Environmental Engineering Reference
In-Depth Information
the availability of rock which can be quarried to yield free draining rockfill often leads to
economic dam design.
Most igneous and many metamorphic rocks (even low grade metamorphics), e.g. gran-
odorite, diorite, granite, basalt, rhyolite, andesite, marble, greywacke quartzite and some
indurated siltstones and sandstones will, when fresh, yield free draining rockfill.
Some metamorphic rocks, e.g. phyllite, schist, gneiss and slate, may break down under
compaction to yield a poorly draining rockfill even though it may be dense with a high
modulus. The amount of breakdown depends upon the degree to which foliation or cleav-
age is developed in the rocks.
Most highly weathered, and many moderately weathered igneous and metamorphic
rocks, will not yield totally free draining rockfill.
The spacing of the bedding and joint planes influences the size and grading of rockfill
obtained from a quarry. Blasting may be varied to yield the required sized product, but
this is not always practicable.
Thick beds of sandstone within a sequence of thinner bedded siltstone and sandstone
are likely to yield oversize rock, which would require either secondary breaking in the
quarry or sorting and disposal on the embankment.
Often a substantial amount of the rockfill for a dam will come from “required excava-
tions”, i.e. from the spillway, foundation for the dam, inlet and outlet works etc. This is
in principle desirable as the effective cost of the rockfill could be only the cost of place-
ment (and any additional haulage costs). However, the rock quality from these excava-
tions may not be ideal (due to rock type, weathering, method of excavation) necessitating
changes to the embankment zoning to accommodate the material. Also the timing of pro-
duction of rock from required excavations may not be ideal from the viewpoint of dam
construction and in the event, may not be as scheduled. Some flexibility in zoning is desir-
able to allow for such circumstances. Such flexibility is also useful to allow for different
quality of rock being obtained from required excavations from that anticipated at the
time of designing the embankment.
Most sedimentary rocks, e.g. sandstone, siltstone, shale and mudstone generally tend to
break down under compaction, even when fresh, and yield poorly draining rockfill. In
these circumstances it may be necessary to incorporate zones of free draining rockfill to
ensure the embankment rockfill as a whole is capable of remaining free draining, e.g.
Figures 8.9 and 8.10 .
The use of so called “random” rockfill zones in a dam section also facilitates use of
rock, which does not yield free draining rockfill.
8.4.1.3 Filters and filter drains
A source of high quality sand and gravel is necessary for construction of filters. It may be
necessary to obtain these materials from many kilometres' distance (50 km is not unusual),
despite the haulage costs.
Filter aggregates may be obtained from alluvial sand and gravel deposits, or from quar-
ries. Generally suitable aggregates are of igneous and, less commonly, metamorphic ori-
gin. It is unusual to manufacture filters from sedimentary rocks, as these rocks are usually
not sufficiently durable and often have poor shape (as measured by flakiness index).
For large dams it is usually necessary to establish a separate crushing and screening
plant for manufacture of filter and concrete aggregates. It is sometimes necessary to let a
separate early contract to begin the manufacture and stockpiling of aggregates, as their
production rate controls dam construction progress.
Where sources of filter aggregates are far from the dam and for this or other reasons fil-
ters are expensive, the width of filter zones may be reduced by using spreader boxes (see
Chapter 9).
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