Rock mass classification systems - Geotechnical Investigation and Design Tables - page 238

Table 18.21 Average tunnel advancement estimate using Q value (adapted from Barton, 2006).

Rock mass quality, Q
value

0.01 0.01-0.1 0.1-1.0 1 - 10 10 - 100 100 - 1000

Poor Poor/Fair Good

Description Exception. Extremely Very OK/Very Ext./Exc.

Delays due to support required Lack of joints

Tunnel boring machine

10

10-40 40-200 200-140 140-80 80-40 m/wk

Drill and blast 10 10-25 25-50 50-120 120 m/week

18.22 Relative cost for tunnelling using Q values
•

The lower quality rock would require greater tunnel support and hence costs.

•

The Q value has therefore been used by Barton to estimate the relative tunnelling
cost.

Table 18.22 Relative cost estimate using Q value (adapted from Barton, 2006).

Rock mass quality,
Q value

0.01 0.01-0.1 0.1-1.0 1 - 10 10 - 100 100 - 1000

Poor Good

Poor/Fair

Description

Exception. Extremely Very OK/Very Ext./Exc.

Delays due to support required Lack of joints

Relative cost 1100% 1100-400% 400-200% 200-100%

100%

Relative time 900% 900-500% 500-150% 150-100% 100%

18.23 Prediction of cohesive and frictional strength using
Q values

•

Barton used the Q value to estimate the rock strength based on the relationships
shown in the Table below.

•

The Hoek - Brown failure criterion can be used to directly assess specific shear
strength situations based on the relationship major (

3 ) principal
stresses, and other material characteristics as shown in Figure 9.2. (Hoek et al.,
2002)

σ

1 ) and minor (

σ

•

σ 1 = σ 3 + σ ci σ 3 / σ ci + s) a (m b

•

a = 0.5 for hard rock