ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 4 doc

Norwegian method of tunnelling 135

Table 10.1 Essential features of NMT (Barton et al., 1992).

S.No. Features

1. Areas of usual application:

Jointed rock giving overbreak, harder end of scale (qc = 3 to 300 MPa)

Clay bearing zones, stress slabbing (Q = 0.001 to 10 or more)

2. Usual methods of excavation:

Drill and blast, hard rock TBM, hand excavation in clay zones

3. Temporary rock reinforcement and permanent tunnel support may be any of the

following:

CCA, S(fr) + RRS + B, B + S(fr), B + S, B, S(fr), S, sb, (NONE)

* Temporary reinforcement forms part of permanent support

* Mesh reinforced shotcrete not used

* Dry process shotcrete not used

* Steel sets or lattice girders not used, RRS and S(fr) are used in clay zones and in weak,

squeezing rock masses

* Contractor chooses temporary support

* Owner/consultant chooses permanent support

* Final concrete lining are less frequently used; i.e., B + S(fr) is usually the final support

4. Rock mass characterization for:

* Predicting rock mass quality

* Predicting support needs

* Updating both during tunnelling (monitoring in critical cases only)

5. The NMT gives low costs and

* Rapid advance rates in drill and blast tunnels

* Improved safety

* Improved environment

Notations: CCA = cast concrete arches; S(fr) = steel fiber reinforced shotcrete; RRS = reinforced ribs of

shotcrete; B = systematic bolting; S = conventional shotcrete; sb = spot bolts; NONE = no support needed.

10.3 DESIGN OF SUPPORTS

The Q-value is related to the tunnel support requirements with the equivalent dimensions

of the excavation. The relationship between Q and the equivalent dimension of an exca￾vation determines the appropriate support measures as depicted in Fig. 10.1. Barton et al.

(1974) have identified 38 support categories (Fig. 10.1) and specified permanent sup￾ports for these categories. The bolt length l, which is not specified in the support details,

136 Tunnelling in weak rocks

can be determined in terms of excavation width, B in meters using the following equations

of Barton et al. (1974).

l = 2 + (0.15 B/ESR), m for pre-tensioned rock bolts in roof (10.1)

l = 2 + (0.15 H/ESR), m for pre-tensioned rock bolts in walls of height (H) (10.2)

and

l = 0.40 B/ESR, m for the untensioned rock anchors in roof (10.3)

l = 0.35 H/ESR, m for the untensioned rock anchors in walls (10.4)

Table 10.2 (Barton et al., 1974) suggests the type of bolt, its spacing and the thickness

of conventional shotcrete for a given rock mass quality Q, equivalent span B/ESR, RQD/Jn

and Jr

/Ja values. For design of wall support system of a cavern, Q should be replaced

by Qw. In case of shaft, Qw may be used for designing the support system for equivalent

span (or diameter or size of shaft/ESR) and corresponding bolt length from equations (10.1)

or (10.3) (Barton, 2001). Many supplementary notes are given at the end of Table 10.2.

Other practical recommendations on shotcrete are compiled in Table 10.3.

It should be realized that shotcrete lining of adequate thickness and quality is a long￾term support system. This is true for rail tunnels also. It must be ensured that there is a

good bond between shotcrete and rock surface. Tensile bending stresses are not found

to occur even in the irregular shotcrete lining in the roof due to a good bond between

shotcrete and the rock mass in an arched-roof opening. Rock bolts help in better bonding.

Similarly, contact grouting is essential behind the concrete lining to develop a good bond

between the lining and rock mass to arrest its bending. However, bending stresses may

develop in lining within the faults.

Rock has ego (Extraordinary Geological Occurrence) problems. As such, where cracks

appear in the shotcrete lining, more layers of shotcrete should be sprayed. The opening

should also be monitored with the help of borehole extensometers at such locations par￾ticularly in the squeezing ground. If necessary, expert tunnel engineers should be invited

to identify and solve construction problems. At this point in time, NTM does not suggest

the tunnel instrumentation in hard rocks, unlike NATM.

