February_1968 — Page 39

stations were non-cumulative and over the total tunnel lengths agreement was remarkably close (see table 4:1).

Tunnel Excavation

Measurement

As the tunnel headings progressed, it was necessary to measure cross- sections at frequent intervals in order to see if the excavation conformed in size and shape to the design section. These measurements were particularly needed in the main tunnel where the entire length had to be fully lined with concrete.

Any excessive overbreak in the rock outside the clearance line would increase the volume of lining concrete above normal requirements but, on the other hand, any underbreak of rock inside the clearance line would not allow either a sufficient concrete thickness or, in the worst cases, suffi- cient space for the erection of the shutter. In the sections where under-

Table 4.2 Tunnel Surveys

Closing Errors at Points of Break-through (Measurements in Feet and Decimals)

Tunnel Section

Length of Chg. Align- Excavation of from Portals Dir. Misclose

ment

Chainage Misclose

B. M. Level Misclose

AA-AB

AB-AD

AD-AE

AE-AJ

AR-AN

AK-AO

BA-BC

BA-BB

13510 7 0.05

9584 4 0.21

10548 4 0.12

7848 2 0.03

5642 3 0.71

4100

0.17

12479 1 0.29

1981 2 0.10

-0.31 0.01

+0.14 0.06

-0.10 0.06

-0.25 0.10

-0.19 0.08

+0.27 0.05

-0.04 0.05

+0.16 0.03

break occurred, the face had to be trimmed back to the clearance line by re-blasting before the shuttering could be placed in position.

Due to the large size of the main

tunnel

Direction of Tunnel Headings

From Portal AA & Portal AF

From Portal AF & Portal AG

From Portal AG & Portal AH

From Portal AH & Portal AJ

From Junction AR From Portal AK

From Portal BA & Portal BC

From Portal BA

a photographic method of measurement was used called the "photo-profil". This system had already been used successfully to measure cross-sections on other tun- nel projects in the Colony; it consists of a light projector (Experfi) powered

PHOTOPROFILE OF MAIN TUNNEL AC-AD-AE-AJ by a 6-volt car battery, a 3 ft. trian-

CENTRE LINE

SPRINGING LINE

TUNNEL

AE - AJ

ALIT

·EXCAVATED SECTION

CLEARANCE LINE

HORSESHOE SECTION TYPE IA

CHAINAGE

SPRING LINE LEVEL

DATE

SCALE

19+

18.770

10-10-1965

I° TO 10'-0"

NOTE: THE INSIDE LENGTH OF BASE OF TRIANGLE IS 3 FEET.

Fig. 4:2 Photoprofile of main tunnel AC-AD-AE-AJ. Theo. cross section area: 807.9 sq. ft. Actual gross sectional area: 891.2 sq. ft. Percentage of overbreak: 10.3.

Far East Architect & Builder February, 1968

gular metal rod fixture and a simple 11⁄2 plate camera with an F1.4 lens.

In this method, a theodolite tripod is first set up on the tunnel centre- line at the required chainage and the projector lamp, together with the triangle fixture, is mounted on the top. The projector is then levelled up and adjusted so that it traces a beam of light about 3-in. wide around the tunnel at right angles to the centre- line.

The camera is then set on the cen- tre-line and at the same height as the projector at a predetermined distance according to the scale at which the photo is to be reproduced. A light box with removable plastic numerals is set up beside the projector giving the tunnel designation, chainage, the height at the base of the triangle as determined by levelling, and the date. The tunnel lights are then switched off and the beam of light around the tunnel is photographed at a five minute exposure time.

The photographer is supplied with a transparent template with the tunnel section clearance line, the centre-line and the springing line drawn to the same scale as the photo. This tem- plate is then placed over the negative and positioned according to the cal- culated level of the springing line at that particular chainage and the level of the triangle base as given on the photo. The resulting print then shows the shape of the tunnel, as excavated, superimposed over the clearance line. springing line, and centre-line (see fig. 4:2).

Meeting of Headings

The first break-through was in the subsidiary tunnel BA-BC on June 23, 1965. The eighth and final break- through was in the main tunnel sec- A tion AA-AB on August 2, 1966. summary of closing errors at the points of break through is given in table 4:2.

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