Geoscience Reference
In-Depth Information
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Continuity may be relative to the type of structure, loading or cutting.
•
Discontinuities considered continuous under structures if it is equal to the base
width, when sliding can be possible.
3.13 Rock defect symbols
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Typical symbols only. Each consultant has his or her own variation.
Table 3.13
Defect description.
Defect type
Surface roughness
Coating or infill
Macro-surface geometry
Micro-surface geometry
Bp - Bedding parting
St - Stepped
Ro - Rough
cn - clean
Fp - Foliation parting
Cu - Curved
Sm - Smooth
sn - stained
Jo - Joint
Un - Undulating
Sl - Slickensided
vn - veneer
Sh - Sheared zone
Ir - Irregular
cg - coating
Cs - Crushed seam
Pl - Planar
Ds - Decomposed seam
Is - Infilled seam
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The application of this data is considered in later chapters.
•
For example, friction angle of an infill fracture
for a
rough fracture. But the orientation and continuity of the defects would determine
whether it is a valid release mechanism.
<
for a smooth fracture and
>
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The opening size and number of the joints would determine its permeability.
3.14 Sedimentary and pyroclastic rock types
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The grain size and shape as used to describe soils can be also used for rocks.
•
Sedimentary rocks are the most common rock type at the earth's surface and sea
floor. They are formed from soil sediments or organic remains of plants and ani-
mals that have been lithified under significant heat and pressure of the overburden,
or by chemical reactions.
•
This rock type tends to be bedded.
•
Pyroclastic Rocks are a type of igneous rock. Pyroclasts have been formed by an
explosive volcanic origin, falling back to the earth, and becoming indurated. The
particle sizes thrown into the air can vary from 1000 tonne block sizes to a very
fine ash (Tuff).
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Even for rocks in a similar descriptor other factors may determine its overall
strength properties.
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For example, Sandstone, Arkose and Greywacke are similarly classed, but
sandstone would usually have rounded grains, which are one size, Arkose
would be Sub - angular and well graded while Greywacke would be angular
and well graded. This results in an intact Greywacke being stronger than a
sandstone.
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