30 Sep 01
be clearly stated in the specification. Cleared and grubbed material is often placed beneath the landfill
cover in the lower regions of the random fill layer.
B-4. Geotechnical Design.
a. Settlement .
(1) General. Settlement of the cover system is the result of consolidation of both the waste
material and the foundation soils. Settlement is attributable to relocation of soil particles and landfill
debris, physicalchemical changes from corrosion and oxidation, and bio-chemical decomposition. The
magnitude of settlement depends on a number of factors, including the following:
Thickness of waste or refuse.
Type of waste (e.g., construction debris and municipal refuse).
Density or void ratio of wastes.
Amount of decomposable materials.
Weight of final cover.
Type of foundation soils.
Stress history (landfill operational history).
It is important to note that many sites require a combination of remedial technologies. For example,
ground water pump and treat systems are often used in conjunction with final covers. If pump and treat
systems are part of the remedial action, the effect of lowering the water table needs to be considered in
the settlement analysis. Lowering the water table will create higher effective stresses in the previously
saturated strata, which may result in a greater degree of consolidation of these soils and larger settlement
of the landfill cover.
(2) Results of Settlement . Excessive total or differential settlement of the cover system can have
the following effects:
Increased permeability of the clay layer because of cracking.
Slope instability because of steepened side slopes.