Lesson 7: Preparing Landfill Designs & Specifications

Patrick Walsh and Philip O'Leary

This is the seventh lesson in the independent learning correspondence course on municipal solid waste (MSW) landfills. One lesson in this 12-part series will be published in Waste Age magazine each month throughout the year.

If you are interested in taking the course for two continuing education credits (CEUs), send a check (payable to the University of Wisconsin) for $149 to Phil O'Leary, Department of Engineering Professional Development, University of Wisconsin, 432 North Lake Street, Madison, WI 53706. Phone (608) 262-0493. E-mail: [email protected]. website: www.wasteage.com.

Course registration can occur at any time until December 2006. Previous lessons will be sent to you.

Click here to download PDF of Course 7 with figures. (Requires Adobe Acrobat Reader) Learning Objectives:

The goal of siting a sanitary landfill is to provide long-term environmental protection that is economically efficient and complies with applicable regulations. Very few potential landfill sites are ideal. But the landfill's design phase allows managers to overcome site deficiencies using proven engineering techniques. A well-developed design plan will make construction, operation and closure less technically difficult and more cost-effective.

Environmental Regulations

Because of the high degree of risk from improper waste disposal, landfill design and operation are heavily regulated. Federal Resource Conservation and Recovery Act (RCRA) Subtitle D requirements place restrictions on locating landfills near airports and in flood plains, wetlands, fault areas, seismic impact zones, and unstable areas.

Other federal agencies also have standards that may affect siting, such as Federal Aviation Administration (FAA) regulations intended to minimize bird hazards to aircraft.

Subtitle D regulations also establish national minimum standards for landfills that receive household waste. These national minimum standards are incorporated into each state's landfill permitting standards. However, states also may impose more restrictive criteria.

Under RCRA, the U.S. Environmental Protection Agency (EPA), Washington, D.C., regulates solid waste landfilling with regard to:

State regulations vary widely. Nevertheless, most states require a regulatory body approve landfill engineering plans. State regulations often specifically address geologic or other factors unique to a region.

In addition, state and local governments also regulate other landfill-related issues. These include conditional use zoning, highway issues (heavy loads and traffic), water discharge/water quality control, mining regulations (excavations), building permits, fugitive dust and emissions controls, and closure permits. All applicable regulations should be considered when planning a landfill.

Government standards generally can be classified into: engineering design standards and performance standards. Engineering design standards are similar to building codes, which describe how a facility must be built. For example, a new landfill may be required to have a 6-foot high fence surrounding the facility. Design standards are prescriptive and don't offer much flexibility.

Performance standards, on the other hand, are applicable over a facility's life and require a certain level of environmental control. For example, the state agency regulating groundwater quality may specify the maximum allowable contaminant concentration that may be present in the groundwater below or adjacent to the site. Site operators must incorporate the necessary control systems to comply with the groundwater standard. Performance standards allow for some design flexibility provided the standard is met.

Despite the pervasiveness of federal, state and local landfill regulations, these criteria are legally viewed as minimum performance requirements. Therefore, even if all the applicable regulations have been followed, if a problem arises, the landfill developer still may be liable for the legal consequences. Claiming compliance with regulatory standards has not been an effective defense against pollution damage claims.

To increase public acceptance and reduce long-term risk, many landfill developers now design facilities that exceed regulatory requirements in some key aspects.

Identifying Goals

Admittedly, the sanitary landfill design process is complex and lengthy [See “Sanitary Landfill Design Steps” on right]. Prior to physically designing the landfill, the developer should determine the facility's goals in cooperation with landfill users, regulatory authorities and neighbors. Clearly agreed upon goals may lessen public resistance to the site and may help formulate a more targeted design process.

Design goals for sanitary landfills could include:

Final Site Use

In addition to setting goals, deciding how the property will be used after landfill closure will help guide the site design. Good planning at the earliest possible stage will minimize cost and maximize the site's usefulness upon closure.

The landfill's final use should be compatible with nearby land use as well as the limitations of the landfill to support structures. Most closed landfills are used for recreational purposes, such as golf courses, nature preserves or ski hills. Consideration also must be given to existing landform compatibility, settlement allowances and drainage patterns. Anticipating future residential or commercial growth also will help deliberations.

General Design Considerations

Landfill design is communicated to regulatory officials through several documents. The landfill design package should include plans, specifications, a design report and an operator's manual — all of which are submitted to regulatory agencies. A general cost estimate also should be developed, even though it may be a rough estimate.

Plans and specifications typically include:

Additionally, a design report typically includes four major sections:

Waste Characteristics

Waste characteristics will provide important design information for determining operating procedures. Waste type affects the handling techniques, and waste quantity determines site lifetime, daily operating procedures and cover requirements. A waste characterization study should precede landfill siting work, but additional information may be needed while the facility is being designed. For example, certain waste types may be used as daily cover or onsite road base.

When preparing a profile of the wastes that will be received at the new landfill, pay attention to sources that may be unknowingly mixing hazardous waste with solid waste. In suspicious cases, hazardous waste testing procedures may be necessary. Systematic load checking during site operation also should be planned.

The types and number of vehicles that transport solid waste to the site should be tabulated, too. Traffic information will be useful for later analysis of roadways and access points.

Specifying Design Basis

The design basis is a tabulation of the general performance requirements that the facility must satisfy to achieve project goals. It includes the facility's capacity, waste flow rates, traffic counts and principal environmental controls. Tabulating the design basis in this manner communicates to the project design team and others, such as regulatory review specialists, about the nature and size of the proposed landfill.

