Welcome to the SCS Advice from the Field blog series.
Airspace 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 waste operating company’s books.
The larger the airspace, the larger the non-monetary asset, and the larger future cash potential in the account.
Therefore, it is extremely important to design landfill footprints optimally in consideration of planned operations at the site, and design landfill features maximizing airspace within the selected landfill footprint.
Optimization takes into consideration the land available for development, including the various facilities and systems necessary for operations. The type of design, depth of landfill, base slopes, leachate collection pipe slope, perimeter berm geometry and size, slopes of landfill side slopes, terraces on slopes, and many other parameters determine the airspace volume available to the landfill operator. The designer’s goal is to provide the most volume to the landfill operator.
How does the operator know that a proposed design is maximizing airspace?
If SCS is the site designer, the maximization of airspace is inherent in proposed designs for permitting. On numerous occasions, when SCS is not the site engineer, our designers have proposed a re-design of landfill features to maximize the airspace within its permitted footprint. Under these circumstances, it is not easy to convince a landfill operator of the benefits of SCS’s proposal. Naturally, one assumes a designer would not propose a lesser design on paper and carry it through the high cost of permitting, so it is common for the landfill operator to express doubts about our proposed changes. Once the operator and SCS review the technical design changes in detail, the demonstrated value becomes apparent. It is not a simple process, but on every occasion, we have successfully increased the airspace for the client, increasing potential revenue for millions of dollars beyond the originally permitted amounts.
Driven by the success of our clients, it is our culture to serve our clients completely as trusted professionals making your challenges our own. SCS is proud to say that at the date of this publication, our designers have created over $400,000,000 of additional financial benefit out of thin air for clients at a dozen landfills with more efficient landfill base grades that maximize airspace and cost less to construct.
As we move toward our 50th year, we hope to continually improve, evolve, and strive to maximize airspace at more landfills, adding value to our clients’ bottom line. Contact our nearest office if you are interested in a landfill evaluation for maximizing airspace and reducing construction costs. As always, our SCS authors are available to answer your questions or comments.
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.
When the Federal Coal Combustion Residual (CCR) rule went into effect in 2015, it was a new regulatory layer on top of a widely varying landscape of state regulations affecting CCR management in impoundments and landfills. Some states already had significant regulations on the books for CCR impoundments and/or landfills, while others did not.
Where state regulations existed, they varied widely from state to state. While a few states have moved toward closing the gap between state and Federal CCR requirements, many utilities continue to face confusing and conflicting requirements coming from different regulatory programs as they move ahead with managing their CCR facilities.
In her paper entitled State vs Federal CCR Rule Regulations: Comparisons and Impacts, Nicole Kron shares state-versus-federal regulatory challenges utilities have encountered during landfill design and management, impoundment closure, and groundwater monitoring and reporting since the implementation of the Federal CCR rule. For example, some sites have completely distinct groundwater monitoring programs under state-versus-federal rules, with different well locations, well depths, and monitoring parameters for the same facility. She highlights unique approaches to bridging regulatory gaps and resolving regulatory conflicts between state and Federal CCR requirements. Ms. Kron also provides insights gained on the long-term potential for regulatory resolution of these issues based on discussions with state regulators in multiple states.
About the Author: Nicole Kron has nearly a decade of experience in the environmental consulting field. Her experiences focus on groundwater quality analysis of sites contaminated with coal gasification byproducts, coal combustion byproducts, chlorinated solvents, petroleum products, metals, and PCBs. Her experience includes managing team task coordination, groundwater modeling, and statistical analysis of CCP/CCR sites. She is experienced in planning and performing soil and groundwater contamination investigations, air monitoring, well design and installation, and soil and groundwater sampling.
Today’s landfill design professionals can help eliminate unsafe configurations and institute features that can proactively warn of and minimize hazards for operator and customer safety. Designers consider subgrade conditions, geotechnical factors and regulatory requirements when specifying how steep a landfill may be constructed.
The practical aspects of landfill operations and maintenance play a significant role in slope configurations since the landfill must provide safe access to monitoring points, environmental control features, and mowing.
