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 4.2.7.8
If a canyon floor is reasonably flat, the first fill in a
canyon site may be carried out using the trench method operation discussed
previously. Once filling in the flat area has been completed, filling starts
at the head end of the canyon and ends at the mouth. An important considera-
tion is that since the canyons and ravines are formed by water erosion,
landfilling may involve a water course. This practice prevents the accumula-
tion of water behind the landfill. Wastes usually are deposited on the canyon
floor and from there are pushed up against the canyon face at a slope of about
2 to 1. In this way, a high degree of compaction can be achieved. Compacted
densities as high as 1200 lb/yd3 have been reported. Even higher densities
have been recorded in the lower portions of the landfill as the height of the
fill increases.
4.2.7.9 Pit and quarry landfill sites are always lower than the
surrounding terrain, so control of surface drainage is often the critical
factor in the development of such sites. Also, borrow pits and quarries
usually do not have adequate soil or geological properties for landfilling
because they display high permeability and fracturing. As with canyon sites,
pit and quarry sites are filled in multiple lifts, and the method of operation
is essentially the same. A key to the successful use of pits or quarries is
the availability of adequate cover material to cover the individual lifts as
they are completed and to provide a final cover over the entire landfill when
the final height is reached. Because of settlement, it is usually desirable
to fill pit and quarry sites to a level slightly above that of the surrounding
terrain. The depression method is also not readily amenable to liners and
leachate collection system.
4.2.7.10 Evaluation of Seepage Potential (Tchobanoglous, Theisen,
and Eliassen 1977). Core samples must be obtained to evaluate the seepage
potential of a site that is being considered for a landfill. Sufficient
borings should be made so that the stratigraphic formations under the proposed
site can be established from the surface to (and including) the upper portions
of the bedrock or other confining layers. At the same time, the depth to the
surface water table should be determined along with the piezometric water
levels in any bedrock or confined aquifers that may be found.
4.2.7.11 The resulting information is then used to (1) determine the
general direction of groundwater movement under the site, (2) determine
whether any unconsolidated or bedrock aquifers are in direct hydraulic
connection with the landfill, and (3) estimate the vertical seepage that might
occur under the landfill site.
4.2.7.12 Drainage and Seepage Control Facilities. In addition to
the seepage analysis , it is also necessary to develop an overall drainage
plan for the area that shows the location of storm drains, culverts, ditches,
and subsurface drains as the filling operation proceeds. In some cases it may
also be necessary to install seepage control facilities.
4.2.7.13 To ensure the rapid removal of rainfall from the completed
landfill and to avoid the formation of puddles, the final cover should have a
slope of about 1%. Where relatively impervious cover material such as clay is
used, lesser slope values may be feasible. The theoretical amount of water
that could enter the landfill per unit area in a 24-h period for various cover
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