SCS Advice From the Field: Temporary Caps for Landfills – A Good Financial Option?

November 1, 2017

 

Temporary Caps
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.

Cost Burden
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:

  • Eliminating routine mowing
  • Eliminating maintenance of storm water swales on slope
  • Eliminating soil erosion during storm events
  • Preventing rainwater from becoming leachate (i.e., leachate reduction)
  • Controlling odors from the temporarily capped area
  • Improving efficiency of gas collection from the temporarily capped area
  • Improving the aesthetics of the slope (e.g., masking leachate seeps or patchy vegetation)
  • Gaining additional airspace as waste settles during the postponement period

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:

  • Routine mowing of the slope
  • Maintaining storm water swales and temporary downchute pipes
  • Maintaining soil erosion occurring during storm events
  • Managing higher leachate generation caused by rainwater infiltration
  • Maintaining slope aesthetics ( leachate seeps and patchy vegetation)
  • Managing odors from the open areas

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:

Contact the author: Ali Khatami or your local SCS Engineers’ office.

 

 

 

 

Posted by Diane Samuels at 6:00 am
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