Environmental Engineering Reference
In-Depth Information
Figure 9.31.
Construction of inclined chimney drain by placement on downstream slope of earthfill.
Figure 9.32.
Filter zone placement ahead of other zones which is generally desirable.
filter during placement can be reduced by spreading it from a movable steel plate
placed adjacent to the trench (Charlton and Crane, 1980). This also reduces the risk
of collapse of the trench under the surcharge load of the filter material.
(e) For smaller dams, an inclined chimney drain can be constructed by dumping the filter
on the trimmed downstream slope of the earthfill core as shown in Figure 9.31. The
filter can be compacted by rolling up the slope or by running rubber tyred equipment
up against the slope. The downstream earthfill (or rockfill) is then placed in layers
adjacent to the filter. In this way thin layers of filter (say as thin as 0.3 m normal to the
slope, 0.5 m horizontal) can be placed.
Horizontal filters can be placed in layers as thin as 150 mm or as thick as about 500 mm
after compaction (the upper limit being determined by compaction requirements).
However, for thick layers of filters (e.g. 400 mm), it is considered preferable to place
2
400 mm layer since, if segregation occurs during
placement, it is less likely that coarse zones will coincide with each other in the two
layers.
200 mm layers rather than 1
9.6.2
Sequence of placement of filters and control of placement width and thickness
Filters generally should be placed ahead of the adjacent earthfill or rockfill zones as
shown in Figure 9.32.
This is desirable because it allows good control of the width of the filter zone compared
to the specified width and reduces the risk of contamination of the filter zone with mate-
rials from the adjacent zones and from water eroding off adjacent areas generally.
Figure
9.33
shows the alternative which is not recommended by the authors or such organisa-
tions as USBR (1987). However, both systems are used successfully by experienced
contractors.
