Robert C. Bachus and Kwasi Badu-Tweneboah

Subtitle D is making landfill history while causing managers to come up to code or go out of business. The details can be confusing, but the goal is simple: to protect the quality of the environment.

While the most appropriate materials, methods and standards are often debatable, few will doubt that waste must be contained to protect the environment.

Containment refers to controlling waste, leachate,landfill gas, rain and surface water. Caps, covers and liners are the three basic components of a landfill containment system. The uppermost element, a landfill cap, is usually placed on the top and on the side slopes after all active landfilling has ended or as waste reaches final grades in the cells.

Landfill covers are used at various times during the site's active life. Daily cover is placed over waste at the end of each day and intermediate cover is placed when an area will not receive waste or a cap for a relatively long time (typically two to six months).

The liner is the bottom part of the landfill containment system and is installed after excavation for the landfill cell and prior to waste placement. The materials used in the landfill containment system are selected for specific reasons (see table); each material has one or more functions which affect the system's performance.

Failure of one or more components may lead to system failure.

The landfill capping system reduces the infiltration of precipitation while controlling leachate and gas migration. The cap is the most visible remnant of a closed landfill, and the component which is most susceptible to eliminate fluctuations in precipitation and temperature. The capping system must be designed to:

* Adequately shed precipitation at the surface of the cap to control infiltration and subsurface drainage;

* Provide a means of establishing a low maintenance vegetative cover that supports plants with roots; and

* Provide a sufficient barrier for upwardly migrating landfill gas and the outward migration of leachate.

The topsoil and vegetation used on the landfill cap serve a valuable function. The vegetation should be a native, drought-tolerant species.

The capping system includes hydraulic barriers below the topsoil which drastically limit the infiltration and absorption of water. Closed landfills often are constructed at a higher elevation than the surrounding area. As a result, wind and the constant exposure to sunlight can increase evapotranspiration and aggravate the low moisture conditions in the cover soil.

The topsoil should be able to hold limited water supplies to support the vegetation and allow the roots to develop large lateral growth. Particularly on the side slopes, the topsoil and vegetation must provide erosion protection from the anticipated sheetflow of surface water down the planar side slopes.

Usually, a protective soil layer is beneath the topsoil or is part of the topsoil layer. Its primary function is to provide either a relatively low hydraulic conductivity barrier or a protective buffer from frost damage and/or root penetration for an underlying soil, geomembrane or geosynthetic clay liner (GCL).

The protective soil will be subjected to water which infiltrates from the ground surface and often is exposed to a wide range of temperatures. In cold climates, this layer is susceptible to frost, freezes and thaws which may be complicated by the relatively warm temperatures in the waste.

If the layer is relatively permeable and it overlies a low hydraulic conductivity layer, seepage can adversely affect the cover's stability. A geonet composite can help control the water infiltrating the cap and can improve the lining system's stability.

Some landfill designs include a low hydraulic conductivity soil to provide a primary hydraulic and gas barrier; other designs include a geomembrane, GCL or a composite of geomembrane/soil or geomembrane/GCL as the barrier layer.

Consider the following when selecting a barrier layer:

* Hydrogeologic conditions and regulatory requirements;

* Availability of materials;

* Design and construction conditions;

* Anticipated weather conditions;

* Odor and gas management; and

* Costs.

The barrier layer is usually underlain by the intermediate cover soil layer which serves as a foundation support for the cap. However, with many designs, depending on the hydraulic conductivity of the intermediate cover, the cover soil layer is often incorporated into the barrier layer. When a geomembrane is used and when gas is actively extracted, a gas collection layer is placed in lieu of the final soil cover or a thick intermediate cover soil.

This underdrain layer may consist of sand or an underdrain geocomposite drainage layer. The layer's function is to collect the gas which may accumulate under different portions of the barrier layer. This layer may also be used to control lateral leachate seepage.

The capping system's components on side slopes usually are similar to those on the landfill's top slopes. However, several design and construction issues will influence which materials are selected. For instance, stability of the lining system must be evaluated. Textured geomembranes, developed to improve the stability of geomembranes on slopes, are often used in design.

Conducting interface direct shear tests using the design's components is recommended. Testing conditions should model the field conditions as closely as possible. A geogrid or other geosynthetic reinforcing material is often used in some designs on steep side slopes or when relatively weak interface conditions are encountered.

The effects of surface and subsurface water flow on cover system stability must be evaluated. Surface water should be designed to sheetflow down the slopes to collection swales where the channeled water can be controlled. Water that infiltrates through the cover soils will flow parallel to the side slopes, particularly if a low hydraulic conductivity barrier is installed on the side slope; this ultimately may have significant adverse impacts on stability.

To control internal seepage, geocomposite drainage layers or geotextile/geonet composites have been used in lieu of sand, which is often difficult to place on side slopes. Unless geotextiles are covered rapidly, they may degrade. Since construction on landfill side slopes can be difficult, using reinforced GCLs may be a beneficial alternative to placing low hydraulic conductivity soil.

