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State regulatory agencies normally require landfill slopes reaching final grades to close within a certain period. This requirement leads to closing landfill slopes in phases, normally referred to as partial closure. Generally, partial closures start from the bottom of the landfill slope up to a certain elevation, with geosynthetics in the final cover temporarily anchored along the partial closure’s sides and upper boundary. Engineers propose different designs for securing the lower boundary of partial closures at the bottom of the landfill slope. Some engineers propose an anchor trench outside the bottom lining system anchor trench to secure the final cover geosynthetics. Others specify welding the cover geomembrane to the bottom lining system geomembrane.
Experience with anchor trenches at the bottom of the landfill slope for the final cover geosynthetics has not been positive because of these issues:
To eliminate the issues above, engineers weld the final cover geomembrane to the bottom lining system geomembrane for cases when there is a bottom lining system below the waste. The welding completely seals the landfill interior space from the outside environment and keeps regulated materials, such as waste, leachate, and gas, within the sealed system. Of course, the engineer should design proper means to address these behind the sealed system; designs may include:
Leachate toe drain system is a concept originally developed by SCS and incorporated into landfill final cover designs over the past 20 years. Unfortunately, many solid waste engineers are unaware of the need for LTDS, so their designs lack this important feature. LTDS saves a tremendous amount of repair money in the long run by avoiding complications for landfill operators.
A rainwater toe drain system removes water that moves laterally within the final cover geocomposite toward the slope’s bottom. The RTDS includes a perforated HDPE pipe encased in gravel and wrapped in geotextile. Also, install the RTDS on terraces along the depression on the interior side of the terrace. Along the landfill slope’s bottom, position the RTDS behind a HDPE flap welded to the final cover geomembrane. The RTDS is sloping with high and low points along the RTDS alignment. Lateral drain pipes located at low points remove water from the RTDS to the perimeter ditches.
Other designs involving extending the geocomposite to daylight at the slope surface cause problem such as those listed below:
Similar issues can also occur at the outlet of such systems on landfill terraces, making the RTDS a superior design.
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, Construction Quality Assurance, and Elevated Temperature Landfills. He has over 40 years of research and professional experience in mechanical, structural, and civil engineering.
About the Author: Ali Khatami, Ph.D., P.E.
There are several hundreds of Municipal Solid Waste (MSW) landfills in the United States. Many of these landfills are anticipated to remain active for decades to come, and Federal and state rules require slopes reaching permitted final elevations to be closed within 180 days. This means partial closure of slopes is part of the operational requirements of MSW landfills.
Federal and State Rules
Subtitle D of the Resource Conservation and Recovery Act (RCRA), enacted on October 21, 1976, requires the final cover of MSW landfills to include a barrier layer with hydraulic conductivity that is substantially equivalent to or less than the hydraulic conductivity of the bottom liner. State-level regulations developed following the enactment of the federal law also required similar standards for MSW landfills. Many states, pursuing the federal guidelines, require at a minimum, the bottom lining system of MSW landfills include at least one primary barrier layer consisting of Polyvinyl chloride (PVC), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE). Naturally, the final cover barrier layer should also be PVC, HDPE, LLDPE as well.
According to the Federal and state regulations, following the completion and closure of a MSW landfill, the facility owner maintains the landfill for a minimum of 30 years beyond the final closing date. Extension of the long-term care period beyond the 30-year post-closure period is a hot subject among solid waste professionals. Some states have already implemented matrices for such time extensions; it is anticipated that the remaining states will require similar extensions for MSW landfills over the next several years. Even if regulatory agencies approve completion of the post-closure period for a specific landfill, the landfill’s final cover system is expected to perform for many more years to come. Otherwise, environmental issues associated with a lack of performance may force the regulatory agency to spend money for repairs no longer available through a financial instrument.
Long-Term Performance Designs
For the past few decades, SCS has specifically designed and permitted final cover systems with special features to prolong the final cover system’s performance beyond the post-closure period of the landfill. The final cover system designs:
Less Maintenance
The first partial final cover with these features was constructed in 1998, and since then, many more partial closures with these types of features have been constructed. All partial closures are performing satisfactorily without failure. Regular maintenance of the final cover vegetation and occasional cleaning of drainage swales, which are common maintenance activities, have been the only measures taken by the operators of the facilities with these final cover systems.