The design basis may require revisions if unforeseen circumstances cause significant changes in the landfill plan.

Site Layout Development

The landfill's layout will be strongly influenced by the site's geology. The potential for gas and leachate migration and the suitability of the soil for landfill base and cover material should be a particular concern. The site layout begins with geotechnical information, which includes data on the surrounding site geology, hydrology and soils. This data usually is collected during the site selection process, then supplemented during subsequent investigation.

Soil-boring logs and other data describing subsurface formations and groundwater conditions are diagrammed to present an interpretation of the subsurface conditions at the planned site [See “Subsurface Conditions Along a Cross Section of a Landfill Under Construction”on page 54]. Soil-boring logs help to show the extent of each formation extrapolated between the boreholes. The depths to bedrock and the groundwater table also are shown. Many more boring logs and additional cross sections at regular coordinate intervals in several (minimum of two) directions typically are required to properly locate the waste disposal area within the developing site.

Preparation of Drawings

The base map usually shows the landfill location in relation to surrounding communities, roads and other features. A site map shows:

Contour maps show drainage patterns adjacent to and through possible disposal sites. Areas with excessive slope or direct overland flow from a site to surface waters must be carefully evaluated.

Subsurface formations and groundwater conditions will influence the landfill's design features in the leachate collection system and liner requirements. A formation's geotechnical characteristics will determine its suitability as a construction material.

Site plans should describe landfill development phases in chronological order. Landfills usually are planned to be developed, constructed and operated in phases of one to two years each. Dividing the project into phases minimizes the amount of open landfill surface and reduces the potential for precipitation to accumulate. As each phase is completed, that landfill portion can be closed and final cover material can be placed over the waste. Another phasing advantage is that it makes premature landfill closure, in the event of an environmental problem, more practical and economical. In a well-planned development, the landfill's end-use can be implemented in the completed sections while other areas remain open for disposal.

Concurrent with plan development for liners, covers, service roads and embankments, soil cut-and-fill balances must be calculated. The best designs minimize soil transfer at the site. Substantial volumes of earth will be required for cover material and possibly for liners.

Some regulatory agencies mandate screening berms or fence construction around the landfill's active areas. The extra soil needed for berm construction must be accounted for when planning excavation work. The berm height will depend on the sight lines into the landfill from adjacent areas.

When practical, design phases should be laid out so that excavated soil is immediately used. When stockpiling is necessary, the work should be organized so that stockpiled soil is undisturbed until needed or used to surcharge completed areas. Stockpiled soil should be covered whenever possible to prevent wind and precipitation erosion.

After completing the phasing diagrams and earth work balances, a table should be prepared to summarize the waste disposal and earth volumes that will be contained within each landfill phase.

Determining Working Face and Phase Dimensions

The operating plan should detail the configuration of the landfill's working face and helps to define terms [See “Subsurface Conditions Along a Cross Section of a Landfill Under Construction” on page 54 for a typical cross section of a portion of a municipal landfill, including the working face]. The “working face” is the area presently being worked, with new refuse being deposited and compacted into it. Once the working face has been completed and daily cover material provided, it is a completed cell, or “daily cell.” A “lift” is composed of the adjacent daily cells that form one layer of the landfill. Lift thicknesses generally are 8 to 20 feet. Larger landfills that accept more refuse per day have higher lift thicknesses. “Daily cover material” [See “Subsurface Conditions Along a Cross Section of a Landfill Under Construction” on page 54] is applied over the working face and can extend over the horizontal surface at the top of each daily cell, depending on how long the cover will be exposed to the environment. If the landfill is not expected to receive additional wastes, closure must begin within 30 days of the final receipt of waste. This closure requirement ensures that a proper cover is installed at the landfill.

The minimum width of the working face or daily cell should be at least wide enough to accommodate as many trucks or vehicles as are expected to be at the landfill. Typically, 10 to 15 feet per truck or vehicle are used in designing. Clearly, it is not a good operating practice to have extremely wide working faces to accommodate the peak flow of trucks that may occur once or twice a day. A tradeoff must be made between the width of the working face and the area needed to queue vehicles entering the site during peak hours.

The working face should remain as small as possible to avoid attracting birds and creating visual problems for passersby, and to contain blowing paper. Keeping freshly deposited refuse in a well-defined and small working face is a good indication of a well-operated landfill.

Phase Diagrams

The site plan should chronologically illustrate the developing landfill's features. The landfill's end-use can begin on completed sections while other areas in the landfill still are being used for disposal.

Phasing diagrams show the landfill's evolution at different stages through the site's life [See “Landfill Construction Plan: Intermediate Phase” above]. Phases should be developed for key times in sufficient detail to ensure that the operator knows what is to be done at any point. The engineers and management must be assured that the site is proceeding according to plan so that contracts can be let or finances arranged for construction. Regulatory bodies also must be assured that landfill operators are following the plan and that the site will be completed as designed at the agreed-upon time. The dimensions of each phase are determined by several factors. Generally, each phase accommodates 2 to 3 years of refuse volume.

Phil O'Leary and Patrick Walsh are solid waste specialists at the University of Wisconsin-Madison. Lesson 8 will discuss landfill slope stability and seismic issues.

Sanitary Landfill Design Steps

Source: Conrad, et al., 1981 with additions by the authors, Solid Waste Landfill Design and Operation Practices, EPA Draft Report, 1981