Environmental Engineering Reference
In-Depth Information
Figure 17.9.
Cutoff trench details.
It is not always necessary to treat the whole of the contact between the earthfill core and
the foundation as “cutoff”, and only part of that area is excavated to cutoff foundation
requirements. Figure 17.9 shows a central core earthfill embankment and for this and
other earth, and earth and rockfill embankments the following guidelines are given.
17.4.1
Cutoff width W
For a cutoff in rock, the width depends on the quality of the rock, e.g. for low permeability
non erodible rock, W/H may be as low as 0.25 but would usually be 0.5 or more. There have
been cases where W/H
0.05 to
0.1 is quite common (Fitzpatrick et al., 1985). For cutoff foundations on more permeable
erodible rock W/H may be taken as the full core width, commonly
0.1 has been used. For concrete face rockfill dams W/H
0.5 H and up to 1 H.
For large high consequence of failure dams it is common to be conservative and the full
core contact is taken to cutoff foundation conditions regardless of the rock foundation.
The cutoff width should be not less than 3 m even for small dams (the width of exca-
vation and compaction equipment) and preferably 6 m.
For a cutoff in soil, it would be common to make W/H
1. The idea is to give a lower
seepage gradient for these more erodible conditions.
It is emphasised that, if the depth to an acceptable cutoff is excessive, these guidelines
may be waived, provided other measures are adopted, e.g. multi-line curtain and consoli-
dation grouting, cutoff walls constructed under bentonite and erosion protection in the
base and sides of the cutoff.
17.4.2
Batter slope
The batter slopes for the cutoff trench should be:
- Not steeper than 0.5 H:1 V, and preferably 1 H:1 V so that the earthfill can be com-
pacted against the sides of the trench;
- Sufficiently flat to avoid arching effects in the cutoff trench. As a guide, if h/W > 0.5,
the batter slopes should be reduced to 1 H:1 V or 1.5 H:1 V;
- The batter slopes may be determined by stability during construction, e.g. by joints in
rock, the strength of the soil or the presence of groundwater. For soil, it would not be
uncommon to require batter slopes between 1 H:1 V and 1.5 H:1 V and possibly flatter.
17.4.3
Setting out
For setting out and construction purposes it is desirable to position the cutoff trench a fixed
distance from the dam centreline, or by means of a series of fixed points.
Figure 17.10a
