CEPIS/OPS/OMS

Landfill closure and long-term care

The last of a 10-part correspondence course prepared by
the Department of Engineering Professional Development, University
of Wisconsin-Madison, in cooperation with Waste Age.

Efficient use of a landfill site following closure requires long-range planning. The best strategy is to plan for the eventual site use before the landfill is constructed and operated. Stating a planned use during site selection may also reduce possible opposition to a new landfill. Potential uses for closed sanitary landfills include: nature or recreation park; animal refuge; tennis court; golf course; ski or toboggan hill; parking lot; or commercial or industrial building site.

Planning is particularly important where construction of a building on or near the landfill site is anticipated. Design features such as locations of structures requiring special support, recreational facilities requiring specific topography, and gas control systems to protect future buildings should be anticipated during landfill operation. Areas should also be set aside for groundwater monitoring. Planning for a potential ground water contamination problem by setting aside space for installation of remediation equipment or structures is also a good strategy.

Depending on planned site use, factors that can be modified are: cover thickness, slope, cover/waste ratio, degree of compaction, use of additives and cements, selective disposal, and setting aside undisturbed structural pads. The consequences of changing plans for landfill site and use will usually require costly modifications, such as the removal of settlement prone cover and waste layers.

BY PHILIP O'LEARY AND PATRICK WALSH
Walsh and O'Leary are solid waste specialists
with the University of Wisconsin-Madison.


Characteristics of completed site

When planning final site use, the critical factors that must be considered are: settlement, foundation characteristics, control of leachate and gas, vegetation, and final grade. Depending on the end use selected, efficient site access may also be a consideration.

Waste settlement and consolidation is a major consideration when constructing facilities on top of a closed landfill. At older sites, where poor initial compaction was achieved and large amounts of precipitation were allowed to percolate into the waste, settlement was quicker and more extensive. Approximately 90% of the settlement will occur in the first five years, although it may continue for 25 or more years at a slower rate. A site with in-place densities of about 800 pounds per cubic yard is more subject to rapid and extensive settlement-compared to a well-run site, with in-place densities of 1,400 pounds per cubic yard.
Decomposition rates can be significantly affected by the amount of water in the landfill. For example, a landfill with limited available water for biochemical decomposition will settle more slowly. Recently implemented landfill standards, which require tightly controlling precipitation entry through the landfill cover, results in very slow decomposition and therefore settlement. Efforts to promote decomposition by leachate recycle should also speed up the settlement process. Settlement is also affected by the amount of cover material used. A landfill with a cover-to-waste ratio of 1:4 is expected to settle more than a landfill with cover to waste ratio of 1:3. The composition of solid waste influences the subsidence rate. A landfill with a high percentage of demolition and construction waste is expected to settle less than a landfill with high organic content.

Because different wastes have different decomposition rates and compaction potential, differential settlement should be anticipated. The final plan should provide for periodic inspections of the landfill and, when necessary, provide for regrading to prevent ponding.

Monitoring the vertical movement of the individual levels of intermediate covered waste can give an initial indication of the eventual settlement rate. Conditions on the finished surface of a landfill will be dependent upon the conditions of each lift within the landfill.

Maximized initial compaction, coupled with minimized heterogeneity, should result in the earliest ultimate consolidation. Surcharge loading where heavy materials, usually soil, is piled onto the site will accelerate consolidation. Volume reduction caused by biological decomposition may be expected to result from wetting of the solid waste.

Settlement due to volume reduction creates cracks in the cover material. Settlement can occur within a few days of filling or can extend over many years.


Procedures for site closure

Table 1 shows the operational procedures to be performed when either the entire landfill, or a phase of it, has been filled to capacity. Phased closure is recommended.

When planing a landfill, the design intent is to limit entry of water-to reduce leachate generation. Specification of cover characteristics may be based on results of water balance calculations described in Lesson 4 (see Waste Age, July 1991). Correct cover placement is an important control technique. Two feet or more of final cover, consisting of an initial two-foot layer of relatively impermeable soil followed by top soil, is usually recommended. The use of geomembranes to further restrict water entry into the closed site is becoming a more frequently utilized control technique. Construction techniques which ensure that quality installation is achieved will enhance the cover's ability to resist percolation into the landfill.

