AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
The structure of the landfill cover varies through the different stages of landfill operations. The
initial phase of waste cover is daily cover, a layer (usually about six inches thick) that is placed on
top of the active fill area at the end of each working day. Soil is typically used as daily cover,
but other types of materials may be used, such as textile covers (manmade fabrics or tarps rolled
over the top of the fill area) or stabilized organic materials such as wood chips, shredded green
waste, and compost. Intermediate cover is a thicker layer of soil (typically 12 inches) that is
applied to inactive areas of a landfill that will not be used for an extended period of time (e.g.,
several months or years) but are not ready to be permanently closed. The final cover system is a
more structurally complex, engineered cover system installed once a landfill or portion of a
landfill has been completed and permanently closed. Under Subtitle D, the final cover system
must meet a prescriptive low permeability cover design or an alternative cover design that
complies with performance-based goals. The final cover system is engineered to tie into the liner
system around the perimeter of the landfill, providing effective containment of the waste. This
section focuses on the containment attributes and environmentally protective management
components of final cover systems.
3.4.1 Design of Final Cover Systems
Cover construction will reflect climatic and site-specific performance goals, but all engineered
cover systems are designed with components to fulfill the following three primary functions:
• Infiltration Control: Soil and/or geosynthetic layers (termed the “barrier layer” in
prescriptive cover systems) are designed to control infiltration of rainwater or snow;
• Control of Surface Water Drainage: A soil or geosynthetic drainage layer is designed to
divert surface runoff above the infiltration control layer in a controlled manner to minimize
ponding and to promote flow to a constructed conveyance ditch; and
• Vegetative Support: An uppermost, organic-rich soil layer is designed to promote
vegetative growth to resist erosion, optimize slope stability, and provide conditions
consistent with the surrounding natural ecosystem if possible.
Subtitle D provides a prescriptive cover system design. However, because the
regulation mandates environmental performance in final cover system design, various
types of cover system designs can be used to fulfill the primary functions listed
above. Some alternative cover designs not only provide protection of the
environment equivalent to a Subtitle D prescriptive cover, but offer additional benefits, including:
• Methane Destruction: An uppermost, oxygen-rich soil layer can be very effective at
reducing greenhouse gas (GHG) emissions to the atmosphere by natural oxidation (i.e.,
biological conversion of methane to carbon dioxide and water); and
• Enhanced End Use Flexibility: Some alternative final cover systems with enhanced
vegetative growth can offer improved long-term slope stability and erosion resistance
MD10186.doc 45 29 March 2009