Environmental Engineering Reference
In-Depth Information
'U' beams
Plow blade fitted with cutting
edge on this side only
3m
SIDE ELEVATION
Fine rock zone side
Clay core side
REAR ELEVATION
Chain with hooks
at end for drawing
spreader
Cross bracing
Height of screeded filter 75mm above
required compacted thickness. Width
of spreader set 150mm greater than
the specified width of filter.
4.5m
PLAN
Figure 9.29.
Typical spreader box and screed (adapted from Charlton and Crane, 1980).
Figure 9.30.
Construction of vertical chimney drain by excavation through earthfill.
manufacture of filters is high and there is a need to keep widths to a practical minimum.
The possible exception is the rare occasion when naturally occurring sand and gravel
deposits satisfy filter design requirements and are therefore relatively inexpensive.
The following is offered as a guide to practical minimum widths:
(a) Filters upstream or downstream of an earth core, when constructed by end-dumping
off a truck should be at least 2.5 m and preferably 3 m wide.
(b) If a spreader box such as that shown in Figure 9.29 is used, a minimum width of
1.5 m is practicable. The filter material is dumped off the truck into the spreader box,
which spreads the filter out of its base as it is pulled along by a small bulldozer.
(c) If filter materials are very scarce or high cost, formwork can be used to contain bands of
filters as narrow as one metre. Sherard et al. (1963) show an example of such placement.
This is very unusual and would only be contemplated in exceptional circumstances.
(d) For homogeneous or zoned earthfill dams with a vertical chimney, a relatively narrow fil-
ter (as narrow as 0.75 m or 1.0 m) can be constructed by placing the earthfill for up to 2 m
over the filter layer, and then excavating through the earthfill with a backhoe or excava-
tor to expose the filter, as shown in Figure 9.30. Careful cleanup of the surface of the
exposed filter is necessary and the filter is compacted with small vibrating sleds or other
compaction equipment. The depth “h” is best limited to say 1.5 m to reduce the risk of
collapsing of the trench and allow access of men into the trench. Contamination of the
