Environmental Engineering Reference
In-Depth Information
Downie Slide in British Columbia (Piteau et al., 1978) and the Tablachaca Slide in Peru
(Arnao et al., 1984; Deere and Perez, 1985).
The landsliding usually occurs by slope failure along weathered foliation surfaces or
foliation shears as shown on
Figure 3.17
. Bell (1976, 1982) describes how the develop-
ment of a river valley in New Zealand has involved landsliding along dipping foliation
surfaces.
In steeply dipping schistose rocks failure by toppling is also relatively common, the top-
pled slabs or columns being separated by joints or shears along the foliation direction.
Riemer et al. (1988) describe a complex toppling failure at San Pablo, Peru.
Where the schistose rocks contain abundant tectonically-formed defects in other orien-
tations, many other failure models have been recorded. However in most of these it is
likely that the low shear strength of the schistose rocks along their foliation surfaces has
contributed to the development of slope failure.
In long, high slopes in mountainous areas some failures of schistose rocks appear to
have occurred by buckling of, and eventually shearing through, the foliation. The buck-
ling is facilitated by the low shear strength of the foliation surfaces which allows multiple
shear displacements to occur along them. Examples of this type of slope failure are dis-
cussed in Beetham et al. (1991), Riemer et al. (1988), Radbruch-Hall et al. (1976),
Nemcok (1972) and Zischinski (1966, 1969).
Examples of landsliding in schists forming slopes around the reservoir of Clyde Dam in
New Zealand are discussed in Gillon and Hancox (1992), Riddolls et al. (1992) and
Stapledon (1995).
3.4.7
Schistose rocks - check list of questions
- Degree of anisotropy, and its effect on the project?
- Low durability in exposed faces?
- Particle shapes and strengths inadequate for filter, concrete or pavement materials?
- Suitability for use as rockfill?
- Foliation shears?
- Kink bands?
- Mica-rich layers?
- Unstable slopes?
3.5
MUDROCKS
Included under this heading are all sedimentary rocks formed by the consolidation and
cementation of sediments which are predominantly clays or silts or clay-silt admixtures.
The common rock types are:
- Claystone (predominantly clay sizes);
-Siltstone (predominantly silt sizes);
- Mudstone (clay-silt admixtures);
- Shale (any of the above, but fissile due to well-developed cleavage parallel to the bedding).
The sediments may have been deposited in either marine or fresh water conditions and
usually have been derived from erosion of older rocks. In some cases they contain parti-
cles of volcanic origin. Possible cementing agents include calcite, silica, iron oxides and
evaporite minerals such as gypsum, anhydrite and halite (common salt). Mudrocks occur-
ring in or associated with coal-bearing sequences often contain abundant carbonaceous
material and sulphide minerals, e.g. iron pyrite.