After cover placement, the area should be immediately vegetated to prevent erosion. Table 2 describes recommendations for establishing vegetation on a closed site. A timetable should be prepared to ensure that the following features are inspected at regular intervals:

Table 1. Site Closure Checklist

Pre-planning:

  • Identify final site topographic plan
  • Prepare site drainage plan
  • Specify source of cover material
  • Prepare vegetative cover and landscaping plan
  • Identify closing sequence for phased operations of on-site structures
  • Specify engineering procedures for the development of on-site structures

Three months before closure:

  • Review closure plan for completeness
  • Schedule closing date
  • Prepare final time-table for closure procedures
  • Notify appropriate regulatory agency
  • Notify site users by letter if they are municipalities or contract haulers; by published announcement if private dumping is allowed

At closure:

  • Erect fences or appropriate structures to limt access
  • Post signs indicating site closure and alternative disposal sites
  • Collect any litter or debris and place in final cell for covering
  • Place cover over any exposed waste

Three months after closure:

  • Complete needed drainage control features or structures
  • Complete as required gas collection or venting system, leachate containment facilities, and gas or groundwater monitoring devices
  • Install settlement plates or other devices for detecting subsidence
  • Place required thickness of earth cover over landfill
  • Establish vegetative cover

Building on a landfill

Engineering techniques are available for constructing many types of facilities over a closed landfill. Depending upon the type of facility being installed, it may be necessary to extend pilings through the landfill and into the underlying base material. In certain instances, it may be possible to employ special foundations or footings to support the load being placed upon the fill.
Major design considerations for building on a landfill include consolidation-time-settlement relationships and load-bearing capacities developed from field and laboratory data. Specialized techniques are also available to help the designer identify the appropriate structural techniques.

Bearing capacity is the ability to support foundations and heavy equipment. Although the hearing capacity of the landfill will depend on the operation, typical values ranging from 500 to 800 pounds per square foot have been reported. Low bearing capacity can be overcome by increasing the soil thickness used for final cover. In this way, soil resists both punching and rotational shear. The minimum recommended soil thickness is 1.5 times the width of structural footings.

Placing and compacting special waste under the areas planned for buildings is also an option. For example, fly ash and bottom ash may be compacted in one-foot lifts in assigned areas while other solid waste is distributed elsewhere in adjacent areas. Incorporation of stabilizers into the soil during cover placement can improve the bearing capacity of the landfill. Common chemicals mixed with soil include lime, portland cement, and various organic chemicals. The desired effect may be strengthening by cementation, waterproofing, or dispersion for greater density and lower permeability.

Pressure-injection grouting of lime slurry is also an option for increasing bearing capacity. A 20,00O-square-foot building was constructed in Fort Worth, Tex., in 1968 on an area that was previously a sanitary landfill. The landfill extended 10 feet below the building. Remedial stabilization of the building was accomplished in 1977. A slurry of two pounds of lime and four pounds of fly ash to one gallon of water was injected.

Where other means of achieving a supportive foundation on a solid waste landfill are unsuccessful, a system of piles or piers can be used to support important structures. Even with these extreme measures, problems may not be eliminated. The landfill may continue to settle around and away from the stable structure and incidentally cause the piling to take additional, unanticipated loads through mobilization of negative skin friction.

When constructing facilities on a landfill, special consideration should be given to the placement and building service entrances of the sewer, water, electric, and gas utilities.

If trees will be located on the landfill cover, special consideration for having a deeper soil cover at the appropriate locations should be provided for when closing the landfill. Figure One shows a special planter developed to hold roots and retain moisture for trees planted on landfills.

When designing structures on a landfill, one must also be aware of gas-venting techniques that will be needed to prevent the accumulation of methane gas within structures. These controls may include vent pipes, suction blowers, or various means of natural ventilation and are described in Lesson 3 (see Waste Age, June 1991).

Often, drainage problems can result in accelerated erosion of a particular area within the landfill. Differential settling of drainage control structures can limit their usefulness and may result in failure to properly direct storm water off the site. Failure to maintain the physical integrity of the Iandfill cover promotes additional infiltration into the landfill and eventually results in increased leachate production.


Remedial gas & leachate control

Quantities of gas and/or leachate may be found migrating away from the landfill if control mechanisms fail or were not installed. The control systems for gas and leachate management require continuous attention after closure. Groundwater monitoring wells and gas probes should be incorporated into the design to check control system performance. Remedial actions for controlling gas and leachate involve implementing some type of engineering control system along the perimeter of the site and, in some cases, within the landfill itself. In many cases, remedial actions are much more expensive than if the gas or leachate had been properly managed with facilities installed when the landfill was being developed.

