Geology Reference
In-Depth Information
0.5
0.4
c
=
80 kPa
φ
=
46
°
0.3
0.2
Rock is very blocky, with poor quality joint
surfaces (GSI
=
35)
uniaxial compressive strength variable but
take as 5 MPa
m
i
taken as 15
0.1
0
0
0.1
0.2
0.3
0.4
Normal stress, MPa
0.5
0.6
0.7
0.8
weathered. There are corestones of very strong tuff but these are sepa-
rated and surrounded by highly and completely weathered materials
that are much weaker. There are many joints and some of these have
kaolin in
ll. In this case, there are no structural mechanisms for transla-
tional failure along daylighting joints, and it is a clear candidate for
where a Hoek-Brown/GSI approach might help the assessment. From
the GSI chart, one might best characterise the mass as
'
'
very blocky
with
'
'
joint surfaces. The rock type is tuff, so the mi
i
value is 15 (for
granite it would be 33). The dif
poor
cult parameter is intact strength. In this
case, the corestones have UCS values in excess of 100 MPa, but for this
assessment I have taken into account the strength of the weakest mate-
rial making up this slope and, on balance, an average of 5 MPa is
considered conservative. Using a spreadsheet from Hoek et al.(1995)
modi
ed for low stress conditions, the curve shown in
Figure 5.17
is
obtained. On that basis, for a potential slip surface at a depth of about
10m (vertical stress say 0.27 MPa), appropriate strength parameters
might be c = 80 kPa and phi = 46 degrees, as shown. Carvalho et al.
(2007) discuss the assessment of rock mass strength where the intact
rock has relatively low uniaxial compressive strength in more detail.
Rock mass modulus is very dif
cult to predict with any accuracy, and
measurements in boreholes or even by large in situ tests need to be
considered critically and certainly should not be used directly in design
without due consideration of the rock qualities of the zone tested
(including relaxation) vs. the larger mass volume. Back calculations
