Environmental Engineering Reference
In-Depth Information
Figure 17.11.
Slope modification in the cutoff foundation to reduce differential settlement and cracking
of the earthfill core.
Figure 17.12.
Slope modification and seam treatment in the cutoff foundation for interbedded sand-
stone and siltstone, near horizontal bedding.
-To allow maintenance of positive pressure of the earthfull on the abutment;
-To limit cracking of the earth core due to differential settlement over large discontinu-
ities in the abutments, e.g. as shown in Figure 17.11.
To allow earthfill to be compacted and to maintain positive pressure on the abutments,
steep small scale foundation surfaces should be flattened to about 0.5 H to 1 V either by
excavation or by backfilling with concrete. Figures 17.12 and
17.13
give examples of this
type of treatment. Some authorities require a flatter slope, e.g. Thomas (1976) and
Wallace and Hilton (1972) suggest the use of 0.75 H:1 V. Others, e.g. Acker and Jones
(1972), accept steeper slopes (0.25 H:1 V). USBR (1984) suggest 0.5 H:1 V. The policy for
Thomson dam was to be guided by the persistence of any slopes, particularly with slopes
within
30° to the upstream-downstream direction for which 1 H:1 V slope was sought.
Significantly high slopes were made not steeper than 0.75 H:1 V. Sharp changes in grade
near the tops of any slope were “rounded out”. Thomson is a very large dam and it might
be argued greater conservatism is warranted in these cases. The authors are of the opin-
ion that 0.5 H:1 V is satisfactory provided the earthfill is compacted wet of optimum with
rubber tyred equipment used to “squeeze” it into position.
In some geological environments these requirements for slope modification may neces-
sitate virtually the whole of the cutoff area being excavated or covered with concrete, e.g.
Figure 17.4
. It should be noted that in some cases there will be tendency for the rock to
pluck out on joints, repeating the oversteepening problem. In these cases backfill concrete,
or possibly presplitting with light blasting, may be necessary.
Large scale slope modification such as that shown in Figure 17.11 to avoid differential
settlement and resultant cracking of the earthfill core has been used on many dams, e.g.
