Tunnel Excavation
After reaching agreement on bench mark vakies and the co-ordinates at the portal control points, excavation of the tunnel headings could then proceed
The tunnel centre-line was produc- ed by setting up a theodolite on the portal control point, backsighting on a distant trig station and then turning off the calculated angle to fix the required tunnel bearing. The centre- hine traverse thereafter was extended through the tunnel by hearing and distance measured from the control
point.
Centre-line traverse stations were located at bends and junction points and at 300 ft. to 400 ft. intervals along the straight sections. These stations consisted of steel plates set in concrete on the tunnel invert and the centre point was fixed by drilling a small hole in the plate.
The driving of the tunnel headings was controlled by the centre-line car-
56
traverse
ried forward from these stations and all subsequent erection of shuttering for concrete lining was set-out from these points.
During the tunnel driving opera- tions, it was always too difficult to put in permanent centre-line stations any closer than 300 ft. to the face owing to the proximity of the drilling platform and other excavation plant. Temporary centre-line points made, however, by stretching a length
were
of wire across the tunnel between two hooks set in each side about 5 ft. above the invert; a ring was then at- tached at the centre point to hold a ranging rod.
Two or three points fixed in this way were sufficient to mark out the centre-line at the face for the drilling crew and, after use, the wires could simply be wound up until required again. Level bench marks were established at intervals of approxi- mately 300 ft., consisting of an iron rod fixed into the side of the tunnel. Frequent cross-checks were carried
Table 4.1 Subsidiary Tunnel BA-BC Centre-Line Traverse Measurements (In Feet and Decimals)
Heading from Portal BA
Heading from Portal BC
Measure- ment by Steel Tape
Measure- ment by Subtence Bar
Differ-
ence
408.92 -.09
434.24 +.01
353.87
} 798.65 +.02
-.02
Traverse Station
Measure- ment by
Steel Tape
Measures ment by Subrense Bar
Differ
ence
Traverse Station
BA 10
BC 10
478.50
478.51 +.01
BA 11
BC II
345.73
345.76 +.03
409.01 434.23
BA 12
BC 12
277.96
277.93
-.03
353.87
BA 13
BC 13
648.43
648.39
J
-.04
407.00
BA 14
BC 14
105.56
105.54 -.02
391.63
BA 15
BC 15
379.98
380.05 +.07
BA 16
BC 16
418.38
418.36 -.02
BA 17
BC 17
439.62
439.58
.04
BA 18
BC 18
561.83
561.80 -.03
BA 19
BC 19
371,90
371.86 -.04
473.70 )
287.72
224.44 ) 274.08
386.69
761.40
BA 20
BC 20
417.53
417.49
.Onf
444.96
BA 21
BC 21
478.88
478.84
.04
202.40
BA 22
BC 22
617.20
617.28
+.08
383.48
BA 23
BC 23
313.40
BC 24
310.12
BC 25
266.02
BC 26
323.57
313.36 -.04 310.20 +.08 266.06 +.04 323.64 +.07
BC 27
291.04
BC 28
745.68
} | 1036.68
-.04
BA 23
Totals 5541.50
Steel Tape
12,464.54
498.42 —.10
386.70 +.01
445.05 +.09
} 585.90 +.02
5541.39 -.]} Totals 6923.04 6923.09 +.05
Total Distances BA 10 To BC 10
Subtense Bur
12.464.48
Calculated from Triangulation
12.464.58
out to agree on alignment and level as the excavation progressed.
Instruments
Wild T2 theodolites reading direct- ly to one second of arc were used for the triangulation survey and setting- out of the tunnel centre-line traverse. In the tunnel the T2 was used in
Wild conjunction with
traversing equipment consisting of two extra tripods, battery illuminated targets and attached tribrachs interchangeable with the theodolite.
With the above system of travers- ing, very precise centering could be carried out over the centre point, this being particularly necessary when ex- tending the traverse from the southern subsidiary tunnels into the main tun- nel and from the main tunnel into AR-AN (see fig. 2:1 paper 2, FEA & B December 1967).
At these junctions the subsidiary tunnels curved around into the main tunnel, thus reducing the effective length of the traverse legs at these points to between 60 and 100 ft. This meant that some thousands of feet of tunnel heading had to be con- trolled from the bearing of this very short length. Consequently several repetitions of the angle readings were observed and cross-checked at these points to achieve the maximum pos- sible accuracy.
Chainage Measurement in Tunnels
The precise measurement of the tunnel centre-line traverse was carried out by two different methods, the contractor using steel tapes and the C.R.E. using the invar subtense bar. The steel tapes of 100 metres length had been given a certificate from the Swedish Government Control Depart- ment and corrections were made ac- cording to this standardisation. These tapes were also checked and agreed with the Hong Kong standard.
When measuring in the tunnel, spring balances were attached to give the required tension and temperature readings were taken at the same time. In the main tunnel, the tape was sus- pended in a catenary but, in the smaller tunnels where rail tracks had been laid, the tape was accepted as fully supported along the timber sleepers. Lengths between traverse stations were measured four times, twice in both directions.
The invar subtense bar, in conjunc- tion with the T2 theodolite, was used to measure lengths of up to 300 ft. (theodolite to subtense bar), observing three random readings to maintain the required accuracy.
By using these separate methods of measurement, agreement was, there- fore, reached with different basic sources of error being inherent. In the case of the subtense bar, any in- accuracies were mostly due to ob servation and instrument errors and no other corrections were required. It may be noted that the differences of measurements between traverse
Far East Architect & Builder February, 1968