Landfills, especially large regional landfills, are huge enterprises with many different operations ongoing daily. A landfill’s tangible assets are equipment, buildings, machinery, construction materials in the ground, or stockpiled to support various operations. Of all these, the most significant asset is the permitted airspace. It’s undoubtedly a non-tangible asset when permitted, but gradually this asset gets consumed as it turns into revenue.
Creating landfill airspace during a design/permitting process involves the operator hiring a landfill engineer to develop the concept of the airspace, prepare an appropriate design with engineering methods, and obtain a permit for it through regulatory agencies. In a sense, a portion of your future revenue is in the hands of your landfill engineer. You depend on this engineer to create the maximum amount of airspace, generating the maximum amount of revenue for your operation over time. Your engineer is supposed to be your trusted partner, and you are investing an enormous amount of capital for the design, permit, and construction based on the work performed by the engineer.
In some instances, the operator leaves most of the technical decision making to the engineer. On other occasions, the operator is in the loop during the engineer’s design, but the operator is not heavily involved in the nuances of the disposal cell’s layout in consideration of the existing terrain. In either case, the engineer is significantly responsible for achieving the maximum amount of airspace. The multi-million dollar question is whether you could have had another 3 million or 5 million cubic yards of additional airspace in your permit. How do you check if your landfill engineer maximized airspace in the design?
Assuming proper training, most landfill engineers can design adequate landfills. Still, very few landfill engineers have the unique talent and experience that can maximize airspace within specific design parameters. You, as the operator want engineers with a proven track record of maximizing airspace in their landfill designs, and do not let relationships or political nuances affect your judgment during selection because tens of millions of dollars of additional revenue are at stake.
A trained landfill engineer may miss details that a highly qualified engineer would not. Incidentals here and there, if recognized and accounted for, can add significant airspace to the design. These details vary from site to site, and it’s up to the engineer to recognize the benefits of geometric and regulatory opportunities to add to the covered airspace. These details could be in the form of:
Special geometries for the landfill slopes,
The lateral extent of waste limits,
The landfill footprint placement within the terrain,
The extent of excavation for establishing bottom grades for disposal cells,
The relative position of base grades with respect to the groundwater elevations,
Combining leachate collection sumps among two or more disposal cells,
Steeper slopes to increase airspace while staying within the bounds of regulatory requirements,
Positioning peripheral systems in a different way to benefit from additional land to add to the landfill footprint,
Considering future expansion down the road and planning appropriately, and
Other nuances that an expert considers.
The operator chooses the project manager or the primary engineer for the design of a greenfield landfill or an expansion to an existing landfill, knowing that the work performed by the selected engineer could potentially add to or take away hundreds of millions of dollars from the bottom line of your enterprise. So, pick your engineer based on the engineer’s prior design track record and make sure the engineer is an expert in maximizing landfill airspace.
SCS is an expert, highly experienced landfill designer – relied on by many landfill operators as a trusted partner. Our culture is to serve our clients as if their project is our own, and we do not consider ourselves successful unless our clients are satisfied. These close relationships help us serve the majority of our clients on a long-term basis, with decades of continuous service and value.
SCS will gladly evaluate scenarios for your landfill expansions that you are planning to design and permit, and provide you with a preliminary estimate of airspace gain and revenue that an SCS design could bring, potentially increasing your primary asset by another tens of millions of dollars. Now that’s a value statement!
About the Author: Ali Khatami, Ph.D., PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.
The industry is designing and building more substantive drainage features and larger collection systems from the bottom up, that maintain their integrity and increase performance over time, thus avoiding more costly problems in the future.
Waste360 spoke with three environmental engineers about what landfill operators should know about liquids’ behavior and what emerging design concepts help facilitate flow and circumvent problems such as elevated temperature landfills, seeps, and keep gas flowing.
