Environmental Engineering Reference
In-Depth Information
roughly 1 m layers and compacted only by construction traffic, did not start to ravel at
any stage, except for the odd stone moved at abutment-dam contact lines. This behaviour
of this coffer dam showed clearly how well an embankment of quality rockfill, “com-
pacted” in lay layers, will cope with extreme conditions without overtopping. In this case,
the dam was deliberately breached before it overtopped.
15.2.2.2
Layer thickness and compaction
Zones 3A and 3B are placed in layers of the order of 1 m and 1.5 m to 2 m thick respec-
tively, which results in a gradation of permeability and a lower modulus for Zone 3B,
which is acceptable since the water load is largely taken by Zone 3A. Rolling for good qual-
ity rockfill is usually by 4 (up to 8) passes of a 10 tonne vibratory steel drum roller. Cooke
(1993) indicates there is no evidence that rollers heavier than 10 tonne give better results.
To avoid embankment deformation Mori (1999) notes that based on experience at Xingo
(140 m high), Aquamilpa (180 m high) and Tianshengqiao 1 (180 m high). Zones 3A and
3B should not have a very different moduli of compressibility, should have a near vertical
interface between them and a high embankment should not be built in stages. This is dis-
cussed in Section 15.6. Cooke (1993) suggests that lower strength (
30 MPa) rockfill
should be compacted in thinner (0.6 m to 0.8 m) layers to encourage breakdown to give
greater “strength” (a higher modulus).
Hydro Electric Commission of Tasmania (HEC) (Carter et al., 2000) place Zone 3A in
1m layers for good quality rockfill and 300 mm to 600 mm layers for lower quality but
well graded material. Zone 3B is compacted in 1.5 m layers to take large rock.
Zone 2E is required to act as a filter to Zone 2D, and is therefore placed in relatively
thin layers (0.4 m to 0.5 m, the same as Zone 2D). This also ensures a high modulus.
Zone 2E is obtained by selecting suitable material in the quarry, or by passing rockfill
over a grizzly. The filter requirement (compared to Zones 2D and 3A) is usually readily
achieved by this process, and further treatment is not required.
The rockfill grading in Zone 3A (and to a lesser extent in Zone 3B) is important. A well
graded rockfill will compact to a higher modulus than a poorly graded fill, and the
amount of fines needs to be limited if the rockfill is to be truly free draining.
Cooke (1984) suggests that the grading specification should be:
- Maximum size shall be that which can be incorporated in the layer and provides a rel-
atively smooth surface for compaction;
- Not more than 50% shall pass 25 mm sieve;
- Not more than 6% shall be clay size particles (this is taken to mean silt and clay).
Cooke (1993) gives not more than 10% finer than 0.075 mm).
In Sherard and Cooke (1987), it is suggested that it is better to specify:
- Not more than 20% finer than 4.76 mm;
- Not more than 10% finer than 0.075 mm (i.e. silt and clay).
They suggest that if the rockfill has a higher proportion of fines, “the final evaluation
of suitability can be made on the trafficability of the rockfill surface when the material is
thoroughly wetted. A stable construction surface under travel of heavy trucks demon-
strates that the wheel loads are being carried by a rockfill skeleton. An unstable construc-
tion surface, with springing, rutting, and difficult truck travel, shows that the volume of
soil like fines is sufficient to make the rockfill relatively impervious. Where the surface is
unstable, the fines dominate the behaviour and the resulting embankment may not have
