Geoscience Reference
In-Depth Information
•
This surface sliding is common as the topsoil is meant to promote vegetation
growth and has been loosely placed on the compacted embankment/slope.
•
The short-term conditions governs the soil thickness. Greater thickness usually
results in gullying and slumping of the topsoil. Once the vegetation has been
established the overall slope stability and erosion resistance increases.
14.23 Design of slopes in rock cuttings and embankments
•
The slopes for embankments and cuttings are different even for the same type of
material.
•
Materials of the same rock type but different geological age may perform
differently when exposed in a cutting or used as fill.
Table 14.23
Typical slopes in rock cuttings and embankments (adapted from BS 6031 - 1981).
Types of rock/geological age
Cuttings:
Embankments:
Resistance to weathering
Safe slopes
Angle of repose
Sedimentary
•
Sandstones: strong, massive
70
◦
to 90
◦
38
◦
to 42
◦
Very resistant
Triassic; Carboniferous; Devonian
Sandstones;Weak, bedded
50
◦
to 70
◦
33
◦
to 37
◦
Fairy resistant
•
Cretaceous
Shales
45
◦
to 60
◦
34
◦
to 38
◦
Moderately resistant
•
Jurassic; Carboniferous
Marls
55
◦
to 70
◦
33
◦
to 36
◦
Softening may occur with time
•
Triassic; Cretaceous
Limestones; strong massive
70
◦
to 90
◦
38
◦
to 42
◦
Fairly resistant
•
Permian; Carboniferous
•
Limestones; weak
70
◦
to 90
◦
33
◦
to 36
◦
Weathering properties vary
Jurassic
considerably
Chalk
45
◦
to 80
◦
37
◦
to 42
◦
Some weathering
•
Cretaceous
Igneous
80
◦
to 90
◦
37
◦
to 42
◦
•
Granite, Dolerite,Andesite, Gabbro
Excellent resistant.
Basalt
Basalts exfoliate after long
periods of exposure
•
Metamorphic
60
◦
to 90
◦
34
◦
to 38
◦
Gneiss, Quartzite,
Excellent resistant
•
•
Schist, Slate
Weathers considerably
•
Angles referred to the horizontal.
•
Consider if weaker layer underneath.
•
Even in weather resistant rocks, tree roots may open joints causing dislodgement
of blocks.
14.24 Factors affecting the stability of rock slopes
•
The stability of rock slopes is sensitive to the slope height.
•
For a given height the different internal parameters may govern as shown in the
table.
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