The engineers cover adopting best practices and emerging design concepts to facilitate flow. They cover topics such as directing flow vertically to facilitate movement to the bottom of the landfill, drainage material, slope to the sump percentages, vertical stone columns, installing these systems at the bottom before cells are constructed, and increasing cell height to prevent the formation of perched zones.
Ali Khatami, one of the engineers interviewed, has developed standards for building tiered vertical gas wells that extend from the bottom all the way up. He frequently blogs about landfill design strategies that his clients are using with success. His blog is called SCS Advice from the Field. Dr. Khatami developed the concept of leachate toe drain systems to address problems tied to seeps below the final cover geomembrane. These seeps ultimately occur in one of two scenarios, each depending on how the cover is secured.
Landfill Gas Header: Location and BenefitsBy continuing to design gas header construction on landfill slopes, all of the components end up on the landfill slope as well. You can imagine what type of complications the landfill operator will face since all of these components are in areas vulnerable to erosion, settlement, future filling, or future construction. Additionally, any maintenance requiring digging and re-piping necessitates placing equipment on the landfill slope and disturbing the landfill slope surface for an extended period.
AIRSPACE, the Landfill Operators’ Golden EggAirspace is a golden egg, the equivalent to cash that a waste operating company will have overtime in its account. With each ton or cubic yard of waste received at the landfill, the non-monetary asset of airspace converts positively to the bottom line of the …
Gas Removal from Leachate Collection Pipe and Leachate SumpKeeping gas pressure low in and around the leachate collection pipe promotes the free flow of leachate through the geocomposite or granular medium drainage layer to the leachate collection pipe and improves leachate removal from the disposal cell. Using gas removal piping at leachate sumps is highly recommended for warm or elevated temperature landfills where efficient leachate removal from the leachate collection system is another means for controlling landfill temperatures.
Leachate Force Main Casing Pipe and Monitoring for LeaksLandfill operators may add a casing pipe to their leachate force main for additional environmental protection. Consequently, the leachate force main is entirely located inside a casing pipe where the leachate force main is below ground. In the event of a leak from the leachate force main, liquids stay inside the casing pipe preventing leakage …
Pressure Release System Near Bottom of LandfillsPressure Release System Near Bottom of Landfills – Essential Component for Proper Functioning of the Landfill Drainage Layer. Landfill designers are generally diligent in performing extensive leachate head analysis for the design of the geocomposite drainage layer above the bottom geomembrane barrier layer. They perform HELP model analyses considering numerous scenarios to satisfy all requirements …
Landfill Leachate Removal Pumps – Submersible vs. Self-Priming PumpsSelf-priming pumps can provide excellent performance in the design of a landfill leachate removal system. Landfill owners and operators prefer them to help control construction and maintenance costs too. A typical system for removing leachate from landfill disposal cells is to have a collection point (sump) inside …
High-density polyethylene pipes have been used for landfill leachate collection and conveyance lines for several decades because of the chemical compatibility of HDPE material with many different types of liquids and chemicals. Designing a leachate collection system for a landfill disposal cell involves numerous engineering analyses of different components involved in collecting and conveying leachate. One of the important engineering evaluations is a determination of structural stability of HDPE leachate collection pipes at the bottom of the landfill.
Structural Stability of HDPE Pipe
Modern landfills are gradually becoming larger and deeper; deeper landfills will naturally impose a higher surcharge loading on the HDPE leachate collection pipes below the waste column. Engineering methodologies for the structural stability evaluation of HDPE pipes with significant surcharge loading have been around as long as HDPE pipes have been in production.
There are three criteria used when evaluating the structural stability of HDPE pipes; wall crushing, wall buckling, and ring deflection. Wall crushing can occur when the stress in the pipe wall, due to external vertical pressure, exceeds the compressive strength of the pipe material. Wall buckling, a longitudinal wrinkling in the pipe wall, can occur when the external vertical pressure exceeds the critical buckling pressure of the pipe. Ring deflection is the change in vertical diameter of the pipe as the pipe deforms under the external pressure. Empirical formulas by HDPE pipe manufacturers or researchers are available to check each criterion.