In the over-stressed brittle hard rocks, rock anchors should be installed to make the

reinforced rock arch a ductile arch. Thus, a mode of failure is designed to be ductile from

the brittle failure. Hence, failure would be slow giving enough time for local strengthening

(or retrofitting) of the existing support system.

10.4 DESIGN OF STEEL FIBER REINFORCED SHOTCRETE

Wet process SFRS has the following advantages (Barton et al., 1992).

(i) high application-capacity rate upto 25 m3 per hour,

(ii) efficient reinforcement,

Table 10.2 Recommended support based upon NGI rock mass quality Q (Hoek & Brown, 1980).

Rock mass quality Q or Qw or Qav or

Qseismic

Equivalent dimension (Span/ESR)

Block size (RQD/Jn)

Inter-block strength (Jr

/Jn)

Approx. support pressure (proof ), MPa

Spot reinforcement with untensioned

grouted dowels

Untensioned grouted dowels on grid

pattern

Tensioned rock bolts on grid with

spacing

Chainlink mesh anchored to bolts at

intermediate points

Shotcrete applied directly to rock,

thickness indicated

Shotcrete reinforced with weld-mesh,

thickness indicated

Unreinforced cast concrete arch,

thickness indicated

Steel reinforced cast concrete arch,

thickness indicated

Notes by Barton et al. (1974)

Notes by Hoek and Brown (1980)

1000–400 20–100

<0.001

≤ 1 a

400–100 12–88 0.005

≤ 1 a

100–40 8.5–19

>20 0.025

≤ 1 a

100–40 8.5–19

>20 0.025 2.5–3 m

100–40 14–30

>30 0.025 2–3 m

100–40 14–30

>30 0.025 1.5–2 m

≤ b

100–40 23–72

>30 0.025 2–3 m

100–40 23–72

>30 0.025 1.5–2 m

≤ b

40–10 5–14

>10

>1.5 0.05

≤ 2

40–10 5–14

>10

<1.5 0.05 1.5–2 m 2

40–10 5–14

>10

>1.5 0.05 1.5–2 m 2

40–10 5–14

>10

<1.5 0.05 1.5–2 m 20–30 mm 2

40–10 15–23

>10 0.05 1.5–2 m

≤ 2,3 b

40–10 15–23

>10 0.05 1.5–2 m 50–100 mm 2,3 c

40–10 9–15 0.05 1.5–2 m

≤ 2,4 b

40–10 15–40

>10 0.05 1.5–2 m

≤ 2,3,5 b

40–10 15–40

>10 0.05 1.5–2 m 50–100 mm 2,3,5 c

Continued

Table 10.2—Continued Rock mass quality Q or Q

w or Qav or

Qseismic

Equivalent dimension (Span/ESR)

Block size (RQD/Jn)

Inter-block strength (Jr

/Jn)

Approx. support pressure (proof ), MPa

Spot reinforcement with untensioned

grouted dowels

Untensioned grouted dowels on grid

pattern

Tensioned rock bolts on grid with

spacing

Chainlink mesh anchored to bolts at

intermediate points

Shotcrete applied directly to rock,

thickness indicated

Shotcrete reinforced with weld-mesh,

thickness indicated

Unreinforced cast concrete arch,

thickness indicated

Steel reinforced cast concrete arch,

thickness indicated

Notes by Barton et al. (1974)

Notes by Hoek and Brown (1980)

