The 14-72-km-long Fionnay-Riddes tunnel had to cross difficult rocks
10.9 Classification of jointed rock masses for tunnelling
Estimate of required rock support based on rock characteristics
Bieniawski's Engineering Classification classifies rock masses irrespective of the engineering job to be done. Most other classifications of jointed rock masses are centred on tunnel design and construction, the main problem being the estimate of the required rock support, a problem of ever-growing importance. In 1872 the contractor Louis Favre started the excavation of the St Gotthard railway tunnel: the tunnel lining represented 25% of the tunnel- ling costs. For the St Gotthard Road tunnel, presently being excavated under the same mountain pass, the cost of the lining represents 45 % of the costs (Lombardi, 1972).
Two different lines of approach have been developed for correlating the characteristics of rock masses and the required tunnel support. A first attempt could be described as the correlation of geomechanical parameters (unconfined compressive strength, defect systems) with types of rock support.
Statistical analysis of case histories is important. A second approach could be called the engineering approach. It is based on the calculated probable deformations of the rock masses - depending on rock characteristics, shape of the tunnel and location of the relevant tunnel section relative to the tunnel heading at the time the rock support becomes efficient - and the deformation of the loaded and strained rock support.
(a) An early approach by Lauffer (1958) correlates rock types, the active unsupported span of rock and the time that this span takes to fail (stand-up time). An active unsupported span is the width of the tunnel or the distance from support to the face, if this is less than the width of the tunnel. Figure 10.47 reproduces, with some modifications by Bieniawski, Lauffer's diagram.
Lauffer mentioned different factors influencing rock mass stability during tunnelling. They are schematically reproduced in fig. 10.48 (after Lauffer, 1958 and Muller, 1963). Lauffer's interesting suggestion on conditions for unsupported rock stability is too restrictive. The necessity for considering rock supports is obvious.
282 Underground excavations
0.2
10" 10" 10 102 103
stand-up time (hours)
104 105 106 Fig. 10.47 Rock mass classification for tunnelling (modified after Lauffer).
parallel to strata
time (a) Orientation of tunnel axis
a 9
time (b) Form of cross-section
time
(c) Excavation method time
id) Support method
Fig. 10.48 Factors influencing rock mass stability during tunnelling (schematically after Lauffer and Miiller).
(b) Before Lauffer, Terzaghi (1946) was interested in an inquiry concerning rock supports in railway tunnels supported by steel ribs with wooden block- ing. Some experts believe the figures given by Terzaghi to be over-conservative in the better quality of rock, but the figures appear quite relevant to present- day practice when excavating medium-size tunnels in very difficult rock conditions, and are in fact quite widely used. In table 10.6 the support pressures have been tabulated for nine classes of rock mass defined by Ter- zaghi. It is assumed that the table refers to tunnels with height H = B9 the tunnel width, 5 = 5m and 10 m. The last column of the table refers to the RQD (Deere) and the mass quality Q, defined by Barton (1974), to be dis- cussed in a further paragraph.
(c) Rock support in tunnels was also one of the major concerns of Bien- awski, when developing his Geomechanics Classification (see section 6.7).
Table 10.7, reproduced from his 1973 paper, summarizes his recommendations concerning tunnel supports.
(d) A Norwegian team, headed by Barton (1974) was not convinced by the efforts of Terzaghi, Bieniawski and others, who published their findings at about the same time. Barton et al produced an alternative rock classification and an Excavation Support Ratio (ESR) based on a very great number of case histories of tunnels, in particular an extensive, detailed survey by Cecil (1970) and another by Cording (1972). This information is analysed in a long document which, because of the great many details it gives, is very difficult to summarize.
Barton et al. chose six parameters to describe the rock mass quality, Q:
RQD Jr Jw
Q =
Jn Ja SRF Where:
RQD = rock quality designation (Deere, 1963);
jn s= joint set number;
Jr = joint roughness number;
Ja = joint alteration number;
jw = joint water reduction factor;
SRF = stress reduction factor.
The authors' suggested values for these six parameters are given in table 10.8.
The stress reduction factor, SRF, is an important parameter, when calculating the rock mass quality, Q. It takes into account special features of rock weaknesses which have a severe weakening effect on the whole rock mass.
Geologists and geophysicists alike know the dangers of some isolated weak rock seams and of contact zones between seams. The SRF does account for them (Cecil, 1970).
The quotient (RQD//n) represents the overall structure of the rock mass.