SDR 11 vs. SDR 17 HDPE Pipe
When a structural stability evaluation involves high surcharge loading on the pipe, an engineer may automatically select SDR 11 HDPE pipe without going through an evaluation process. The engineer’s reasoning is that the higher wall thickness of SDR 11 pipe, as compared to SDR 17 pipe, is the logical choice because it provides a higher level of structural stability to the pipe. In the case of wall bucking and wall crushing, where the pipe strength in these two criteria is inversely proportional to the SDR value, the engineer is making the right choice. The strength is greater for the lower SDR value that represents thicker pipe wall thickness; making SDR 11 stronger than SDR 17.
However, in the case of ring deflection, the pipe strength is not a function of SDR, but a function of another parameter called allowable ring deflection. The allowable ring deflection value varies from one SDR to another and is generally reported by pipe manufacturers. The allowable ring deflection for SDR 17 pipe is greater than all other SDR pipes, which makes SDR 17 pipe stronger when considering ring deflection. SDR 17 pipe is also the most commonly used HDPE pipe in the landfill industry, being lighter in weight per unit length of the pipe than SDR 11, thus making it less expensive than SDR 11 pipe.
Which Is Best For My Landfill?
SCS Engineers recommends that landfill engineers consider SDR 17 pipe as the first choice for use as a leachate collection pipe below the waste column, and then other SDRs if SDR 17 does not pass the three structural stability criteria mentioned above.
Read more blogs by Ali Khatami, click here and type “Advice from the Field” in the search box.
As a designer, I’ve been hired to correct inconsistencies between the gas system and the landfill too many times. Today’s blog is about the most important factors that all landfill gas designers should consider for a gas system to coordinate efficiently with the landfill design as permitted. This is a partial list of best practices developed at SCS Engineers.
Considerations for Design of Gas Collection Systems for Landfills:
1. Include the final cover layers in the gas design details where gas wells are installed near the landfill final surface. This inclusion will help the designer to specify proper heights for gas wells, proper depths for gas headers and lateral pipes, and proper heights for condensate sumps within the lined area of the landfill. Otherwise, locations of these elements may end up being in conflict with the location of various layers of the final cover system to be constructed later.
2. Always leave pipes exiting the liner boundary at the perimeter of the landfill at least 1 ft above the anchor trench shoulder. When the final cover is installed, it would be impossible to install a geomembrane boot over at the cover geomembrane penetration point of a pipe that is in contact with the bottom lining system geosynthetics over the anchor trench shoulder.
3. If flow control valves are located below the final cover near the perimeter of the landfill, design a vertical casing around the valve tall enough that future final cover can be booted to the vertical casing and access to the valve would be possible. Do not use corrugated material as casing because it would be difficult to place a geomembrane boot over corrugated casings. The designer should require sealing the void inside the casing pipe to prevent landfill gas release or oxygen intake through the void. If the control valve is located above the final cover, the designer should specify a proper height for the casing pipe that access to the valve stay above the final cover surface.
4. Locating flow control valves near the landfill perimeter and within the lined area should be in consideration with the future location of a rainwater toe drain system at the toe of the slope that will be constructed when the final cover is constructed.
5. Condensate sumps installed before construction of the final cover should be tall enough to accommodate construction of the final cover system around the condensate sump with sufficient space to boot the final cover geomembrane to the exterior walls of the condensate sump. Miscellaneous stub outs on the condensate sump should be designed in consideration of having enough space for the geomembrane boot in the future.
6. Pipes connected to a condensate sump (such as compressed air line, discharge force main, power conduits, etc.) should be positioned such that boots can be placed on each line at the penetration point of the pipe through the final cover geomembrane. Boots may not be placed on pipes clustered together. If boots are placed on a pipe cluster, the designer should require sealing the voids between the pipes within the boot to prevent landfill gas release or oxygen intake through the voids.
