Site investigation - Introduction to Tunnel Construction - page 57

Table 2.18 Suggested excavation support ratios (ESR) (after Barton and Grimstad
1994, from Barton et al. 1974)

Type of excavation ESR

A Temporary mine openings 2.0-5.0

B Permanent mine openings, water tunnels for hydropower 1.6-2.0

(excluding high pressure penstocks), pilot tunnels, drifts and
headings for large openings, surge chambers

C Storage caverns, water treatment plants, minor road and railway 1.2-1.3

tunnels, access tunnels

D Power stations, major road and railway tunnels, civil defence 0.9-1.1

chambers, portals, intersections

E Underground nuclear power stations, railway stations, sports 0.5-0.8

and public facilities, factories, major gas pipeline tunnels

The equivalent dimension, D e , plotted against the value of Q, is used to
define a number of support categories in a chart published in the original
paper by Barton et al. (1974). This chart has been updated a number of
times to directly give the support requirements. Grimstad and Barton
(1993), for example, modified it to reflect the increasing use of steel fibre
reinforced sprayed concrete in underground excavation support. Figure
2.24 is reproduced from this updated chart.
In a further development, Barton (1999) proposed a method for
predicting the penetration rate and advance rate for TBM tunnelling. This
approach is based on an expanded Q-method of rock mass classification
and average cutter force in relation to the appropriate rock mass strength.
The parameter Q TBM can be estimated during feasibility studies, and can
also be back calculated from TBM performance during tunnelling. This
method is briefly described in Appendix A (section A.2.1).
Barton (2002) provides some other useful correlations for the Q-value
to assist site investigation and tunnel design. For example, a relationship
between Q-value and seismic velocity (V p ) as used for some geophysical
site investigation techniques (see section 2.3.2.1) is given in equation 2.12.

V p 3.5 + log Q (2.12)

where V p is in units of km/s. This relationship was developed from tests
in hard rock, but this has been developed further for application to weaker
and harder ground conditions. This has been achieved by normalizing the
Q-value using 100 MPa as the hard rock norm. The relationship for the
normalized Q-value, Q c is shown in equation 2.13.

Q c = Q q u /100 (2.13)

where q u is the unconfined compressive strength of the rock mass.