Geology Reference
In-Depth Information
Figure 5.2 Ground
and polished saw-
cut surface of
Delabole Slate at
high magni
Saw-cut, ground surface (220 grade diamond wheel)
|
0.0025
mm
Surface after more than 3
m sliding in tilt test
0.025
mm
shown in
Figure 5.2,
at a microscopic scale such ground and apparently
flat surfaces are still rough. Each data point in
Figure 5.3
is taken from a
separate test with the sample reground beforehand. The upper line
(inclined at 38 degrees) is the friction angle measured for moderately
weathered (grade III) rock; the lower line inclined at 32.5 degrees is for
slightly weathered (grade II) rock. The reason for the higher strength for
the more weathered surface is because the surface
finish is slightly
rougher, the weathered feldspars being preferentially plucked from the
surface during grinding. The key observation, however, is the precision
of the frictional relationships
-
an increase in strength that is directly
proportional to the level of normal load. Scholtz (1990) reviews the
origin of rock friction and concludes that the additional strength is
derived from deformation and damage to small-scale textural roughness.
It is quite remarkable that this interlocking, non-dilational component
still obeys Amonton
slawsoffriction.
The third contact phenomenon is dilation. Additional work is done
against the con
'
ning normal load during shear as soil moves from a
Saw-cut granite #60 carborundum
1000
y = 0.77x
grade II dry
500
y =0.64x
grade II wet
grade III wet
0
0
500
1000
1500
Normal stress, kPa
cut and ground surfaces of rock. The upper line (stronger) is for moderately weathered granite, the
lower for stronger, slightly decomposed rock. This paradox is explained by the fact that in the grade
II rock the various mineral grains are of similar scratch resistance and therefore the surface takes a
better polish during grinding than the more heterogeneous grade III rock.