The features incorporated into the final cover systems were:
The features above have financial, performance, and stability benefits for the facility for many years to come. So far, such final covers have been constructed on 3H:1V slopes as long as 550 ft. in length with no terraces. Several of the completed final covers were partial closures on a 3H:1V slope, where the next phase was constructed directly above a previous phase with the two phases tied together at the phase boundary.
Want more advice from our designers? Select articles and blogs for further reading:
Landfill slopes that have reached final grades, or will receive waste in the distant future have maintenance challenges. Environmental elements continually affect surface conditions, and remedial work is required routinely to prevent negative outcomes of exposed slopes. Consider using a geomembrane temporary cap to address much of the maintenance. Here’s a list showing how the cap can help:
Landfill Maintenance Challenge
With Geomembrane Temporary Cap
The significant maintenance savings by using a temporary cap make the payoff period for the investment attractive. Based on my experience and site variations, the return on investment is usually three to six years. The period is considerably shorter if your landfill does not have a leachate disposal or treatment system, or deep injection well. The difference is the high cost to have the leachate hauled away.
Temporary caps potentially reduce routine maintenance work, leaving operation staff available for other tasks. The cap provides peace of mind that slopes remain in compliance; regulators don’t need to report non-compliance conditions of exposed slopes during inspection events.
After completing 25 temporary cap projects in the U.S. Southeast alone, we highly recommend using a thick geomembrane. It’s tempting to try to save money using a thinner geomembrane, such as 12 mils or 20 mils, but these can damage more easily and will negatively affect your return. The majority of SCS clients chose to use the recommended 40 mils thick geomembrane, which will survive severe weather conditions.
Ballasting the geomembrane and using the right materials for ballasting is significantly important. We recommend using ultraviolet (UV) resistant rope and sandbags, a tried and true system. UV resistant straps are a decent replacement for ropes. Anchoring mechanisms are also important. We typically recommend using 4×4 treated wood posts at 10-ft spacing, installed in anchor trenches, and tied to ballasting ropes. Depending on the site and operator’s preference, the supporting architecture may be to lay the post horizontally, while tied to the ballasting ropes, at the bottom of the anchor trench buried in the anchor trench’s backfill material.
Over the years, landfill operators have experienced the savings and value that temporary caps bring to landfill operating budgets, and we’re placing more temporary caps every year. If considering this option, SCS can assist you by evaluating the slopes at your site for the caps. We’ll also prepare estimates for the purchase of material and installation costs and estimated time of recovery for your project.
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 Elevated Temperature Landfills, plus Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.
Learn more at Landfill Engineering
Matt Brokaw, P.E. joins the SCS Engineers new office at 3801 Lake Boone Trail, Suite 430, Raleigh, NC 27607, Tel: +1-919-662-3015
SCS Engineers, a top-tier ENR environmental consulting and construction firm, opened a larger office in Raleigh, North Carolina, in late May. The move centralizes the team closer to their clients’ sites to provide full-services. The new office accommodates new team members, including Matt Brokaw. Matt joins the SCS professionals who provide environmental services for solid waste management for the benefit of municipal and private landfills, public works, and recycling.
As a Senior Project Professional, Matt is responsible for the engineering and design of environmental solutions, with a primary focus in solid waste, stormwater management and planning, and erosion and sediment control critical to permitting compliant facilities and ultimately protecting natural resources. Extending the life of a landfill and adding airspace is often critical for the communities SCS clients serve.
The new SCS Raleigh location supports the growing demand for full-service environmental solutions supported by a mix of professionals. As specialized teams, they can help reduce greenhouse gas emissions, capture landfill gases, create renewable energy from by-products, and optimize utilities and businesses using environmental practices that are economically feasible. The firm specializes in permitting for and meeting comprehensive clean air, water, and soil goals. It provides a range of services such as PFAS treatment, solid waste master planning, landfill technology, risk management, groundwater monitoring, pre-closure and landfill closures, and Brownfields remediation.
About SCS Engineers
SCS Engineers’ environmental solutions and technology are a direct result of our experience and dedication to solid waste management and other industries responsible for safeguarding the environment. For more information about SCS, please visit our website at www.scsengineers.com/, contact , follow us on your preferred social media, or watch our 50th Anniversary video.
