final cover

Final Cover System and Landfill Gas Piping – Design Considerations

June 24, 2020

SCS Advice from the Field

Landfills are complex systems with many pipes for liquids and landfill gas running in many different directions. Some of these pipes are at the bottom of the landfill, such as leachate collections pipes, leachate toe drain pipes, pressure release pipes, etc. Other pipes are near the final cover system, either below or above, and closely interact with the final cover geosynthetics. Many of these are for control of landfill gas or leachate seeps at the landfill surface. Pipes may include vertical gas wells, horizontal gas wells, condensate sumps, condensate force main, compressed air lines to gas well pumps and condensate sumps, seep control sumps, electric conduits to condensate sumps and seep control sumps, leachate recirculation force main, stormwater downchutes, etc.

When pipe locations are near the final cover geosynthetics, below or above, or penetrating the final cover, design plans should show details of how the pipes or associated components interact with the final cover components. Lack of sufficient information may cause difficulties years later when scheduling the construction of the final cover. Most often, it becomes evident that many of the pipes constructed years earlier are too short for extending through the final cover.

Another aspect of piping and their interaction with the final cover is conflicts among different pipes, more specifically conflicts among gas pipes and liquid carrying pipes, in and near the final cover system. Liquid carrying pipes may include stormwater downchutes, rainwater toe drain pipes, and leachate toe drain pipes. Stormwater downchutes are usually large diameter pipes extending from the top of the landfill to the perimeter stormwater system. Rainwater toe drain pipes – pipes that receive water from the final cover geocomposite drainage layer, and leachate toe drain pipes – to collect leachate seeps below the final cover geomembrane, are co-located at terraces on slopes and the toe of the slope near the perimeter berm.

A few design considerations can be useful as guidelines during the preparation of design sets to address the relative position of these pipes and the final cover geosynthetics or to avoid conflict among pipes.

  • Include the final cover layers in the gas design details where gas wells installations exist near the landfill’s final surface.
  • If flow control valves locations are below the final cover near the perimeter of the landfill, design a vertical casing around the valve tall enough that booting the future final cover to the vertical casing is possible.
  • Condensate sumps and associated stub outs (such as condensate force main, compressed air lines, or electric conduits) installations 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.
  • Leave pipes exiting the liner boundary at the perimeter of the landfill at least 1 foot above the anchor trench shoulder. This allows the installation of a geomembrane boot on the pipe at the point of penetration through the final cover geomembrane.
  • Flow control valves located 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.
  • Gas pipes located above the final cover geomembrane and crossing terraces or access roads may create conflict with the rainwater toe drain at the terrace or adjacent to the road.
  • Large gas headers located across the slope above the final cover geomembrane may cause conflict with stormwater downchutes.
  • Large gas pipes on top of the final cover geomembrane crossing a tack-on swale may cause conflict with the flow line of the tack-on swale.

The complexity of landfills varies from site to site, and issues related to conflicts among gas and liquids pipes, and pipes and final cover geosynthetics vary depending on the geometry and other landfill features involved at each location. The best way to resolve conflicts before construction is to have a coordinated effort among parties involved in the design to discuss and find solutions to every conflict at the design stage.


 

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

 

 

 

 

 

 

Posted by Diane Samuels at 6:03 am

Congratulations to Phil Gearing – Selected as a 2017 SWANA Young Professional UP NORTH

March 14, 2017

SCS Engineers’ Phillip E. Gearing, PE is a winner of the SWANA 2017 Young Professional Award from the Wisconsin Badger Chapter. The Solid Waste Association of North America honors individuals like Phil who make a significant difference in the solid waste industry.

Phil Gearing, PE, SCS Engineers and recipient of the 2017 SWANA – Wisconsin Young Professional Award.

Phil represents the best of the young professionals working within Wisconsin’s solid waste industry. Clients, contractors, and team members appreciate Phil’s leadership and passion for doing the job right.