Bob Gardner highlights the most important features to consider for landfill cells, including the design and construction phases of the entire landfill’s infrastructure. Bob covers a broad range of topics including:
Many states regulate the maximum design slope, and although these vary, it is up to the landfill designer to take practical, safety and regulatory considerations into account when establishing the slope configuration. Bob recommends working closely with the field staff to incorporate a design that is user-friendly, effective and safe.
About the Author: Bob Gardner, PE, BCEE
It is a general misconception that leachate seeps stop or disappear when slopes receive the final cover. In fact, it is only true if the source of leachate is located directly below the cover, but in most cases, the leachate originates from another location. Continuing seeps eventually reach the bottom of the slope, where two scenarios can happen depending on how the final cover geomembrane is secured at the landfill’s perimeter.
In the first scenario, where the geomembrane is anchored in an anchor trench, liquids will gradually flow underneath the cover geomembrane in the anchor trench and enter the perimeter berm structure. Leachate entering the berm structure softens the berm’s structural fill adversely impacting its shear strength. Additionally, leachate gradually seeps through the berm structure and enters natural formations below the berm and possibly into the groundwater. The operator is alerted when monitoring shows a localized structural failure or a groundwater impact in a nearby groundwater monitoring well.
In the second scenario, where the final cover geomembrane is welded to the bottom lining system geomembrane, leachate seeping out of the slope reaching the toe of the slope accumulates at the toe because it has nowhere to go. Accumulation of leachate behind the final cover geomembrane forces water to gradually move laterally along the landfill perimeter berm behind the final cover geomembrane damaging a larger area behind the final cover. Vertically, more of the area above the toe of the slope becomes engaged by the accumulating leachate. The two obvious consequences are the softening of the soil layer below the final cover geomembrane at the toe of the slope and the water-bedding effect of the area near the toe of the slope.
In the first scenario, the operator has to handle a non-compliance issue, either a failure in the slope or impacts to groundwater. In the second case, the leachate remains contained, but the operator has to address the issue by opening the final cover and removing leachate accumulated behind the final cover geomembrane. The geomembrane opening is closed, and final cover soils are restored after liquids are removed. Both are costly and complicated solutions. Moreover, the problem does not end after completion of the repair because the source of leachate seep is not eliminated.
Landfill operators can require their engineers to design a leachate toe drain system located at the toe of the slope and connected to the leachate collection system at the bottom of the landfill before the final cover geomembrane is installed. The leachate toe drain system is the only way to collect and route leachate to a location at the bottom of the landfill constructed for removal of leachate.
If you are closing a portion of your landfill slope and you find no leachate toe drain system in the construction plans, you can ask for a system to be added to the design plans before the commencement of the construction project.
SCS has significant experience with various types of leachate toe drain system constructed at different locations under various conditions. If you like to know more about the design of leachate toe drain systems or if you are looking for an experienced engineer for the design of your next final cover contact SCS.
Author: Dr. Ali Khatami
Landfill base grades not only make leachate collection and removal possible but also have a significant impact on the amount of landfill airspace. For landfill operators, airspace is the primary asset, because it represents the level of revenue the operator can expect. Airspace is a commodity to be maximized.
Operators expect to get the most airspace from their landfill designer and depend on the engineer to design the grades to maximize it. Placing your trust in an engineer is a noble matter, but as the operator, you check, verify, and confirm that what the designer has engineered is what is needed to provide you with the expected value. An experienced landfill designer looks for ways to provide airspace above and beyond the operator’s expectations.
SCS has been in the business of designing landfills for nearly half a century. We have significant experience in optimizing landfill designs and maximizing airspace. SCS is often retained to design a new expansion to an existing landfill. Upon starting work we analyze the entire facility holistically to see all of the potential ways to maximize airspace around and above the existing landfill. Every cubic yard of additional airspace is a big achievement for our clients and in turn for us.
SCS often evaluates permitted, yet to be developed, base grades for operators. The intent is to determine whether additional airspace can be achieved by applying a different design to the base of the landfill. SCS has turned the science of geometry into mathematical models utilized to quickly evaluate base grades. Specific parameters of the currently permitted base grades are plugged in the mathematical model along with those of the alternative and the model provides quantitative values (cubic yards) of the difference between the permitted grades and the alternative. The values are quickly returned. After modeling, the operator may decide to modify the design to gain the additional airspace based on the alternative design. Contact us to work with our landfill design experts to assist you with an evaluation.