Table 10.6 Estimates of roof support pressures (after Terzaghi and Barton) Designation
(1) Hard, intact (2) Hard, stratified
(3) Massive, moderately jointed (4) Moderately blocky and seamy (5) Very blocky and seamy (6) Crushed
(7) Squeezing rock
(8) Squeezing rock, great depth (9) Swelling rock
Rock load (m)
0 to 0-5 B 0 to 0-25 B 0-25 to 0-35 (B + H) (0-35 to M0) (B + H)
1-10 (£ + H) (M0 to 210) (B + H) (2-10 to 4-50) (B + H)
up to 80 m
Support pressure B=H=5m B
0 0 to 0-6 0 to 0-3 0-3 to 0-9 0-9 to 2-9
2-9 2-9 to 5-5 5-5 to 11-7 up to 20-0
in kg/cm2
= H = 10 m 0 0 to 1-3 0 to 0-6 0-6 to 1-8
1-8 to 2-9 5-7 5-7 to 10-9 10-9 to 23-4
up to 200
RQD (after 100 100 100 80 50 20 20 0 0
Q Barton)
>1200 20-10 50-25 6-2 1-0-4 008-004 003-001 0004-0001 0003-0001 The two last columns are Barton's estimates, based on his formula for RQD (Deere, 1963) and Q (Barton, 1974)
drilling and blasting (after Bieniawski, 1973)
Rock mass class 1 2 3
4
5
Average stand-up time at unsupported
span 10 years 5 m 6 months 4 m 1 week 3 m
5 hours 1-5 m
10 min 0-5 m
Rock-bolts*
Additional Spacing support
1-5-2-0 m Occasional wire mesh in crown 1-0-1-5 m Wire mesh, plus 30 mm shotcrete
in crown as required 0-5-1-0 m Wire mesh, plus
30-50 mm shotcrete in crown and sides Not recommended
Alternative support systems Shotcrete
Additional Crown Sides support
Generally not required 50 mm Nil Nil
Steel sets
Type Spacing
Uneconomic 100 mm 50 mm Occasional wire mesh Light sets 1-5-2-0 m
and rock-bolts, if necessary 150 mm 100 mm Wire mesh and 3 m
rock-bolts at 1-5 m spacing 200 mm 150 mm Wire mesh, rock-
bolts and light steel sets. Seal face. Close invert.
Medium sets plus 0-7-1-5 m 50 mm shotcrete
Heavy sets with 0-7 m lagging,
immediately 80 mm shotcrete
! Bolt diameter 25 mm, length \ tunnel width. Resin bonded fully.
286 Underground excavations
Table 10.8 {abbreviated, after Barton et al.) (1) Parameter RQD; as in table 6.4 (after Bieniawski).
(2) Joint set number, Jn
(a) Massive, few joints (b) One-joint set (c) Two-joint sets (d) Three-joint sets
(e) Three-joint sets, plus random (/) Four or more sets
(g) Crushed rock
(3) Joint roughness number, Jr
(a) Discontinuous joints (b) Rough, irregular, undulating (c) Smooth, undulating
(d) Smooth, planar (4) Joint alteration number, Ja
(a) Tightly healed (b) Unaltered joint walls (c) Slightly altered (d) Silty coatings (e) Soft clay
(/) Over-consolidated clay (g) Swelling clay
0-5- 1 0 2 4 9 12 15 20 4 3 2 1
# 2
0-75
1-0 25°-35°
2 0 25°-30°
3-0 20°-25°
4-0 8°-10°
6-0-8-0 12°-16°
8-0-12 6°-12°
(tan </>2 = roughness coefficient) (5) Joint water reduction factor, Jw
(a) Minor inflow (b) Medium inflow (c) Large inflow
(d) Exceptionally large inflow (6) Stress reduction factor, SRF
(a) Multiple weakness zones (b) Single weakness zone containing
clay, depth less than 50 m (c) Same, depth more than 50 m (d) 'Sugar cube' rock
(e) Heavy swelling rock pressure
10 0-66 0-5 0-2-0-05
100
r
50 2-5 50 100-150
When boreholes are not available, the RQD can be estimated using the follow- ing empirical formula:
RQD = 1 1 5 - 3 - 3 / ,
(Jv = total number of joints per m3 of rock mass) with RQD = 100 for Jv < 4-5.
Barton's formula does not agree with Deere's definition of the RQD, which may cause confusion for Jv < 4-5.
The quotient (/r//a) represents the roughness and degree of alteration of the joint walls or filling materials. The authors write: 'Quite by chance it was found that the function tan"1 (/r//a) is a fair approximation to the actual shear strength that one might expect of the various combinations of wall, roughness and alteration products.'
Table 10.9 is an extract from a table calculated by Barton et al.
Table Jr 4 2 1
10.9 Values of tan' Ja = 0-75
79°
69°
53°
4 45°
IT 14°
1 (JrUa)
12 18°
9-5°
4-7°
Pressure support P. Barton suggests the following formula for the pressure support, P (in kg/cm2) for the roof:
2 0-1/3
^ (in kg/cm2) or approximately
Pro* = m Q-113 (in kg/cm2).
Barton's classification includes in its tables parameters representing the roughness of the joints and the strength of the joint filling, which are absent from Bieniawski's tables. All these figures are based on an extensive analysis of case histories, but such a system of correlations remains empirical.