Environmental Engineering Reference
In-Depth Information
3.5.1
Engineering properties of mudrocks
Most mudrocks when fresh lie in the weak to very weak range as defined on Table 2.4.
The very weak claystones grade into hard, overconsolidated clays. The strongest
mudrocks lie in the medium strong and strong ranges and in most cases these owe their
greater strength to cementation by calcite or silica.
Because of their relatively high clay contents the porosity and water absorption properties
of mudrocks are much higher than those of most other rocks. As a result of this and the
expansive nature of clays, all mudrocks swell and develop fine cracks on prolonged expo-
sure to wetting and drying. The strongest siltstones, which contain appreciable amounts of
calcite or silica as cement, can be exposed for up to a year before cracks are evident. At the
other end of the scale, the weakest claystones and shales develop fine cracks as soon as they
dry out (often only hours or days) and disintegrate with further cycles of wetting and dry-
ing. Some of these materials also swell noticeably on removal of overburden.
The mechanisms involved in deterioration of mudrocks on exposure have been
described already in Sections 2.5.3 and 2.9.1.
Because of their instability when exposed, special care needs to be taken during prepa-
ration of foundations on mudrocks. Treatments range from shotcreting or slush concret-
ing immediately after exposure and cleanup, to an initial cleanup followed by a final
cleanup immediately before placement of concrete or fill.
Taylor and Spears (1981), Cripps and Taylor (1981) and Spink and Norbury (1993)
provide useful information about the engineering properties of mudrocks occurring in the
United Kingdom.
Mudrocks containing iron pyrite or other sulphide minerals can cause severe problems
for dam projects due to rapid weathering of the sulphides to form sulphuric acid and
metallic hydroxides and sulphates. The rapid weathering processes and some of their
effects on dam projects are described in Section 2.9.4. Such effects may include:
- Damage to the fabric of concrete due to sulphate attack;
- Heave of foundations or of excavation sides causing damage to concrete slabs or walls
(Penner et al., 1973; Hawkins and Pinches, 1987);
- Blockage and/or cementation of filter zones or drains;
- Seepage waters which are acidic and rich in iron or other heavy metals;
- Possible lowering of shear strength, of embankments formed by pyritic mudrocks.
3.5.2
Bedding-surface faults in mudrocks
Valley bulging (see Chapter 2, Section 2.5.4 and Figure 2.7) is a common feature in
mudrock sequences, where the beds are near-horizontal. In such situations and in any
other situation in which mudrock sequences have been disturbed by folding, tilting, or
stress relief movements, thin seams of crushed rock develop within the mudrocks, usually
at their boundaries with interbedded stiffer rocks (e.g. sandstones or limestones). These
seams are developed due to interbed slip during the movements, in the same way as folia-
known as bedding-surface faults, bedding-surface shears or bedding-plane shears. They
usually consist mainly of clay, are almost planar and have slickensided surfaces both
within them and at their boundaries. Residual effective shear strengths of laboratory sized
samples (i.e. excluding larger scale roughness effects) are commonly in the range 7° to
12°, with zero cohesion.
Although usually extending over wide areas, bedding surface faults may be only a few
millimetres thick. Such defects are difficult to recover and recognize in diamond drill cores.
This is particularly so when the defect is normal to the axis of the borehole, as even minor
