This type is slower in loading than the rocker shovel but is more mobile. It was chosen in order to allow each loader to serve two faces alternately, access to each pair of faces being served through one of the southern adits. Ultimately the two faces of a pair would be about 7,000 ft. apart. Caterpillar 977 tracked front-end loaders, operated individually, were used at the two ends of the main tunnel.
Drilling of the shot holes was by light drills on air-legs and using drill- ing ladders. This system (as opposed to the use of rigidly mounted heavy drifter drills) permitted the use of comparatively light and cheap jumbos (see Fig. 2.3). In good rock, all tun- nels were driven full face. An un- successful attempt was made to drive full face through bad rock in AB-AA. Elsewhere in the main tunnel, when bad rock was encountered the top half of the tunnel was driven first and the bottom half later (see Fig. 2.4).
Altogether 4,100 ft. was driven in this way in the headings from AB towards AA and from AJ towards AE. Of this only 400 ft. and 1,200 ft. respectively were in rock bad enough to necessitate this method of advance but, having once changed to gallery and bench, it was simplest to go on in that way to breakthrough and remove the bench afterwards.
a
Only one area of bad ground, about 2,000 ft. west of AB, caused serious difficulty. The original tunnel line connected AAC and ABA by straight line and on October 28, 1965 the 25 ft. adjacent to the face col- lapsed in very shattered and weathered ground. The fall extended upwards for about 90 ft. above the tunnel
crown, the surface being 140 ft. above the crown.
During November and December extensive diamond drilling was carried out from the surface along the pro- posed tunnel line and, on the strength of this, the contractor elected to abandon this heading and drive the tunnel on a new line to the north of the bad patch; this is the sector ABA- ABB-ABC. This new heading, begun three months after the fall, soon en- countered bad ground and recourse was made to gallery and bench driv- ing.
Before each advance of gallery, pilot headings about 100 ft. long were driven at the two springings and in these, reinforced concrete beams were cast as footings for the future gallery ribs. After the gallery was driven, the bench was pierced by a system of small pilot tunnels in which further reinforced concrete beams were cast. These beams were the future lateral strip-footings for the down- ward extensions of the steel arch ribs, for a series of transverse struts be- tween them at 30 ft. intervals and for wall columns rising from the strip- footings up to the springing beams.
In this way the 400 ft. of soft ground was traversed without further incident. One further difficulty was caused by the gallery ribs settling well within the clearance line over a length of 100 ft. It was found that the tunnel line could be lowered corres- pondingly over this length provided horizontal air-transfer pipes were in- stalled through the "sag" in the crown to prevent air being trapped at this point.
This was preferable to the costly and slow process of re-ribbing the
TABLE 2.1
·TUNNELLING DATA
crown at the correct level. Had the tunnel been fully drowned at all operating conditions the "sag” would have been a less serious hydraulic problem.
(ii) Temporary Support
Half the rock traversed by the main tunnel and rather more in the subsidiary tunnels required no support during driving. The remainder need- ed some support and in most cases this was provided by sprayed concrete applied soon after blasting. In par- ticularly bad rocks, steel ribs and lagging were used, or steel ribs and sprayed concrete. This was the first instance in Hong Kong of the use of sprayed concrete as temporary tun- nel-support.
Sprayed concrete differs only from sprayed mortar or "gunite" in having a larger maximum size of aggregate (here 3⁄4 in.). The novelty lies in its replacement of steel ribs as temporary support and in the method of applica- tion by "Robot" (see Fig. 2.5).
Quite a thin layer of concrete sprayed on closely jointed or shatter- ed rock immediately after blasting prevents further loosening of the sur- rounding rock. Thus the rock is given strength to act as an arch and support for both itself and the forces acting on it; if left untreated the rock would collapse in separate pieces and so expose a further layer of rock to do the same. Thicker layers of spray- ed concrete give stability to more weathered rock, the necessary thick- ness being decided by experience rather than exact calculation.
The "Robot" machine is simply a means of holding and directing the nozzle of the concrete spray by re-
Main Tunnel
Subsidiary Tunnels.
Description
AA- AB -AC AD
Southern Branches -AE—— AJ AA-AJ AF-AB AG-AD AH-AE
Northern Area
AN-AR AK-AÓ
BA-BC
Length (ft.) Dia. (ft.)
11790 4010 2100
26
30
26
7600 6640 32140 1721
30 30
15
1726 1202
15
5557
4086
12478
15
12
8.5
12
Temporary Support (% of tunnel length)
None
Spray
5542一
43
38
54
47
51
81
63
65
73
43
89
57
62
45
44
46
12.4
20.4
23.8
27.0
52.0
9.2
Bolting
Steel
Steel Spray
Total
0.5
0.1
5.5
1.2
2.3 45
གྲྭ།།|
9.0
2.3
6.2
16.6
5.3
4.8
1.5
0.8
57
62
46
53
49
18.6
37.0
34.6
27.0
56.8
10.7
Details of Blasting
Pull in Sound
rock (ft.) No. of Shot
holes
12 to 18
115 to 120
12
12
12
9
120 to 130
9
50.55 45.55 45.60 38
81
7
51
8
27
45
Explosives Consumed (lb./cu. yd. Volume calculated at clearance line)
1.46 to 1.59
1.39 to 1.56
2.98
3.06 2.99
3.61 5.61
3.25
Rate of Progress (ft. per week)
Over all average
70
Maximum
212
56 139
63
63
53
127
97
124
133
132
115
225
159
234
Normal
165
90
90
100
80
170
130
200
Cost in HK$
per lin.
yd. to nearest
$10
5730
6310
2060
1600 2000
1120
Far East Architect & Builder December, 1967
55