Geology Reference
In-Depth Information
have been made from large projects, including dams and tunnels, and
these data provide the main database for prediction (e.g. Gioda &
Sakurai, 2005). Generally, poor quality, highly fractured rock (up to
RMR = 50) will have a rock mass modulus increasing from soil-type
values of perhaps 500 MPa to about 20GPa with decreasing fracture
spacing and increasing intact compressive strength. As the rock mass
quality improves, so the modulus increases markedly, up to values of
60 GPa or so for good-quality rock with RMR
80. Many authors
have attempted correlations between a variety of rock mass classi
=
ca-
tions (RMR and Q especially) and rock mass modulus, but with
considerable scatter. This is perhaps not surprising given the inherent
dif
culties of 1) trying to represent an often complex, heterogeneous
geological situation as a single quality number and 2) the non-uniform
loading conditions of any project vs.
the measurement system
(de
ciencies of data).
Hoek & Diederichs (2006) carried out a detailed review and pro-
posed optimised equations linked to the GSI classi
cation. The best-
t
equation obtained was:
1 þ e
ðð75þ25D−GSIÞ=11Þ
E
mass
ðMPaÞ¼10
5
ð1−D=2Þ=
where GSI is as taken from the chart in Appendix C
(Table C11
). The
factor D
0 for undisturbed masses, 0.5 for partially disturbed and 1.0
if fully disturbed. Hoek &Diederichs present a more re
=
ned version of
this equation using site-speci
c data for intact strength and modulus,
but in many situations the rock mass will not be uniform, so consider-
able judgement is necessary anyway. Richards & Read (2007) tried
applying the Hoek-Diederichs equations to the Waitacki Dam in New
Zealand, which was founded on greywacke, and found that the mass
modulus was considerably underestimated for a judged GSI of 20, but
examination of their data shows how sensitive any prediction is on the
GSI adopted. As discussed elsewhere, features like joint spacing and
continuity are extremely dif
cult to measure and characterise and very
risky to extrapolate from
field exposures because of variations with
weathering and the structural regime. This all reinforces the need
for considerable judgement and engineering geological expertise in
establishing ground models, and caution when applying any empirical
relationships.
Large-scale pile loading tests can provide data on rock mass deforma-
tion (Hill & Wallace, 2001). They found that published correlations
based on RMR and Q classi
cations overestimated the in situ modulus
for deep foundation design by up to one order of magnitude, but this
was only a signi
cant consideration where the Rock Mass Rating was
below 40 (poor and very poor rock masses), and in such cases site-
speci
c testing might be required. As discussed in
Chapter 6,
the
