Environmental Engineering Reference
In-Depth Information
16.6.3.
Desiccation
Desiccation of CCLs is induced by a change in the water content of the clay that
can occur: i) after construction of the clay liner and before placement of the
geomembrane; ii) after placing the geomembrane and before covering with waste;
and; iii) after placement of waste [ROW 98]. Desiccation due to water evaporation
before the placement of waste or a geomembrane on the CCL can readily be
prevented by adopting appropriate construction procedures and, if it does occur, can
be observed and rectified by removal and replacement of the cracked portion.
A high geomembrane temperature has the potential to cause: i) evaporation of
water from the compacted clay into any air space between the clay and the
geomembrane (e.g. wrinkles and waves); and ii) moisture movements from the
region of higher temperature to the region of lower temperature [ROW 98]. The
aforementioned phenomena can be particularly critical along landfill slopes and in
the case of heating and cooling cycles. In order to avoid desiccation of CCLs after
placement of the geomembrane it is important to avoid, as much as possible, the
formation of wrinkles in the geomembrane and to cover the geomembrane as soon
as possible with the protection layer and/or leachate collection layer [ROW 98].
The temperature of the waste in close contact with the landfill lining system can
have a significant effect on: the rate of clogging in the leachate collection system;
service life of geopipes and geomembrane liners; diffusion through low permeability
liners; and on the desiccation problems of CCLs [ROW 98]. The temperature of the
waste body appears to be related to the water content of the waste and the level of
leachate mounding. This highlights, once again, the importance of the design,
construction and operation of the leachate collection system.
The risk of CCL desiccation after waste placement depends on: the properties of
the clay; properties of the underlying subgrade; overburden pressure; temperature
gradient across the liner; and depth of groundwater [DOL 97, HOL 89, HOL 95,
ZHO 03]. The basic influencing properties of the clay are: the initial saturation
degree; shear strengths in both drained and undrained conditions; and the matric
suction. The thickness of the unsaturated zone below the liner, its unsaturated
hydraulic conductivity and the capillary rise together with the thermal gradient can
play a fundamental role in the potential desiccation of CCLs.
Finally, it is important to underline the fact that the potential water movement
due to a temperature gradient can be controlled by installing a geomembrane below
the CCL as well as above it [ROW 98]. Moreover, it is also important to note that a
drainage layer below the CCL that is not protected by a geomembrane can increase
the potential for desiccation in two ways. First, risk of desiccation is increased by
preventing the movement of underlying groundwater upward into the CCL and