40–10 30–65 >15 0.05 1.5–2 m ≤ 2,6,7,13 b

40–10 30–65 >15 0.05 1.5–2 m 50–100 mm 2,6,7,13 c

10–4 3.5–9 >30 0.10 ≤ 2 a

10–4 3.5–9 >10<30 0.10 1–1.5 m 2

10–4 6–9 <10 0.10 1–1.5 m 20–30 mm 2

10–4 <6 <10 0.10 20–30 mm 2

10–4 10–15 >5 0.10 1–1.5 m ≤ 2,4 a

10–4 7–10 >5 0.10 1–1.5 m ≤ 2 a

10–4 10–15 <5 0.10 1–1.5 m 20–30 mm 2,4

10–4 7–10 <5 0.10 1–1.5 m 20–30 mm 2

10–4 20–29 0.10 1–2 m 100–150 mm 2,3,5 c

10–4 12–20 0.10 1–1.5 m 50–100 mm 2,3 c

10–4 35–52 0.10 1–2 m 200–250 mm 2,6,7,13 c

10–4 24–35 0.10 1–2 m 100–200 mm 2,3,5,13 c

4–1 2.1–6.5 >12.5 <0.75 0.15 1 m 20–30 mm 2

4–1 2.1–6.5 >12.5 <0.75 0.15 20–30 mm 2

4–1 2.1–6.5 <0.75 0.15 1 m 2

4–1 4.5–11.5 >10 <30>1 0.15 1 m ≤ 2 a

4–1 4.5–11.5 <10 >1 0.15 25–75 mm 2

4–1 4.5–11.5 <30 <1 0.15 1 m 25–50 mm 2 c

4–1 4.5–11.5 >30 0.15 1 m 2

4–1 15–24 0.15 1–1.5 m 100–150 mm 2,3,5,8 c

4–1 8–15 0.15 1–1.5 m 50–100 mm 2 c

4–1 30–46 0.15 1–1.5 m 150–300 mm 2,6,7,13 c

4–1 18–30 0.15 1–1.5 m 100–150 mm 2,3,5 c

1–0.4 1.5–4.2 >10 >0.5 0.225 1 m ≤ 2 d

1–0.4 1.5–4.2 <10 >0.5 0.225 1 m 50 mm 2 c

1–0.4 1.5–4.2 <0.5 0.225 1 m 50 mm 2 c

1–0.4 3.2–7.5 0.225 1 m 50–75 mm 14,11,12 c

1–0.4 3.2–7.5 0.225 1 m 25–50 mm 2,10

1–0.4 12–18 0.225 1 m 75–100 mm 2,10 c

1–0.4 6–12 0.225 1 m 50–75 mm 2,10 c

1–0.4 12–18 0.225 1 m 200–400 mm 14,11,12 c

1–0.4 6–12 0.225 1 m 100–200 mm 14,11,12 c

1–0.4 30–38 0.225 1 m 300–400 mm 2,5,6,10,13 c,f

1–0.4 20–30 0.225 1 m 200–300 mm 2,3,5,10,13 c

1–0.4 15–20 0.225 1 m 150–200 mm 1,3,10,13 c

1–0.4 15–38 0.225 1 m 300 mm–1 m 5,9,10,12,13

0.4–0.1 1–3.1 >5 >0.25 0.3 1 m 20–30 mm

0.4–0.1 1–3.1 <5 >0.25 0.3 1 m 50 mm c

0.4–0.1 1–3.1 <0.25 0.3 1 m 50 mm c

0.4–0.1 2.2–6 >5 0.3 1 m 25–50 mm 10 c

0.4–0.1 2.2–6 <5 0.3 50–75 mm 10 c

0.4–0.1 2.2–6 0.3 1 m 50–75 mm 9,11,12 c

0.4–0.1 4–14.5 >4 0.3 1 m 50–125 mm 10 c

0.4–0.1 4–14.5 <4>1.5 0.3 75–250 mm 10 c

0.4–0.1 4–14.5 <1.5 0.3 1 m 200–400 mm 10,12 c

0.4–0.1 4–14.5 0.3 1 m 300–500 mm 9,11,12

0.4–0.1 20–34 0.3 1 m 400–600 mm 3,5,10,12,13 f

0.4–0.1 11–20 0.3 1 m 200–400 mm 4,5,10,12,13 c

0.4–0.1 11–34 0.3 1 m 400 mm–1.2 m 5,9,11,12,13

Continued