materials should be installed and maintained at sufficient thickness and compaction. Haul roads should be terminated below the cover layer and bench roads should be properly maintained to avoid settlement and sloughing. If a subterranean fire does occur, it should be investigated by studying system components and subsurface conditions.

Using the PPT to analyze subsurface conditions at the landfill can provide extensive information and allow for a fast and effective diagnosis of the problem. By measuring gas concentrations and temperatures around the area of concern, conclusions can be made about the cause of the fire and the best approach to extinguish it. Push-in steam injectors and gas collectors can be installed at a low cost to quickly control the subterranean fire and surface emissions.

Furthermore, continued use of steam injection technology at the landfill will result in increased gas production and quality while accelerating airspace recovery.

Reg Renaud is President of STI Engineering, Inc. (Silverado, CA) and has been involved in the development and use of in situ testing instruments used in geotechnical and environmental investigations since 1978. He has been an instructor of in situ testing at several universities. He began developing the Landfill Gas 3-D Profiling Method in 1984. Based on in situ data from these profiles and with the understanding of the dynamic conditions inside landfills he developed the patented Steam Injection Bioreactor Landfill method. From this development he patented the Steam Injected Biomass Reactor (SIBR). Reg can be reached at (714) 649-4422 or visit www.airspacerecovery.com.

Jonathan Chihoski, P.E. is an environmental engineer for the Massachusetts Department of Environmental Protection, as well as an independent consultant. He has more than 10 years of experience in the environmental industry, working in both private and public sectors, on projects including R & D, site assessments, remediation/redevelopment and construction oversight. Jonathan can be reached via email at ads.jtc@gmail.com.

References
1. Municipal Solid Waste Landfills: New Source Performance Standards (NSPS), Emission Guidelines (EG) and Compliance Times. (2018, November 09). Retrieved April 10, 2019, from www.epa.gov/stationary-sources-air-pollution/municipal-solid-waste-landfills-new-source-performance-standards
2. California Air Resources Board. (2018, July 26). Landfill Methane Control Measure Applicability and Guidance. Retrieved April 10, 2019, from www.arb.ca.gov/cc/landfills/decisionguide/decisionguide.htm
3. Steam Injection Landfill Bioreactors, by Reg Renaud 7th World Congress, San Diego, CA. May 2002


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