Table 2. Steps for Vegetating Landfills with Limited Funds

  1. Select an end use.
  2. Determine depth of cover: Cover soil must be at Ieast 60 cm deep for grass establishment and 90 cm for trees
  3. Establish an erosion control program: The soil on recently covered landfills must be stabilized soon after spreading to prevent erosion.
  4. Determine the soil nutrient status: Before or during the grass and ground cover experiments, soil tests should be made for pH, major nutrients (nitrogen, potassium, and phosphorus), conductivity, bulk density, and organic matter.
  5. Determine soil bulk density: Cover soil is frequently compacted by landfill equipment during spreading operations to bulk densities that will severely restrict plant root growth.
  6. Amend soil cover: The soil over the entire planting area should be amended with lime, fertilizer; and/or organic matter according to soils tests before planting. These materials should be incorporated into the top 15 cm of soil.
  7. Select landfill-tolerant species: Grasses and other ground covers can be selected for planting in the soil cover by evaluating the results of the experimental plots established earlier to determine landfill-tolerant species.
  8. Plant grass and ground covers: It is generally desirable to embed the seed in the soil. Mulches can be used as an alternative to embedding the seed but are not as effective.
  9. Develop tree and shrub growth: Trees and shrubs should not be planted for one or two years after grass has been planted. If the grass cannot grow because of gases from the landfill, other deeper-rooted species are not likely to thrive either.

Groundwater that has been contaminated by leachate can be dealt with in many ways. Impermeable barriers constructed of bentonite slurry, cement or chemical grouts, or sheet piling can be installed vertically to prevent groundwater from migrating away from the site, or divert groundwater so that contact with leachate is prevented. These treatment methods are considered passive groundwater control. Pumping of groundwater with subsequent surface treatment is considered an active remedial measure. Leachate generation can also be reduced by placing a more effective cover over the landfill. Implementation of a leachate control program requires an extensive engineering study to determine the most effective and economical approach.
Options for controlling gas migration were described in Lesson 4 (see Waste Age, April 1986).


Paying for long-terms care

Closure and long-term care of a landfill is expensive. While some income may be expected through the sale or lease of the site and possibly through the sale of methane gas, this is not usually enough to pay all costs of closure and long-term care.

Table 3. Long-term Care Fund Deposits and Payments

Fund Deposits

Year Site Life
Year
 

Annual
Payment
To Fund

Fund
Balance
January 1

Interest
Earned
(10%)

Fund
Balance
December 31

1992

1

 

$214.259

$214.259

$21.426

$235,685

1993

2

 

229.257

464.942

46.494

511,436

1994

3

 

245.305

756.741

75.674

832,415

1995

4

 

262.476

1,094.892

109.489

1,204,381

1996

5

 

280.850

1,485.231

148.523

1,633,754

1997

6

 

300.509

1,934.263

193.426

2,127,690

1998

7

 

321.545

2,449.235

244.923

2,694,158

1999

8

 

344.053

3,038.211

303.821

3,342,033

2000

9

 

368.137

3,710.169

371.017

4,081,186

2001

10

 

393.906

4,475.093

447.509

4,922,602

2002

11

 

421.480

5,344.082

534.408

5,878,490

2003

12

 

450.983

6,329.474

632.947

6,922,602

2004

13

 

482.552

7,444.973

744.497

8,189,471

2005

14

 

518.331

8,705.802

870.580

9,576,382

2006

15

 

552.474

10,128.856

1,012.886

11,141,741

Fund Payouts

2007

Site Closure

 

Fund Payment

$1,289.267

Fund Balance

$9,852.405

   

 

Year

LTC

Year

LTC Estimate

(current price)

Annual

Payment

to Fund

Fund

Balance

January 1

Interest

Earned

(10%)

