Geology Reference
In-Depth Information
man-made slope, in the absence of discontinuities, almost irrespective
of height and steepness.
The strength of fresh rock is a function of its mineralogy, internal
structure of those minerals (cleavage), grain size, shape and degree of
interlocking, strength of mineral bonds, degree of cementation and
porosity. Some rocks have intact strength approaching 400 MPa
-
these might include quartzite, welded tuffs and
fine- and medium-grained
igneous rocks such as basalt and dolerite. Corresponding intact moduli
canbeashighas1x10
6
MPa(1x10
3
GPa) (Deere, 1968).
Compressive strength is measured most accurately using very stiff
servo-controlled loading frames, whereby, as the rock begins to fail, so
the loading is paused to limit the chance of explosive brittle failure.
Such test set-ups allow the full failure path to be explored, which can
be important in underground mine pillars where, despite initial failure
in one pillar, there may be suf
cient remnant strength after load is
transferred to adjacent pillars, so that overall failure of the mine level
does not occur. For most civil engineering works, UCS values mea-
sured by less sophisticated set-ups are adequate. Nevertheless, the
speci
cation for UCS testing is onerous, particularly regarding test
dimensions and
flatness of the ends of samples. If these requirements
are not followed, local stress concentrations can cause early failure. If
samples are too short, then shear failure might be inhibited. As noted in
Box 5-1,
there are alternative ways of estimating UCS that might be
adequate for the task at hand.
UCS is the starting point for many different empirical assessments of
rock masses, including excavatability by machinery such as tunnel
boring machines. Other parameters that might need to be quanti
ed
include abrasivity and durability. Appropriate tests are speci
ed in the
ISRM series of recommended methods (Ulusay & Hudson, 2006).
Intact rock modulus is rarely measured for projects and is not
usually an important parameter for design. An exception is in numer-
ical modelling of fractured rock mass, e.g. using UDEC (Itasca), where
this parameter is required, but for this purpose, values are typically
estimated from published charts or even selected to allow the model to
come to a solution within a reasonable time. Models tend to be
insensitive to the chosen parameter.
Intact weathered rock has true cohesion from relict mineral bonding.
In some cases there may be secondary cementation, especially from
iron oxides and the redistribution of weathering products within the
rock framework. At the strong end of grade IV where it can just be
broken by hand UCS might be about 12.5 MPa and cohesion of about
3 MPa might then be anticipated (Hencher, 2006). In practice, such
high values have never been reported. Ebuk, who tested a range of
