Geology Reference
In-Depth Information
East).
then it can give UCS strength of 60 or 70 MPa (higher than structural concrete). Samples of
inherently weaker material (as could be estimated from scratchtesting) or containing vugs or other
flaws, will fail at much lower strengths. A typical range of data is given in
Figure B5-1.4.
If smaller
intact pieces of dolomitised limestone are point load tested selectively, they will, of course, err
towards the higher strength of the rock mass. As a consequence, conversion factors from point load
test to UCS for this rock are usually taken empirically as 8 to 9 (Khalaf, personal communication).
Data converted in this way are included in
Figure B5-1.4.
For more uniform rocks elsewhere in the
world, conversion factors of about 22 are more commonly applied (Brook, 1993). If such a factor
were to be used for the Simsima Limestone, then it would imply strength for the intact limestone,
without
aws, up to about 200 MPa.
Given this very wide range of possible strengths, it would seem unwise simply to rely on a
statistical testing campaign for characterising the rock mass. Far better to try
first to characterise
the rock geologically into units based on the strength of rock materials and then mass char-
acteristics including
flaws, degree of cementation and degree of fracturing. In this case, index
tests (hammer, knife), combined with visual logging and selective testing of typical facies, are
likely to give a far better indication of mass properties than UCS testing alone. To obtain
parameters for the large scale (say foundations) then in situ tests such as plate loading and
perhaps seismic tests would help, as would full-scale instrumented pile testing. Where rock mass
strength is very important, as for the selection of dredging equipment, then it would be very
unwise to take UCS data at face value (as a statistical distribution). As for many tests, there are
numerous reasons why values measured in the laboratory might be unrepresentative of condi-
tions in situ, often too low, and considerable judgement is required if the parameters are
critically important.
Lesson: compressive strength of most rocks can often be estimated adequately by hitting with a hammer
and the use of other index tests; if a hard blow by a hammer cannot break the material, then its strength
probably exceeds that of any concrete structure to be built upon it. Where strength is critical, as in the
selection of a tunnelling machine or choice of dredging equipment, then any test data must be examined