He is a dedicated father of three children and an avid fan of all things Wisconsin, namely Badger sports, Green Bay Packer football, and Brewer baseball. Wisconsin from head to toe! Phil was raised on a dairy farm in Jackson County near Merrillan and attended the University of Wisconsin – Madison where he earned his B.S. in Geological Engineering, Geology, and Geophysics.

Phil serves clients out of the SCS Engineers office in Madison, WI.

 

Read about Phil’s work and SWANA award here.

 

Congratulations! Thanks for your hard work and dedication Up North.

 

 

 

 

 

 

 

Posted by Diane Samuels at 3:00 am

SCS Advice from the Field: How to Increase the Flow Capacity of the Final Cover Drainage Layer Over a Concaved Portion of a Landfill Slope

May 17, 2016

The drainage layers of landfill final covers normally go through a rigorous flow capacity evaluation. This evaluation is necessary to ensure that the volume of water reaching the drainage layer due to percolation of precipitation water through the final cover upper soil layer will not overwhelm the drainage layer in its flow path. If the flow volume in the geocomposite drainage layer is greater than the capacity of the drainage layer, water will exit the geocomposite and enter the overlying soil. The water entering the soil layer can easily saturate the lower portion of the soil layer, which will affect the stability of the slope. The geocomposite should always be designed to have a flow capacity greater than the flow rate of water running through it.

Concave areas of a landfill slope experience flow patterns quite different from slopes that go straight down. Slopes with concaved geometry have an unequal crest and toe lines – the toe line being smaller than the crest line. As a result, the width of the concaved slope decreases as the distance from the crest line increases in the downward direction. The narrowest width of the concaved slope is at the toe of the slope. The drainage layer on the slope experiences the same width change from the crest line to the toe line. This means that the overall width of the channels that carry water within the geocomposite drainage layer decreases toward the toe line, and, therefore, the depth of water in the channels increases. This phenomenon is referred to as flow convergence, and the convergence is toward the vertical centerline of the concaved slope. The flow convergence may be significant enough to increase the water depth in the geocomposite in the vicinity of the vertical centerline of the slope to greater than the thickness of the geocomposite. That, in turn, forces water out of the geocomposite and into the overlying soil, which may result in slope stability problems.

To complement the geocomposite flow capacity along the vertical centerline of the concaved slope in order to accommodate the higher water flow rates in the system, a pipe-gravel-geotextile (a burrito) may be constructed along the vertical centerline of the slope. The burrito, which would be positioned directly over the geocomposite drainage layer, increases the flow capacity of the system at and in the vicinity of the vertical centerline of the concaved slope. The burrito will receive water from the geocomposite where the water depth exceeds the geocomposite thickness. The burrito will be connected to the toe drain system at the toe of the slope, and water in the burrito will be discharged to the toe drain. The water in the toe drain, in turn, leaves the final cover through lateral drain pipes at regular intervals.
It should be noted that not every concaved slope requires a burrito. Some concaved slopes may be fairly wide, and the convergence of water inside the geocomposite may not be significant enough to cause the depth of water to exceed the geocomposite thickness. But, if the concavity of the slope is significant, a severe convergence of water in the geocomposite drainage layer is more likely. In that case, a burrito along the vertical centerline of the concaved slope is highly recommended.

A cautionary construction related note seems to be appropriate at this point. During construction, extra care should be taken to ensure that all geocomposite panels within the boundary of the concaved slope run such that the machine direction of the panels follows a path from the top toward the bottom of the slope. If some geocomposite panels are installed with the machine direction running across the slope width, significant turbulence in the flow will be created at the point where panels running in one direction transition to the panels running in the other direction. The turbulence will reduce the flow capacity of the geocomposite.
If you are planning to install a final cover over a portion of the slope that has concaved geometry and you want your final cover design to properly address flow volumes in the geocomposite drainage layer, please contact us. SCS Engineers has extensive experience with these types of circumstances, and we will gladly review your case and make recommendations. Learn more here.

