Geology Reference
In-Depth Information
Table 5.1 Indicative uncon
ned compressive strengths for some rock, soil and concrete.
Material Uniaxial Compressive Strength, UCS MPa
Natural rock and soil
Fine-grained, fresh igneous rock such as dolerite,
basalt or welded tuff, crystalline limestone
>300
Rings when hit with geological
hammer
Grade I to II, fresh to slightly weathered granite
100
-
200
Dif
cult to break with hammer
Cemented sandstone (such as Millstone Grit)
40
-
70
Broken with hammer
Grade III, moderately weathered granite
20
-
40
Chalk
5
-
30
Readily broken with geological
hammer
Weaker material broken by hand
Grade IV highly weathered granite
Overconsolidated clay
0.6
-
1.0
Dif
cult to excavate with hand
pick
Very stiff clay-rich soil
0.3
-
0.6
Indented with
finger nail
Concrete
High-strength concrete (e.g. Channel Tunnel
liner)
50
-
100
Typical structural concrete
30
-
50
Shotcrete in tunnel
20
-
40
Many ground investigations are wasteful in that they do not target or identify critical geological
features, and laboratory tests are commissioned without real consideration of whether or not they
will be useful.
Example 1
Figure B5-1.1
shows the formation level (foundations) for the Queen
s Valley Dam, Jersey, which was
completed in 1991. The dam was to be an earth dam, which exerts relatively low stresses on its foundations,
compared to a concrete dam such as an arch or gravity dam. With amaximumheight of 24mand an assumed
unit weight of 20 kN/m
3
, the bearing pressure might be of the order of 500 kPa. The author, who was
mapping the foundations, was asked to select samples of core to be sent to the laboratory for uniaxial
compressive strength testing.
Rock over much of the foundation was rhyolite that was extremely dif
'
cult to break by geological
hammer and had an estimated compressive strength of more than 300 MPa. The rhyolite, however,
contained numerous incipient fractures
(Figure B5-1.2),
which would mean that the mass strength
was somewhat lower and, more signi
cantly, would cause samples to fail prematurely in the
laboratory. The author argued that if the samples were sent to the laboratory, the reported result
would simply be scattered with a range from 0 to 300 MPa and what would that tell us that we
didn
t already know? The allowable bearing pressure for rock of this quality (
Chapters 6)
would be
at least
'
five times the bearing pressure exerted by the dam. In the event, the samples were still sent off
to the laboratory for testing (because they had already been scheduled by the design engineers) and
the money was duly wasted.