Contact Dr. Ali Khatami with questions about the model.
Sometimes geosynthetic material specifications for a specific project, i.e., lining system or final cover system, is a performance-based specification which does not specify the type of product for use in construction. What does the engineer need to do when the selected contractor submits a product for approval in accordance with a performance-based specification? What should the engineer do when the owner purchases the material and identifies a product for use based on the performance-based specification?
Specifications that SCS has prepared are performance-based and include a qualifying procedure whether the product is introduced by a contractor or owner. This qualifying procedure is specifically left to the engineer to carry out by laboratory testing of typical samples of the specific product for use in construction. Typical reported values by the manufacturer or test results submitted by the contractor or owner are not acceptable under these procedures. Since the engineer is taking the liability of accepting a specific type of product for his or her project, the engineer should have the right to perform laboratory testing before the product is approved for use in the project, that only makes sense in the world of taking liabilities!
The testing performed by the engineer for qualifying a product do not count toward conformance testing of materials delivered to the site. The qualifying procedures are solely for accepting a certain type of product to be used in the project, but the specific rolls of pre-qualified product manufactured for use in engineer’s project must go through the required conformance testing specified in the specifications before use in the project.
The process of qualifying a product, ordering the qualified product, and performing conformance testing on the pre-qualified materials takes time. Engineers need to consider the amount of time necessary for the involved stages of approval into the construction schedule. If using material purchased by the owner, the owner needs to keep the timeline in mind to allow the engineer to carry out all necessary testing for the approvals to be in place before construction begins.
Repeating the qualifying procedure for a product from one project to the next depends on how the performance-based specification is written. Sometimes, the engineer accepts a product that was qualified for use in a prior project as long as the product has not changed since last used in accordance with statements by the manufacturer. If the performance-based specification includes such options, SCS highly recommends identifying the period between a prior project and the next project in the specification. In some cases, this means the product must go through a qualifying process even if it has not changed for many years but the previous set of qualifying data is older than a certain number of years. The period is based on the engineer’s judgment, but most professionals normally use five years in their specifications. During a five-year period, if the product changes or there are indications that the product might have changed due to recorded changes in certain reported values by the manufacturer, the qualifying process must be followed irrespective of the number of years passed since a recent past project to maintain quality and minimize risk.
Questions? Contact the author, Ali Khatami.
Landfills are getting larger in height and greater in footprint area, but the location of leachate tanks, leachate ponds, or discharge points to an on-site or off-site leachate treatment plant usually don’t change.
A larger footprint means leachate force mains are getting longer and pumps have to work harder to push leachate through the system to a target point. Some operators carry on with the same pumps for decades and don’t monitor the performance of the pumps after expanding the landfill footprint.
SCS highly recommends that you evaluate the performance of the existing pumps again. Such an evaluation may require hydraulic analysis of the entire network of pipes along with pumps, or whatever segment of the network that is affected by the expansion. The effort is minimal in retrospect, but the operator makes sure that the system will function in an optimized zone with minimal wear on the pumps.
Sometimes the hydraulic evaluation may require up-sizing all or certain pumps in leachate sumps because not enough flow can go through the force main due to high friction loss in the expanded leachate force main. Up-sizing pumps may be achievable depending on the type of the leachate sump, i.e., riser system or vertical manholes. If the up-sized pump in a riser system is too long to fit inside a riser system or too long to the point of making routine maintenance too cumbersome, your engineer needs to come up with another idea.
Booster pumps along an expanded leachate force main can certainly be an option. Booster pumps can be the inline or offline type. Install the inline pumps on the actual force main, and position the offline type on the side so that liquids go through bends and elbows to reach the pump, and again through bends and elbows to get back in the force main. In either case, the booster pump adds hydraulic energy to the flow inside the force main to push the liquids at a higher pressure and velocity through the remainder of the force main and to the target point.
