VORTEX- CHAMBER
SHAFT
CONTOUR INTERVAL — 10 ET,"
0 10 20 30 40
50
STREAM
TRASH RACK
HANDRAILING
PLAN
SIPHON
BLOCK
SIN DIA, IRRIGATION PIPE
T
SECTION THROUGH SIPHON BLOCK
3 FT. DIA. AIR SHAFT
DE-AERATION CHAMBER
SECTION THROUGH SHAFT
FIG. 2.2
Typical drop shaft intake
Plover Cove to Tai Po Tau, from where it will be pumped to the treat- ment works at Sha Tin via the Stage I tunnels.
In good rock the decision whether to line was made on a comparison of the total cost of lined and unlined tunnels of equal capacity. The un- lined tunnel had to be larger because the jagged walls of bare rock would reduce the water flow, but the cost of the lining would be saved. In this scheme it was found that for the large main tunnel it was cheaper to line, the lining in good rock being as thin as could be easily placed since its strength was not important. the subsidiary tunnels an unlined section worked out cheapest but lin- ings were included at points where turbulence would occur.
In
In addition the tunnels were lined wherever they passed through patches of bad ground, and the lining ordered varied from 2 in. of sprayed concrete where a subsidiary tunnel met mod- erately bad rock, up to 15 in. of reinforced shuttered concrete where the main tunnel met soft ground.
All tunnels were designed with a nearly flat concrete invert to facilitate vehicle access for maintenance. Shut- tered concrete linings were normally horseshoe section but, in one place in the main tunnel, a circular section was constructed to resist the pressure of soft material on the tunnel. Surface Works
Most of these are intakes, each consisting of a weir across a stream, diverting water into a tunnel. The weirs were designed to divert all but compensation water and major flood flows into the tunnels. In order to limit the rise in upstream level during floods, the spillways at BA and BB embody partialising siphons, and at AN the discharge into the shaft is via siphons. The siphons are used to keep the water level constant and
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thereby control the maximum quan- tity of water diverted into the intake.
At AN the control is provided by the capacity of the siphons them- selves. The two shaft intakes, BB and AN, include specially shaped en- tries, called vortex chambers, which cause the water to rotate as it enters the shaft. The water then clings to the sides as it falls down the shaft. allowing entrained air to escape up the centre (see Fig. 2.2).
In spite of this it is expected that some air will be entrained by the turbulence at the foot of the shaft. At AN where the tunnel may flow full under certain conditions, a de- aeration chamber has been excavated, where the air can separate out and escape to the surface via a ventilation shaft which rises alongside the main shaft.
At BB the shaft serves a free-flow tunnel and a de-aeration chamber has not been provided. The de-aeration chamber and ventilation shafts follow the designs evolved for the Stage I intakes between Tai Po Tau and Sha Tin.
Three small catchwater channels have been constructed to extend the catchments of intakes AG and AH.
A fourth larger catchwater inter- cepts water from the slopes above Tai Mei Tuk village and discharges directly into Plover Cove.
Intake AN consists of a siphon spillway drawing water from a ге- servoir formed by the construction of a 65 ft, high concrete dam under a separate contract. This dam is sited just downstream of the outfall AO in the Hok Tau valley. Tunnel AK-AO in turn conveys water diverted by two dams in the adjoining Lau Shui Heung valley, one of concrete 75 ft. high and one of earthfill 35 ft. high, also built under a separate contract. These three dams are designed to provide irrigation requirements in the Lau Shui Heung and Hok Tau valleys
where they will augment the areas of irrigable land and provide storage to tide over drought periods.
Water surplus to irrigation needs will flow via tunnels AK-AO and AN-AR and the main tunnel into Plover Cove.
Construction
General
Tunnelling began in April, 1964 and driving was completed in July, 1966. The overall average rate of advance was 2,200 ft. per month, or 322 ft. per heading per month, with actual rates of up to 900 ft. per month.
A special new explosive, "Gurite”, developed by Nitro-Nobel A.B. of Sweden was used in the outer ring of holes as a trimming charge to reduce overbreak. This put up the apparent consumption figures as it is a low concentration explosive. Also a new type of detonator with an improved safety margin against lightning dis- charge was used.
Concrete lining began in Septem- ber, 1965 and is expected to be com- pleted in January, 1967. Too many factors affect concreting progress for overall rates to be significant but in the main tunnel the arch shutters, which are the slowest part of the con- creting process, advanced about 1,200 ft. per month.
Tunnel Driving and Temporary Support (i) Driving
Rail-mounted equipment was used in tunnels of up to 12 ft. unlined diameter and tyred or tracked equip. ment in the larger tunnels. Whereas rocker shovels were used in the smaller tunnels, the chief type used in the main tunnels was the Allis Chalmers TL30 tyred loader equipped to tip sideways into Aveling Barford 7 cu. yd. dumpers (9 cu. yd. heaped).
Far East Architect & Builder December, 1967
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