Environmental Engineering Reference
In-Depth Information
16.3.3
Tensile strength of rock foundations
Within the scale of most concrete dams, there are discontinuities in the foundation rock
mass which mean that the tensile strength is zero, or so close to zero that it should be
ignored.
There has in the past been an over-emphasis on the tensile strength of the concrete -
foundation contact. This is virtually irrelevant, because the rock beneath the contact has
zero tensile strength, so even if the contact has some strength, tensile failure can occur in
the foundation immediately below the concrete.
16.3.4
Compressibility of jointed rock foundation
The Youngs modulus of deformation for a jointed rock mass can be estimated from the
Geological Strength Index (GSI) using the method proposed by Hoek et al. (1998) and
Hoek and Brown (1997), using the equation:
GSI
10
s
10
ci
E
10
40
(16.19)
d
Douglas (2003) collated a number of case studies from the literature where the rock
mass modulus was measured by plate load, pressuremeter, dilatometer, flat jack and
jointed block tests and the GSI could be estimated. The results from the case studies,
together with data presented by Bieniawski (1978) and Serafim and Pereira (1983) are
shown in Figure 16.8. The error bars acknowledge the variation of E
d
in the test results.
The uncertainty in GSI was due both to the variability of the rock masses and also to
the conversion from published RMR values to GSI. The compressive strength of the rock
substance lay between 10 MPa and 100 MPa, and the corresponding plots for Equation
70
Bieniawski (1978)
Serafim & Pereira (1983)
Santiago (1986)
Georgiadis (1986)
Gifford (1986)
Pavlakis (1980)
Poulton (1986)
Schultz (1995)
Cheng (1993)
Giovanni (1993)
McDonald (1993)
Littlechild (2000)
Bieniawski (1990)
Ribacchi (1984)
UCS = 100MPa
UCS = 10MPa
1
σ
ci
10
(GSI-10)/40
E
d
60
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
GSI
ci
Figure 16.8.
E
d
versus GSI case study data and Hoek et al. (1998) equation for
100 MPa and
ci
10 MPa.
