LANDFILL BASICS: closure

Neal Poteet and David Pelham

Landfill closures are increasingly common these days, but that doesn't mean the process has become any less complicated.

The first step is preparing a conceptual design that meets current regulations, then submitting a closure permit application to the appropriate regulatory agency. The application should include plans, general specifications, quality control measures and other technical details.

After the permit has been granted, complete the final, detailed construction design and accept construction bids. Construction should be closely monitored by the design engineer and a third-party quality control inspector. When construction is completed, detailed documents on quality control measures and results must be prepared and submitted to the appropriate regulatory agency(ies) for final approval.

Once the project closeout is completed and the regulatory authorities have approved the closure, many jurisdictions place a notation on the property deed as future notification of the closed landfill's existence. Long-term care then begins and must continue for 30 years under federal and most state requirements.

Permitting Involvement The engineer should thoroughly understand the owner's needs from the start. Prior to conceptual design, discuss areas that could lead to overextended scope and cost overruns. It's important to get opinions on design concepts from different individuals and departments involved with the project, particularly for a public project. The engineering department may want a traditional, cut-and-dried design, while the owner's operations department may want something entirely different. These needs must be resolved prior to fee negotiations so that the owner receives the desired end product and the consultant completes the project within budget limits.

Regulatory involvement can also prevent surprises and save time and money during permitting. Regulators should be informed of design concepts prior to the design phase. While regulators can be reluctant to approve a concept if it is not binding, the agency can inform the consultant of possible problems dealing with internal, often unknown policies.

Closing a landfill involves three major design elements: slope stability, drainage and gas controls. Slope stability is usually defined as the general stability of the slope and landfill mass as well as the soil/cap interface stability. Proper slope design is necessary for an environmentally sound closure; otherwise, the owner and the design consultant can face negative publicity. Have a geotechnical expert evaluate its design prior to final design.

During the preliminary design phase, review the choice of the cap material. If the site is near acceptable clay sources, a clay cap might be economically competitive with synthetic materials. In this case, the designer might consider bidding the project with one of the materials as an alternate bid item to increase competition for the suppliers. Otherwise, synthetic materials will probably be more economical due to hauling costs and third-party quality assurance testing of multiple clay layers.

The side slope angles determine the type of synthetic materials needed. Unless the side slopes are flat, the liner surface should be textured. When designing the cap, the interface angle analysis must include the soil in contact with the liner and the degree of saturation.

Soil testing with liner samples and readily available soils under expected saturation conditions is strongly recommended. If soil sources and characteristics are unknown, an engineered soil layer must be incorporated into the design, which will increase construction costs. The project specs should include the desired interface friction angle at the appropriate degree of saturation, depending on climatic conditions and normal stress loading.

Since force-fitting the site to a neat design isn't always feasible, a drainage engineer is crucial for sizing and locating the drainage elements of a closed landfill. The drainage design usually dictates other design aspects and defines much of the site's character.

Erosion controls also require special consideration. Sod, seed and mulch, hydroseed or seed/matting combinations can be planted as surface vegetation. Flat-topped slopes allow greater flexibility in vegetation selection, while side slopes require immediate protection, usually with sod. Although it is expensive at 10 to 15 cents per square foot, sod has an immediate impact on post-closure maintenance.

With EPA's landfill gas control regulations looming, landfill gas design will become a key element of site closures. Since most states have minimal regulations on gas designs, some engineers use EPA's guidelines on spacing the extraction wells. These guidelines may result in more wells than necessary, but it prevents the risk of damage to a synthetic liner when installing additional wells later. This should be discussed with the client prior to final design.

The orientation of the wells also requires careful attention. The landfill operator can install horizontal gas vents, which generally are less expensive than vertical wells installed by a contractor. However, some operators do not want the additional responsibility of installing horizontal vents.

Vertical wells are typically constructed of PVC or HDPE pipes. Contractors typically choose HDPE pipe because they can butt-weld the joints at a staging area and deploy each vent to the installation location as one unit.

Whether clay or synthetic liner caps are installed, each gas vent requires a sealing boot. When using clay caps, a synthetic apron should be placed around the vents to minimize air intrusion at the clay/vent interface. The boot is then welded to the liner or apron and clamped to the vent above the ground surface. Using above-grade clamps allows for adjustments during the post-closure care period to compensate for landfill settlement.

The project manual - which includes the notice for bidding, instructions to bidders and contract with conditions and specifications is a project's bible and should be assembled carefully. Overall success depends not only on correct design, but also on the proper development of specification sections complementing the design. Add to the standard contract supplementary conditions that reflect the specific project needs. All applicable permits should be included in the manual.

It's important to have the engineer on site during construction. Conditions change rapidly and problems or discrepancies can become the source of disputes. The engineer can help the contractor make decisions and address problems that could escalate into construction claims.

Documentation plays a significant role in a landfill closure construction project and is helpful if a claim is filed. An owner or engineer's defense may depend upon the ability to provide proper documentation. Construction record-keeping should be a regular practice, preferably in a standard format to ensure consistency. Daily records should include construction equipment on site, work performed, daily labor force, material deliveries, progress meetings, tests, telephone conversations and correspondence logs.

A Critical Path Method Schedule (CPM) is an essential contract requirement to evaluate monthly pay requests and analyze project work elements. The CPM should be submitted prior to construction to allow sufficient time for review and approval before the contractor's first pay request.

The reality of claims within the construction industry is an unfortunate component of many construction projects, and landfill projects are not immune. The project team should establish procedures prior to the start of construction for dealing with claims. Include contract provisions outlining the contractor's claims procedures. Define time stipulations and general guidelines regarding claim analysis and what the contractor may claim. This subject should be reviewed by legal counsel.

Clearly defining the roles and levels of responsibility of the engineer, designer and third-party quality-assurance teams will ensure total coverage. For proper maintenance, documentation from all parties (including the contractor's daily reports, sub-grade preparation, liner testing, concrete testing, etc.) should be funneled to a single source such as the engineer. The approved construction quality assurance plan should be included in the project manual, making this document part of the construction contract. The contractor's responsibilities regarding specific quality assurance requirements must also be defined.

When completing the contract and closing out the project, typical problems can be minimized with proper planning. All warranty requirements should be carefully listed within the individual sections of the specifications. Misunderstandings over warranties and record drawings can delay the engineer's certifications, which could prolong project closeout and regulatory approvals. The project manual should contain specific closeout procedures to ensure the project's successful completion.

A successful landfill closure project depends on clear communication and cooperation between the design engineer, regulatory agency(ies), owner, contractor and third-party quality-assurance inspection team. Clear procedures and conditions in a well-written project manual, along with regular documentation of conditions and progress during construction and closeout, are the keys to landfill closure.