The cover system includes the daily cover and intermediate cover. For health reasons, daily covers are placed when the cell is not actively used. Daily cover prevents debris from blowing, minimizes vectors and fire threats, controls odor and reduces the chance of surface water and precipitation reaching the waste.

Traditionally, local soil has been used as daily cover, primarily because of its low cost. A portion of the soil would normally be removed and reused. But even if the soil is recovered carefully, a large quantity usually is permanently left in the landfill.

Several states have already approved alternate daily cover materials, which reportedly save landfill space. These covers include synthetic sheets and foam or spray-on products, such as slurry and emulsion-based materials, which are placed over the active face each day. The synthetic sheets are removable for continued waste placement. The foams or spray-on products are extremely compressible and, when the wastes are placed directly on top, they will virtually disappear.

Intermediate cover soil is placed on areas of the active landfill that won't be covered with waste or a cap for a long period of time. The length of time varies from 30 to 180 days.

The reasons for using intermediate cover are the same as those for using daily cover. Intermediate cover usually consists of a layer of soil that may include a portion of the daily cover. The intermediate cover's surface layer should be graded to encourage surface water runoff.

The landfill lining system controls and minimizes the release of leachate using a leachate collection system. In addition, low hydraulic conductivity materials on the side slopes and the landfill base can restrict or significantly slow the infiltration of leachate into the subsurface.

Subtitle D significantly affected the types of materials that can be used in lining systems. In the past, conventional systems generally consisted of a single liner with a layer of compacted low hydraulic conductivity soil. Subtitle D requires using a composite liner or a double liner.

A single liner is a single layer of a low hydraulic conductivity material, generally a compacted soil layer (usually clay) or a geomembrane. The compacted soil liners in old landfills and geomembrane liners for reservoirs are examples of single liners.

A single composite liner is a single layer with two different liner materials in intimate contact with each other (e.g., a geomembrane placed in intimate contact with a compacted soil layer). Synergistic benefits exist from using the two different materials as a composite.

A double liner is two liner materials which are separated by a layer with high hydraulic transmissivity. This layer, referred to as a leakage detection layer, generally consists of a geonet, a drainage composite or a highly permeable sand or gravel.

The proper design allows liquids that pass through the upper, primary liner to rapidly flow on top of the lower, secondary liner within the leakage detection layer under a small hydraulic gradient. Flow is directed to a collection sump where the liquid is removed.

The lining system must be placed on an appropriate foundation. The subsurface conditions beneath the base and side slopes must be analyzed. Slope stability and landfill foundation settlement are usually the two most important considerations that influence the design.

Relatively poor subsurface conditions may be improved by using a reinforcing geosynthetic material, such as a geogrid or high strength geotextile. Once foundation stability has been addressed, construction begins by preparing the subgrade.

The subgrade usually is a native foundation material which is graded to promote drainage to a collection point. The subgrade is smoothed, preparing it for construction and placement of the subsequent liners. The subgrade may have a minimum hydraulic conductivity requirement.

A low hydraulic conductivity material is placed as the hydraulic barrier layer above the subgrade. This layer is generally a single composite, double composite, or double liner. The low hydraulic conductivity component is either a compacted soil, a GCL or a geomembrane.

All liner components must have low hydraulic conductivity and must be compatible with the leachate; compressibility and strength also are important considerations. Most inorganic soils are relatively compatible with leachate; high density polyethylene (HDPE) is often used because it is compatible with leachate.

Usually, the first material placed on top of the liner, a protective soil or a geosynthetic layer, protects liner integrity from damage during construction and waste placement. This layer's materials also are integral to the leachate collection system. They are usually highly permeable and transmissive to minimize the hydraulic head on top of the primary liner. In fact, most regulations require a leachate management system to maintain a maximum of one foot (0.3 m) of leachate on the liner.

This protective layer may consist of both soil and geosynthetic materials, the lower geosynthetic material selected because of its high transmissivity. In these cases, a filter geotextile is required to protect the geosynthetic materials' hydraulic integrity.

In general, cover systems may cost between $50,000 to $150,000 per acre ($125,000 to $370,000 per hectare), while lining systems may cost between $100,000 to $300,000 per acre ($250,000 to $750,000 per hectare).

Landfill operators must strike a balance between regulatory and budgetary demands. Understanding containment systems will help to reach that balance, while serving the public and protecting their future.

Under Subtitle D regulations, the working face of all landfills must include a daily cover. Daily covers control odors, prevent litter from blowing away and keep birds, rodents and insects out of the landfill. When choosing a product, landfill owners should seek daily covers that are cost-effective and will save time.

When comparing alternate daily landfill covers, consider the following:

* Will the product save air-space? If so, owners will achieve an extended landfill life and increased long term revenues.

* Can the cover withstand the specific environmental conditions of your site? Also, can it be applied during rain, snow and high winds?

* Odor Control. Look for a product that will continually suppress odors thereby being more aesthetically acceptable to the nearby community.