quire 4 in. of sprayed concrete as temporary support in a 30 ft. tunnel whereas in a 10 ft. tunnel 1 in. would suffice and in a 6 ft. tunnel only oc- casional patches of 1⁄2 in. thick would be needed.
An alternative method of treatment in rock of the tyype just described is tension rockbolting. This was carried out on a small scale but not extended to general use partly due to doubts regarding the reliability of the bolt anchorages in this rock, but also be- cause, with the contractor's existing equipment and organisation, it would have proved a slower method than spraying. Tension rockbolts were. however, used to support slabby rock in the subsidiary tunnels, and non- tension bolts were used in conjunc- tion with sprayed concrete support in the eastern heading at AJ.
Support by steel arches was con- ventional. Steel lagging was speci- fied, instead of timber lagging more common in Hong Kong. Timber would have been suitable, although not necessarily any cheaper owing to the greater overbreak required to ac- commodate it.
In the diversionary heading at Fung Yuen steel ribs were used, with sprayed concrete applied in place of steel lagging or in addition to partial lagging. Sprayed concrete in this in- stance partially replaced the need for packing the voids behind the ribs with pieces of rock, etc., and after it had set it provided some strength in itself and by virtue of the stiffening of the steel arches.
For this reason the cost is sensitive to any large volumes of overbreak outside the ribs. Where such over- break occurred the voids were filled with hand-packed rocks, sprayed over. and subsequently grouted solid.
This
In bad ground near AJ, near AA and in AB-AA it was considered necessary to strut the invert. was done at AJ and AA by concret- ing-in steel H-beams and at AB-AA by reinforced concrete beams cast in tunnels as already described.
Shaft Excavation
a
were
Two shafts, each 15 ft. dia., constructed together with small shaft at CB on the dewatering tunnel. The shaft at BB is 332 ft. deep and was raised, using an Alimak raise- climber to drive a pilot shaft 61⁄2 ft. square up from the bottom, after which the shaft was reamed out to full size starting at the top. This took about 4 months in all.
The shaft at AN is 263 ft. deep and was sunk from the top to its full cross-section. This shaft has an ex- tensive enlarged section near the top to accommodate an elaborate vortex chamber and took 4 months to sink. Shaft-sinking is both slower and more expensive than shaft raising; for example, it would take about I month longer to sink a 300 ft. shaft than to raise it. Shaft-sinking was adopted at AN in order to meet the comple- tion date, access to the bottom of the shaft not being possible until much later.
Far East Architect & Builder December, 1967
Fig. 2.5: Concrete spraying by 'Robot'
Concrete Lining of the Tunnels
In the main tunnel the lining was placed in three stages, namely kerbs, invert and arch.
The kerbs consisted of two strips of concrete down the sides of the tunnel, keyed to receive the arch. These formed the reference surface for the two later stages.
a
The invert was formed with vibrating screed mounted on rails run- ning along the kerbs.
The arch shutters were mounted on rails running along the kerbs (see Fig. 2.6). Each shutter consisted of a crown section and two hinged wall sections, all carried on a traveller and operated by hydraulic jacks. There were six arch shutters, each about 45 ft, long and capable of up to eight pours in a six-day week. There were never more than four arch shutters in simultaneous use and, in fact, the concreting was held up by the delay in tunnel driving between AA-AB.
Concrete was batched centrally at two batching plants at Tung Tsz (AG) and loaded unmixed into transit mix- ers which mixed and distributed the concrete to the various sites. In the case of kerbs and inverts the mixers placed the concrete direct. For arch shutters concrete was transferred by pumps. In all cases poker vibrators were the principal means of compac- tion.
Arch shutters were fitted with numerous windows to give access for this purpose, but at the conclusion of each pour, when all the windows had been closed, external shutter vibrators
were
used to compact the crown. The use of pumps necessitated a very sandy and wet mix with a high cement and water content. A retard- ing plasticiser was added to the mix to minimise the effect of these fea- tures but, nevertheless, they resulted
in rather high shrinkage and conse- quent occasional cracking of the arch.
These cracks were random in pat- tern and probably followed lines of minimum concrete thickness caused by projection in the rock walls. They were not regarded as serious and their occurrence appeared to be reduced by assiduous water-curing. probably be- cause this reduced the temperature rise in the setting concrete. Curing by sprayed-on membrane was also used, but it may have been less effec- tive since it would not affect tempera- ture rise.
In the subsidiary tunnels there was invariably a
The concrete invert. arch, where the rock was bad, was given either a permanent lining of shuttered concrete or a further coat of sprayed concrete 2 in. or 4 in. thick depending on the rock condi- tion. Where the tunnels were inter- sected by isolated seams of bad rock these were treated individually with The in- reinforced sprayed concrete.
verts were concreted in two stages, lateral trips (or kerbs) being con- structed first and followed by the cen- tral strips. Concrete was transported in the tunnels by rail-mounted agita- tor wagons.
Water flows only 2 or 3 ft. deep in the three southern adits due to their steep gradients. It was therefore only necessary to line the tunnel up to about the springline, and the arch was generally omitted.
Cost of Tunnels
The cost per yard of the tunnels varied along their length according to the temporary support applied during driving and the type of permanent lining. For simplicity the figure given in table 2.1 for each size of tunnel is the overall cost per linear yard.
The main tunnel was lined
57