Environmental Engineering Reference
In-Depth Information
- The rockfill should have a high enough modulus after compaction in the dam to limit
face slab deflections under water load to acceptable values. Creep of the rockfill should
also be small enough to avoid excessive longer term settlements;
-It should be readily available as a quarry run product with a minimum of wastage of
oversize or undersize rock.
A wide range of rock types have produced satisfactory rockfill including granite, basalt,
dolerite, quartzite, rhyolite, hornfels, limestone, gneiss, greywacke, andesite, welded tuff
and diorite. Rocks such as sandstone, siltstone, argillite, schist and shale have been used
but in some cases produce a non-free draining rockfill. This is discussed further in section
15.5.1. Gravels have also been used with success and as discussed below, can lead to very
high modulus fills. Cooke (1984) suggests that “if blasted rockfill is strong enough to sup-
port construction trucks and the 10 tonne vibratory roller when wetted, it may be consid-
ered to be suitable for use in compacted rockfill”. He goes on to point out the need for
drainage zones if the resulting rockfill is of low permeability.
Penman (1982) and Penman and Charles (1976) suggest that for rockfill to be consid-
ered “free draining”, it should have a permeability of at least 10
5
m/sec based on
in situ
tests in the rockfill. This is based on the requirement for adequate permeability to dissi-
pate construction pore pressures in the rockfill, rather than having a high water discharge
capacity from a leaking face slab.
The authors are satisfied rockfill is free draining if water will flow rapidly from a test
pit which penetrates the full layer thickness of the rockfill, when pumped in from a water
tanker.
As discussed by Cooke (1984) and Sherard and Cooke (1987), rockfill placed in the nor-
mal way, i.e. dumped from a truck, and spread by a bulldozer, will result in segregation, with
the coarser particles collecting at the base of the layer, and the finer rock and fines on the sur-
face. Breakdown of the upper part of the layer during rolling creates even more stratifica-
tion. Cooke (1984, 1993) and Cooke and Sherard (1987) point out that far from being a
problem, this stratification is desirable because:
-Tyre wear on trucks is reduced and the smoother surface allows more rapid truck
travel;
- The smooth surface facilitates the rolling operation, spreading the vibrating load and
reducing roller maintenance compared to an irregular rocky surface;
- The lower parts of layers have a high horizontal permeability, facilitating drainage of
leakage or embankment overtopping during construction. The resulting average hori-
zontal permeability is much higher than if the fines were distributed uniformly through-
out a layer;
- The layers create a variation in vertical permeability which will prevent buildup of pore
pressures in the rockfill.
It should be noted that this stratification means that “free draining rockfill” may allow
water to pond on the surface of layers, provided the lower parts have higher permeability.
As pointed out by Cooke (1984), there is no need to scarify the surface of layers of com-
pacted rockfill prior to placing the next layer.
The fourth author was shown in 1997 a video of a 30 m high rockfill (quarried) coffer
dam in Brazil that had been built in 3-4 days, the crest just keeping ahead of a rising
flood. At the start of the coffer dam construction, an impervious zone had been placed on
the upstream face, but with the advert of a flood, the contractor decided to push ahead
with the coffer dam as a simple rockfill dam. The video showed clearly that seepage
appeared on the downstream side from the lower half of each layer. The rockfill placed in