Lessons learned from previously constructed gas collection and control systems teach solid waste professionals valuable lessons about designing for long-term survivability and reducing the maintenance cost of gas system components. The location impacts operating and maintenance costs for various components of gas collection and control systems such as condensate force main, condensate sumps, force main for well liquids, air lines to pumps in gas wells, and gas headers long into the future. As often as possible, design the gas header in the landfill perimeter berm along with the condensate sumps. Landfill perimeter berms constructed in an engineered manner with well- compacted soils and a well-defined geometry provide a long-term cost-effective alternative to earlier designs outside the berm.
For many years, gas headers were designed and constructed outside of the landfill perimeter berm, on the landfill surface. Of course, landfill surface changes as waste elevation increases over time, resulting in many gas headers that now may be 30 feet or more below the current waste surface. Deeply buried gas headers are unreliable at best, and the operator loses access to them as soon as 20 feet of waste covers the header.
Collapsed gas headers buried deep in waste are an expensive challenge when operating a large number of gas wells connected to the gas header, and could cause serious compliance issues. Upon discovery of a collapsed buried gas header, installing a new header is a lengthy process with significant costs, not to mention the hurdles the operator will have to jump addressing noncompliance with their state agency.
The benefits of placing gas headers in the landfill perimeter are:
Since the condensate force main follows the gas header in the perimeter berm to flow to a tank or discharge point, there are additional maintenance benefits.
By 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.
For more information about these benefits and more, please refer to the MSW Magazine article series Considerations for the Piping Network, the author, or contact SCS Engineers at .
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.
Learn more at Landfill Engineering
One general problem that is encountered in traditional designs is the potential for clogging of geotextiles in the vicinity of the leachate collection pipes.
Traditionally, leachate collection pipes are encased in gravel, wrapped in geotextile, and positioned above the leachate collection system geocomposite drainage layer inside a trench or at the trough of the bottom of a cell. In a traditional design, leachate travels through the geonet component of the geocomposite and reaches the leachate trench where the leachate collection pipe is located. Here, leachate must flow out of the geocomposite, through the upper geotextile component, and then through the geotextile wrapped around the gravel, before entering the gravel and eventually flowing through the pipe. The flow through the geotextiles is concentrated in small areas on the two sides of the leachate collection pipe-gravel-geotextile wrap. Considering the large volume of leachate that follows this path over the life of the cell, it is evident why traditional designs are doomed to clog.
The clogging impedes the free flow of leachate from the geocomposite drainage layer to the leachate collection pipe. As the clogging occurs, the leachate must find a new flow path (most likely further back from the collection pipe), and flow out of the geocomposite, through the geotextile wrap at a different location, and eventually enter the gravel and pipe. This new location will eventually clog as well for the same reasons that the initial location was clogged. This process continues until the geotextile within the leachate trench becomes completely clogged and the system loses functionality. Unfortunately, the periodic cleaning of leachate collection pipes (usually every few years) cannot address this issue because the problem is outside the pipe and the high-pressure jets inside the pipes do not reach the clogged locations.
The solution is to eliminate geotextiles from the flow path of the leachate, extending from the geocomposite drainage layer to the leachate collection pipe. Over the past several years, SCS has successfully designed and constructed numerous landfill cells with no geotextile in the flow path of leachate from the geocomposite drainage layer to the leachate collection pipe. The design follows the “Rule of Transmissivities” which dictates that a proper design should provide the free flow of leachate from one medium to another and that only occurs when the transmissivity of the latter medium is equal to or greater than the transmissivity of the former medium. If a design does not satisfy the Rule of Transmissivities, there may be potential for clogging, bottlenecking of flow, and other consequences resulting from impeded flow in the system.
SCS Engineers is a leader in the design of landfill lining systems, and we have experience with issues that may not be familiar to other firms. If you are interested in the design of a leachate collection system at your facility, please contact SCS. Our professional engineers will gladly review your design and make recommendations if needed. We can identify potential issues and improve designs to prevent future problems and maintenance during the life of your facility.
Questions? Contact Ali Khatami, PhD, 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. 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.
Read more here. Rule of Transmissivities at Material Interfaces in Landfill Leachate Collection Systems, in Talking Trash