If you have comments or questions about this article, please contact Dr. Ali Khatami.

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.

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 in the design and permitting of civil and 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. He is also involved in 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.

Posted by Diane Samuels at 6:00 am

SCS Advice from the Field: How to Manage Water Percolating into the Landfill Final Cover Drainage Layer

May 4, 2016

As more and more landfill closure projects were built using geomembranes as the final cover barrier layer over the past 20 years, the issue of handling water in the final cover drainage layer became more prominent.  Precipitation on closed portions of a landfill leaves the landfill basin in three ways: (1) runoff; (2) percolating into the final cover upper soil layer and then evaporating back out into the atmosphere by evapotranspiration; and (3) percolating into the final cover upper soil layer and reaching the final cover drainage layer, flowing through the drainage layer to the bottom of the landfill slope, and leaving the landfill basin through discharge points to the landfill perimeter ditch.  Addressing the runoff component is relatively straightforward because for decades engineers have been designing various types of conveyance systems to handle surface water runoff.  Mother Nature takes care of the second component.

The third component, however, took many years to be engineered properly.  SCS developed one of the best-engineered systems over 18 years ago and perfected the design over the following five years.  The perfected design has been incorporated into the permits of many landfills and implemented at many closure construction events.  As a commitment to the efficiency of the design, SCS has been monitoring the performance of many of these closure projects during rain events to gather data and ensure that nothing unexpected occurs during the more severe storm events.

SCS’s system involves a perforated collection pipe embedded in gravel, wrapped in geotextile, and placed in a depression created by a geomembrane flap near the bottom of the slope.  The geomembrane flap is welded to the final cover geomembrane and supported in a depressed shape by the upper soil layer of the final cover system.  The drainage layer geocomposite ends at the bottom of the depression, delivering the water in the geocomposite into the pipe-gravel-geotextile positioned inside the depression.  Installation of the geomembrane flap is at a sloping grade; therefore, the collection pipe ends up sloping toward a low point where a drain pipe that is perpendicular to the perforated pipe takes the water out of the depression and delivers it to the landfill perimeter ditch.  The system is fairly easy to install and almost guarantees proper removal of water from the final cover drainage layer.   Although other engineers have designed many varieties of such systems, the SCS system has a proven track record with no glitches or side effects at the bottom of the landfill slope.

Some other systems, because of the inherent shortcomings in the design, either don’t remove all of the water from the geocomposite drainage layer, or they clog at the discharge point. Sometimes the water coming through the system adversely affects other landfill components, such as the perimeter berm integrity.  Additionally, complexities during construction of the system can conflict with the storm water down chute pipes or landfill gas pipes that may exist above the cover system geomembrane.

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In addition to writing the SCS Engineers blog series SCS Advice from the Field, Dr. Khatami speaks about SCS blog topics at SWANA national and local chapter conferences. His webinar Design Leachate Collection Pipes to Eliminate Clogging of Geotextiles will be presented on June 29, 2016. Use the links below to learn more about Dr. Khatami’s advanced landfill designs which last longer and help prevent common operational challenges over time.

How to Manage Leachate Seeps Below the Final Landfill Geomembrane Cover

Recommendations for Jet Cleaning Leachate Collection Pipes

Avoid Geotextile Clogging of Leachate Collection Pipes

How to Compensate for the Effect of the Ambient Temperature Variations on the Pressure Changes Within the Pipe During HDPE Pipe Pressure Testing Using Incompressible Fluid

 

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.

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 in the design and permitting of civil and 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. He is also involved in 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.

If you are looking to design a final cover for your landfill, please contact SCS.  We will review your particular needs and the existing conditions at your facility and will recommend a proper design that suits your site conditions.  SCS will also provide you with construction recommendations and an estimate for construction of the system.  Furthermore, SCS will gladly incorporate the final design into your facility permit and prepare construction drawings for implementation of the system.

Posted by Diane Samuels at 6:00 am