Fund Balance

December 1

2008

1

$200.000

$590.433

$9,262.042

$926.204

$10,188.246

2009

2

200.000

631.763

9,556.483

995.648

10,512.131

2010

3

200.000

675.986

9,836.145

983.614

10,819.759

2011

4

200.000

723.306

10,096.454

1,009.645

11,106.099

2012

5

200.000

773.937

10,332.162

1,033.216

11,365.378

2013

6

150.000

621.084

10,744.294

1,074.429

11,818.723

2014

7

150.000

664.560

10,154.163

1,115.416

12,269.579

2015

8

150.000

711.079

11,558.500

1,155.850

12,714.350

2016

9

150.000

760.855

11,953.495

1,195.349

13,148.844

2017

10

150.000

814.115

12,334.729

1,233.473

13,568.202

2018

11

150.000

871.107

12,697.099

1,269.710

13,966.809

2019

12

150.000

932.080

13,034.729

1,303.473

14,338.202

2020

13

150.000

997.326

13,340.876

1,334.088

14,674.964

2021

14

150.000

1,067.139

13,607.825

1,360.783

14,968.608

2022

15

150.000

1,141.838

13,826.770

1,382.677

15,209.447

2023

16

150.000

1,221.767

13,987.680

1,398.768

15,386.448

2024

17

150.000

1,307.291

14,079.157

1,407.916

15,487.073

2025

18

150.000

1,398.801

14,088.272

1,408.827

15,497.099

2026

19

150.000

1,496.717

14,000.382

1,400.038

15,400.420

2027

20

150.000

1,601.487

13,798.933

1,379.893

15,178.826

2028

21

150.000

1,713.591

13,485.235

1,346.523

14,811.758

2029

22

150.000

1,833.543

12,978.215

1,297.822

14,276.037

2030

23

150.000

1,961.891

12,314.146

1,231.415

13,545.561

2031

24

150.000

2,099.223

11,446.338

1,144.634

12,590.972

2032

25

150.000

2,246.169

10,344.803

1,034.480

11,379.283

2033

26

150.000

2,403.400

8,976.883

897.588

9,873.471

2034

27

150.000

2,571.639

7,301.833

730.183

8,032.016

2035

28

150.000

2,751.653

5,280.363

528.036

5,808.399

2036

29

150.000

2,944.269

2,864.130

286.413

3,150.543

2037

30

150.000

3,150.368

175

18

193


Expenses incurred at closure and during a 20-year, long-term care period, for example, can be a significant portion of the overall cost of operating a site. Under recently issued Subtitle D regulations of the Resource Conservation and Recovery Act, the federal government requires that closure and long-term care costs be identified, and a plan established for their financing when operating a solid waste disposal facility. Several states have had similar requirements for a number of years requiring long-range financial planning and responsibility by the landfill owner.

Table 3 summarizes the payments that a landfill authority could make into an account over the life of a site to finance closure and long-term care. When the site has been completed, funds would be withdrawn to pay for closure and then a sum would be withdrawn annually to pay for long-term care. Essentially, this approach is a savings account for the landfill authority. Several features should be noted. First, payments into the account have been computed in such a way that the amount of money deposited is adjusted for 7% inflation each year. When the landfill was being planned, it was estimated that the closure costs would be $500,000. However, since the closure did not occur until fourteen years after the cost estimate was made, the estimate was adjusted to reflect 7% inflation. Therefore, when closure is completed, the landfill authority withdraws $1,289,267, or the equivalent of $500,000, after inflation.

Long-term costs are estimated in a similar fashion. Each year the authority draws money for long-term care, with the fund being exhausted in 30 years.

The preceding example illustrates one possible way a landfill authority can accumulate the necessary funds during the active life of the fill to pay for closure and long-term care. Many other alternatives have also been suggested such as trust funds, escrow accounts, or bonds. This example shows in a simple fashion, how much money is needed when closing and providing for long-term care of a landfill Specific regulatory procedures are more extensive and should be consulted before putting together the long-term financing plan for a landfill.

Factors to consider when choosing a funding method include political sentiments within the community, the tax consequences to operators of private landfill facilities and the desirability of tying up large sums of money where the operating authority appears to already have sufficient financial resources.

Designing and funding a landfill for a 10-or 20 year life followed by a 20-or 30-year long-term care period also requires that the landfill authority be in existence as many as 50 years. Many things can change during this time, and the need for long-term viability must be considered.

Source: Joe Spear, Sanitary Landfill Design Course
             University of Winconsin-Madison, October 1-3, 1991


Lesson assignments

  1. Why does a landfill require continuing care?
    Give examples of site features which will require maintenance.

  2. What are the causes of landfill subsidence?

  3. How can the bearing capacity of a landfill be increased?

  4. What kind of remedial action can be taken for groundwater pollution from a closed landfill?


Authors' note

This is the final lesson in the 1991-92 Solid Waste Landfills Independent Study Course. More than 200 persons are officially enrolled in this course.

Course registrants should prepare for the final exam by reviewing the previous lessons. Each participant who successfully completes both exams will receive a certificate and be awarded two continuing education units (CEUs).
In addition to the current authors, Philip O'Leary and Patrick Walsh, previous editions of this Solid Waste Landfills Correspondence Course have been authored by John Reindl, Berrin Tansel, and Rick Fero. Their contributions to the preparation of this series are hereby acknowledged and appreciated.

Special thanks also go to Kris Winneke Judy Fabor, and Jean Zwaska of the University of Wisconsin who carefully revised and edited the manuscripts.

Persons wishing to enroll in this correspondence course, or any of the other courses on recycling, composting, or waste-to-energy, shoutd contact Judy Fabor, Department of Engineering Professional Development, University of Wisconsin-Madison, 432 N. Lake Street, Madison, WI 53706 (608) 262-1735. The fee is $90 for each course.