Operators need to be aware of the dynamic nature of the leachate piping network and the role of booster pumps in dynamic environments. After landfill expansion, with new cells coming online -increasing leachate generation, and when closing landfill slopes -decreasing leachate generation over time, the flow in the force main may change. Sometimes booster pumps have to be up-sized or down-sized depending on flow and pressure in the system.
Questions? Contact the author Ali Khatami.
We continue SCS’s Advice from the Field blog series with guidance from an article in MSW Magazine by Daniel R. Cooper, Jason Timmons, and Stephanie Liptak.
The authors of a recent article in MSW Management Magazine present engineering ideas that provide for more efficient construction of a GCCS. Gas system operators will benefit by having fewer pumps to operate and maintain and shallower headers that are more easily accessible. Odor management will be easier along with other benefits.
Read the full article here to learn about the design elements for maximizing long-term benefits, impacting: bottom liners, location of the blower/flare station, leachate risers, extraction well targets, and external header piping.
Most often, landfill gas system design is added to an existing landfill cell, but the co-authors of this article explain the benefits of considering the LFG system during the landfill bottom liner design process. Doing so during the early stages of the landfill’s life, make it possible to improve collection efficiencies, lower operating costs, and save time in the future.
Continue to the full article, Planning Ahead for the Bottom Liners, published in MSW Magazine’s November 2017 issue and learn about the co-authors from Sarasota County, Florida Public Utilities and SCS Engineers.
Temporarily capping landfill slopes is becoming a common measure for landfill operators. There are many benefits to closing landfill slopes with geomembrane on a temporary basis. One of the benefits is delaying construction of the final cover. Following is a discussion of the steps that should be taken to determine whether temporarily capping the slope with geomembrane and postponing the final cover construction is a better financial/operational decision.
Constructing the final cover is costly, and it is considered an unavoidable expense that has no return on the money spent. Therefore, some operators perform a financial evaluation to determine whether the final cover construction costs can be delayed (provided, of course, that such delays are acceptable to the regulating agency). When evaluating whether to delay the final cover, the cost of maintaining the slopes during the postponement period should be considered. The operator must look at the financial aspects of either closing the slopes with a temporary geomembrane or of leaving the slopes open during the postponement period.
Temporary Capping Option
The benefits of temporarily capping the slopes during the postponement period may include:
The other side of the coin is the expense associated with the temporary cap. There may be repair costs associated with the geomembrane every few years in order to ensure that the temporary cap remains intact.
Leaving Slopes Open Option
The option of leaving the slopes open during the postponement period involves maintenance expenses such as:
The benefits of leaving the slopes open are twofold: first, the operator will save the costs of constructing the temporary cap; and second, the operator will gain additional airspace as waste settles during the postponement period.
Experience with the Temporary Capping Option
As discussed above, both options provide the benefit of gaining additional airspace during the postponement period. Constructing a temporary cap involves the costs of materials and installation, including the geomembrane and the ballasting system that keeps the geomembrane in place. Generally, the financial and non-tangible benefits of a temporary cap that remains in place five years or longer are more attractive than leaving the slopes open; therefore, most operators choose to install a temporary cap. The next step in the financial evaluation should be comparing the costs of the temporary cap to permanently closing the slopes without postponement.
Final Step in the Financial Evaluation
The next question is whether it makes financial sense to postpone the construction of the final cover.
Waste settlement during the postponement period and the resulting airspace are considered the determining financial factor in choosing the right option. If the present worth value of the airspace generated from waste settlement during the postponement period is greater than the cost to construct the temporary cap at the present time, then the temporary cap option would make financial sense; otherwise, the final cover should be constructed without postponement.
It should be noted that the length of the postponement period plays a very important role in this financial equation. Longer postponement periods have the potential for a greater gain in airspace. Another incentive that should be factored into the financial evaluation is the potential return on the money set aside for the final cover construction during the postponement period.
To assist with this financial evaluation, landfill operators are encouraged to discuss these options with their landfill engineers. Settlement models can be performed to calculate the amount of airspace that may be generated during the postponement period as well as the present worth value of the generated airspace. The returns on the final cover construction costs during the postponement will just be “icing on the cake.”
Read the related Advice From the Field blogs from the landfill and LFG experts at SCS Engineers: