In the Estimate for the completion I have allowed rates less than the average for the whole of last year as the costs during the year showed a fairly steady decrease.
The central part of the tunnel though it cannot be left altogether unlined can have the thickness of the lining very much reduced and a saving made in brickwork quantities compared with the outside lengths.
A rough estimate for the completion is :-
Lining Shaft at North Side, $ 6,000 Heading driving, 105,000 Widening out, 500,000 Lining, 410,000 Depreciation of plant, 100,000 $1,121,000This brings the total cost of the tunnel to $3,000,000 or $1,300,000 more than my Estimate of 1907.
This great increase above the Estimate is principally due to the unusual hardness of the rock met with.
The unusual hardness of the rock came as a great surprise. The interior of the hill consists of quite a different class of rock from that usually met with in the Colony. It is not the same kind of granite at all. In places it combines great hardness with the addition of numerous faults and backs which causes the drills to jamb. The quantity of explosives used is very great compared with other tunnels and this above all other causes, made the greatest difference between the estimated and actual cost.
In September 1908 the South Face heading reached such hard rock that the consumption of dynamite reached thirty pounds per foot run which was about three times what was estimated. This increase applies both to the heading and the widening and shows how misleading comparisons with tunnels in other parts of the world were. This hard rock has continued almost continuously at the South Face but not quite so badly at the North Face.
The great hardness of the rock necessitated very large gangs of blacksmiths to keep the drills sharp and also caused much greater wear and tear on the rock drills than was estimated for.
When writing my last yearly report I also had hopes that it would not be necessary to line the central portion of the tunnel except with a light flying arch and probably not even that. However owing to the hard and massive nature of the rock it is necessary to build side walls from which to spring the arch for the roof as a good bench cannot be dressed in the rock. The rock though very hard is full of faults and backs which render it liable to shake loose and come down with the vibration of the passing trains. Any such accident would be very serious and the risk is not worth the saving obtained by leaving the tunnel without lining.
Damages done by Typhoons were originally charged to a separate unestimated sub-head but afterwards this damage was charged against the works concerned which was chiefly Beacon Hill Tunnel.
In Tunnel No. 3 the headings were completed and all fully widened out and lined except a length of forty-five feet in the centre. The two portals were nearly completed. The length of this tunnel is 329 feet.
One fifteen feet length of lining and one face remained to be completed of Tunnel No. 4 on 31st December, 1908. This tunnel is 170 feet long.
Taipo Tunnel (No. 5) gave a great deal of trouble at the South Face. This side of the hill consists of yellow clay full of water which kept slipping into the cutting approaching the Face of the Tunnel. Great difficulty was experienced in making a start at heading driving as time after time the hillside slipped and blocked the entrance.
It was not until a length had been completely lined outside the slips that a safe entrance could be effected. However all danger is now over. During the year a length of 573 feet of heading was driven out of a total of 924 feet and 158 feet 6 inches lined of which 90 feet is of flying arch type as the rock is very hard.
Tunnels 3 and 4 should be completed for the estimated amount but Tunnel No. 5 may exceed by a little owing to the great expense incurred at the South Face.
3
In the Estimate for the completion I have allowed rates less than the average for the whole of last year as the costs during the year showed a fairly steady decrease.
The central part of the tunnel though it cannot be left altogether unlined can have the thickness of the lining very much reduced and a saving made in brickwork quantities com- pared with the outside lengths.
A rough estimate for the completion is :-
Lining Shaft at North Side,
Heading driving,
Widening out,
Lining,....
Depreciation of plant,
..$ 6,000
105,000
500,000
410,000
100,000
$1,121,000
This brings the total cost of the tunnel to $3,000,000 or $1,300,000 more than my Estimate of 1907.
This great increase above the Estimate is principally due to the unusual hardness of
the rock met with.
The unusual hardness of the rock came as a great surprise. The interior of the hill -consists of quite a different class of rock from that usually met with in the Colony. It is not the same kind of granite at all. In places it combines great hardness with the addition of numerous faults and backs which causes the drills to jamb. The quantity of explosives used is very great compared with other tunnels and this above all other causes, made the greatest difference between the estimated and actual cost.
In September 1908 the South Face heading reached such hard rock that the comsump- tion of dynamite reached thirty pounds per foot run which was about three times what was estimated. This increase applies both to the heading and the widening and shows how misleading comparisons with tunnels in other parts of the world were. This hard rock has continued almost continuously at the South Face but not quite so badly at the North Face.
The great hardness of the rock necessitated very large gangs of blacksmiths to keep the drills sharp and also caused much greater wear and tear on the rock drills than was estimated for.
When writing my last yearly report I also had hopes that it would not be necessary to line the central portion of the tunnel except with a light flying arch and probably not even that. However owing to the hard and massive nature of the rock it is necessary to build side walls from which to spring the arch for the roof as a good bench cannot be dressed in the rock. The rock though very hard is full of faults and backs which render it liable to shake loose and come down with the vibration of the passing trains. Any such accident would be very serious and the risk is not worth the saving obtained by leaving the tunnel without lining.
Damages done by Typhoons were originally charged to a separate unestimated sub-head but afterwards this damage was charged against the works concerned which was chiefly Beacon Hill Tunnel.
In Tunnel No. 3 the headings were completed and all fully widened out and lined ex- cept a length of forty-five feet in the centre. The two portals were nearly completed. The length of this tunnel is 329 feet.
One fifteen feet length of lining and one face remained to be completed of Tunnel No. 4 on 31st December, 1908. This tunnel is 170 feet long.
Taipo Tunnel (No. 5) gave a great deal of trouble at the South Face. This side of the hill consists of yellow clay full of water which kept slipping into the cutting approaching the Face of the Tunnel. Great difficulty was experienced in making a start at heading driving as time after time the hillside slipped and blocked the entrance.
It was not until a length had been completely lined outside the slips that a safe entrance could be effected. However all danger is now over. During the year a length of 573 feet of heading was driven out of a total of 924 feet and 158 feet 6 inches lined of which 90 feet is of flying arch type as the rock is very hard.
Tunnels 3 and 4 should be completed for the estimated amount but Tunnel No. 5 may exceed by a little owing to the great expense incurred at the South Face.
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