Environmental Engineering Reference
In-Depth Information
remedial works being done, the upstream edge of a crest could be seriously threatened,
particularly in dams with continuously high storage level.
13.2.1.4
Rock quality and quarrying
The rock used for rip-rap must be able to withstand the repeated mechanical abrasion and
wetting and drying action of the waves.
Rocks in the strong to extremely strong range are usually suitable for rip-rap if they can
be quarried in intact blocks of sufficient size. Rock types which are commonly used when
fresh include:
- Quartzite and sandstone;
- Limestone, dolomite and marble;
- Granite, diorite and gabbro;
- Basalt and andesite;
-Gneiss.
Most rocks containing siltstone, shale and claystone would be unsuitable for rip-rap
because they would break down (slake) under repeated wetting and drying.
Ideally the rock should meet the durability requirements for concrete aggregates, but
many rocks which do not meet these requirements have performed satisfactorily.
Laboratory tests which provide an indication of long term durability include:
- Apparent specific gravity and absorption;
- Petrographic examination;
- Methylene blue absorption;
- Accelerated weathering test such as wetting and drying, or sulphate soundness.
Fookes and Poole (1981) discuss the durability requirements for rip-rap.
Rip-rap is commonly obtained from rockfill quarries by stockpiling oversized rock
from each shot. If a special quarry is required for rip-rap, selection of potential sites must
bear in mind that the spacing of persistent joints in the rock mass should be appreciably
greater than the dimensions of the required blocks. It is possible to get a good initial indi-
cation of the likely sizes and durability of blocks obtainable from an outcropping source
by observing the joint spacing in the outcrops and the size and condition of surface boul-
ders and/or scree derived from them. Experience suggests that the size of excavated block
will rarely exceed the joint spacing.
At the site, or sites, selected for detailed exploration a special objective of the
exploratory programs should be determination of the pattern of joints in the rock mass,
i.e. the number of sets of joints and the orientation, persistence and spacing of joints in
each set. In this regard, geotechnical mapping of surface rock outcrops and trench expo-
sures (even if extremely weathered) is important, as it is usually difficult to determine the
joint pattern at depth from drill cores.
During core logging, particular attention should be given to:
- The location of any zones in which the rock appears to be affected by chemical alter-
ation (e.g. in granitic and basaltic rocks - see Chapter 2, Sections 2.7 and 2.9.3 and
Chapter 3, Section 3.2.3). Such zones often contain minerals which weather rapidly on
exposure;
- The distinction between minor joints which may extend for less than 500 mm, and
major joints which extend for many metres;
- The mineral type(s) which form the joint cements, and the apparent strength of the
joints. In general, only quartz-cemented joints are likely to be strong enough to resist part-
ing during quarrying and later when blocks are in service.
