Environmental Engineering Reference
In-Depth Information
the properties desired for a pervious rockfill zone”. As pointed out by Cooke (1993) the
purpose of the specification is to be able to reject loads of predominantly soil or rock with
large amounts of fines.
Sherard and Cooke (1987) point out that there is no technical need for rock in the rock-
fill to have a high compressive strength. In their experience, rockfills constructed of rocks
with compressive strength of 30-40 MPa are no more compressible than those of higher
strength. However as discussed in Section 15.2.3 the rock strength does have some influ-
ence on the rockfill modulus.
Rocks with very high compressive strength lead to higher quarrying cost and wear on
equipment. They conclude that any rock with a (soaked) unconfined compressive strength
of 30 MPa or more is adequate. Lower strength rocks may be used with special zoning
provisions as discussed in Section 15.5.1.
15.2.2.3 Use of gravel as rockfill
Cooke (1984) discusses the use of gravel (really sandy gravel or sandy gravel with some
silt) as rockfill and maintains that, if available, this material can be very suitable. He
points out that:
- Gravel is often more economically handled than rockfill (lower excavation and loading
cost, and less wear on tyres and rollers);
- Compacted gravels commonly have high modulus of compressibility, up to 5 to 10
times that of some compacted rockfill. In his experience since face slab movements vary
roughly inversely with the modulus, and directly with the square of height of the dam,
gravel fills are desirable for higher dams;
- If a significant fines content is allowed, then chimney drains, abutment drains, filters
and intermediate drainage layers will be required.
Mori (1999) recommends that alluvial gravel should be used if possible in the upstream
third of the dam to limit deformation and cracking of the face slab.
Amaya and Marulanda (1985) describe the use of gravel for the 125 m Golillas Dam in
Colombia. In this dam the bulk of the fill (i.e. Zone 2 in Figure 15.5) was natural river
gravel. The grading is shown in Table 15.5 .
Figure 15.5.
Golillas Dam (Amaya and Marulanda, 1985, reproduced with permission of ASCE).
 
 
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