7. Gas pipes located above the final cover geomembrane and crossing terraces on landfill side slopes may create conflict with rain water toe drain at the terrace. The designer should design terrace crossings such that future conflicts can be avoided.
8. Gas pipes crossing an access road on the landfill slope may cause conflict with a ditch adjacent to the access road at the final cover surface. Location of the gas pipes, either below or above the final cover geomembrane, should be designed in consideration of the final cover features that will exist in the crossing area in the future.
9. Gas pipes located above the final cover geomembrane and crossing an access road on the landfill slope may cause a conflict with a rainwater toe drain system above the final cover geomembrane running parallel to the access road at the toe of the slope next to the access road.
10. Gas pipes located above the final cover geomembrane and crossing the access road on the landfill slope may cause road grade problems at the final surface. Specific depressions across the access road width may have to be designed for larger pipes to prevent grades problem at the finish surface.
11. Gas pipes located above the final cover geomembrane may cause conflict with storm water downchutes that will be installed above the final cover geomembrane. Special depressions may have to be designed to place downchutes below gas pipes on the slope. Placing gas pipes above downchute may cause a problem with the flow of condensate in the line.
12. Sometimes horizontal gas collection pipes come out of the landfill side slopes and extend down the slope to a gas header or some other component of the gas system. If the pipe segment on the slope is going to be below the final cover geomembrane, then it must be placed deeply enough in the waste that it would not have any conflict with the final cover system components, such as leachate toe drain systems, terraces, access road ditches, etc. If the pipe segment is going to be above the final cover geomembrane, extension of the horizontal pipe connecting to the pipe segment on the slope will be designed such that the horizontal pipe can penetrate the final cover geomembrane and extend down the slope while located above the final cover geomembrane. Extension of the pipe on the slope above the final cover geomembrane should not cause any conflict with the final cover components, such as rainwater toe drains at terraces or at the toe of the slope next to the perimeter berm, downchute pipes, terrace or access road grades, etc. The elbow at the connection of the horizontal pipe to the pipe segment on the slope and above the cover geomembrane is critical because a geomembrane boot must be installed at the penetration point.
13. If tack-on swales are used on the landfill slopes, gas pipes on top of the final cover geomembrane may cause conflict with the flow line inside the tack-on swales. Large headers should cross tack-on swales at the high-end point of adjacent swales to prevent flow problems in the swale.
14. If tack-on swales are used, the location of wells for drilling purposes should be chosen to be outside the tack-on swale structure.
15. If a gas header located above the final cover geomembrane and crossing a terrace or access road where the terrace or access road is sloping toward the landfill, condensate flow through the gas header may become an issue. Special depressions across the terrace or access road may need to be designed such that condensate can flow in the proper direction.
SCS Engineers is a leader in the design of landfill gas and landfill lining and final cover systems. We evaluate these issues and many others during our landfill gas design work; our clients pay only once for construction of the system and do not have to spend additional money in the future to fix a system that could have been constructed correctly in the first place. Learn more here.
Dr. Khatami has acquired extensive experience and knowledge in the areas of geology, hydrogeology, hydrology, hydraulics, construction methods, material science, construction quality assurance (CQA), and stability of earth systems. Dr. Khatami has applied this experience in the siting of numerous landfills and the remediation of hazardous waste contaminated sites.
Dr. Khatami has been involved with the design of gas management systems, hazardous waste impoundments, storage tank systems, waste tire processing facilities, composting facilities, material recovery facilities, landfill gas collection and disposal systems, leachate evaporator systems, and liquid impoundment floating covers. He has also been involved in the design and permitting of civil/environmental projects such as surface water management systems, drainage structures, municipal solid waste landfills, hazardous solid waste landfills, low-level radioactive waste landfills, leachate and wastewater conveyance and treatment systems.
Contact Dr. Khatami directly to answer questions and comments.
Posted by Diane Samuels at 6:00 am
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