ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 6 pps

Tunnel instrumentation 237

0 25 50 75 100 125 150

0

10

20

30 3m dia. Tunnel

2

1 3

3m

1

2

3

Time, Days

Borehole Extension, mm

Fig. 14.8 Variation of borehole extension with time.

Giri Hydeltunnel through crushed phyllites which squeezed due to high cover pressure

of about 300 m. Two extensometers of 5 and 2.5 m depths were installed on the left wall

and three extensometers of 7.5, 5.0 and 2.5 m depths were installed on the right wall.

No extensometer could be installed on the roof. Tunnel closures were also measured.

The data were analyzed and radial displacements ur were plotted against radial distance

r for various time intervals as shown in Fig.14.9. The convergence of ur−log r plots at

point indicates stabilization of the broken zone between 200 to 300 days after excavation.

The broken zone radius (b) at this period was found to be 20.7 and 20.3 m on the left and

right wall, respectively. (It can be noted that the radial displacements vs. time curves tend

to converge at some radial distance which is believed to be the interface between broken

zone and elastic zone within a squeezing ground condition.) The steel ribs buckled after

300 days. This produced a spurt in radial displacements and the broken zone started widen￾ing again as indicated by the divergence of ur−log r plots in Fig.14.9. The example clearly

shows the usefulness of multi-point borehole extensometers to monitor the development

of broken zone around a tunnel under squeezing ground conditions.

3.0

2.0

1.0

0

0 2 3 510 20

b = 20.7

800

400

300

200

100

50

20

Days

(a) Left wall

Radial Distance, m

Radial Displacement, cm

3.0

2.0

1.0

0

0 2 3 5 10 20

b = 20.3

800

400

300

200

100

50

Days

(b) Right wall

Radial Distance, m

Radial Displacement, cm

20

Fig. 14.9 Variation of radial displacement with radial distance within phyllites in Giri Hydeltunnel

(a = 2.12 m and b = radius of broken zone in squeezing ground).

238 Tunnelling in weak rocks

0 400 800 1200 1600 2000 2400

Time, days

−0.50

0.00

1.00

2.00

3.00

Relative displacement between anchors, mm

0.73mm

0.024mm/month

0.00mm/month

Anchors 1 and 2

Anchors 2 and 3

Installation of

longer rock bolts

7.8.84

1985 1986 1987 1988 1989 1990

25.2.91

23m

57m

EL(-)12m

1 EL 45m

2

3

Agglomerate

Band EL 48.5m

47m

EL 93m

Fig. 14.10 Monitoring agglomerate band behavior with the multi-point borehole extensometer in

the roof of a large underground cavity, India (Goel, 2001).

14.8.6 Observation by borehole extensometer in large underground cavity

In one of the large underground opening projects, for example, it has been possible to

monitor the roof displacement of 0.024 mm/month (Fig. 14.10). The deformation remains

continued for almost 30 months. At this point of time, additional supports of longer rock

bolts were installed and subsequently it was observed that the roof movement/displacement

had stopped.

14.9 LAYOUT OF A TYPICAL TEST SECTION

Layout of an extensively instrumented zone is shown in Fig.14.11. Measurements taken

consist of following robust and valuable instruments.

(i) Radial support pressure by pressure cells

(ii) Load on support by load cells

(iii) Depth of loosened rock mass by multi-point borehole extensometers and

(iv) Rock closure and support deformation by tape extensometer.

Tunnel instrumentation 239

Pressure Cells

Load Cells

Pin for Support Deformation

0.33m

4-Point B.H.

Extensometer

3.0m dia.

Fig. 14.11 Layout plan of a typical instrumentation zone.

Strain in support can be measured by strain gauges.

Instrumentation in the lined and concreted zone should consist of the following:

(i) Stress meters

Embedded in concrete

(ii) Strain meters

Besides the above mentioned instrumentation, following data should also be

collected:

A. Geology – mapping, fracture spacing and orientation, width of fracture zone,

alteration and groundwater

B. Rock mass quality (Q), rock mass rating (RMR) and geological strength index

(GSI)

C. Geophysical observations – seismic activity, in situ stresses and their orientation,

micro-seismic activity inside opening.

Significant researches have been done on the basis of field data from the instrumented

tunnels in past. One is missing great opportunity by avoiding the tunnel instrumentation

and not collecting new field data, specially in complex geological conditions.

REFERENCES

Fairhurst, C. (1994). Lecture. Civil Engineering Department, I.I.T., Roorkee

Goel, R. K. (2001). Status of tunnelling and underground construction activities and technologies

in India. Tunnelling and Underground Space Technology, 16, 63-75.

Kastner, H. (1962). Statik does Tunnel - and Stollen Baues. Springer Verlag, Berlin/Gottingen/

Heidelberg.

Merrill, R. H. (1967). Three component borehole deformation gauge for determining the stress in

rock. Bu. Mines Rep. of Inv., 7015, 38.

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15

Tunnelling machines

“Any manager of a project must understand that his success depends on the success of

the contractor. The contractors have to be made to succeed. They may have many

problems. We cannot always talk within the rigid boundaries of a contract document. No,

without hesitation. I go beyond the contract agreement document.”

E. Sreedharan, Managing Director, Delhi Metro Rail Corporation

15.1 GENERAL

The age-old drill and blast technique is still being used in poor countries due to choice

for labor-friendly policies. The time has come for change. We should prepare ourself men￾tally for change and for a fast rate of progress also. The applications of modern techniques

like NATM and NTM involving automated excavation methods are the need of time.

Fig. 15.1 depicts a variety of methods of excavation as a function of strength of rock

material (Jethwa, 2001). Table15.1 shows comparative study of the available techniques

for tunnelling vis-à-vis some of the important parameters like cost, advance rate of tun￾nelling, utilization of money and geometric requirements of a tunnel. A judicious selection

of tunnelling technology may be made with the help of Table 15.1 depending upon the

culture of a nation. Some nations in Asia prefer to evolve slowly for sustainable growth

for a very long time.

15.2 SYSTEM’S MIS-MATCH

An effort to increase the rate of tunnelling requires a system’s approach. The system

in totality should be improved, specially the weakest link which is the installation of

support system in weak rock masses. For example, excavation by a road header will

be meaningless if steel-arch supports are not replaced by SFRS (steel fiber reinforced

shotcrete) support for weak rock masses. A tunnel boring machine is stuck in a thick fault

or shear zone in a complex unknown geological condition, burying the machine. Excessive

failure of tunnel face causes jamming of excavating head. So the choice of selection of

Tunnelling in Weak Rocks

B. Singh and R. K. Goel

© 2006. Elsevier Ltd