78 The N.Z. Journal of Science and Technology. [April 
To understand the scheme at present proposed, fig. 1 should be referred 
to. This shows the provision of two diversion tunnels, each 25 ft. 3 in. 
in diameter, curving in a large sweep well away from the abutments of the 
main dam, with two temporary dams—one on the up-stream side to turn 
the water into the tunnels, and one on the down-stream side to prevent the 
water cutting back on to the main-dam site from the point of discharge 
from the diversion tunnels into the river-channel again. A dry river-bed 
will thus be provided for the building of the main dam. 
The diversion tunnels will start at water-level on the down-stream side, 
though a good block will be left at first to guard against flooding the tunnels 
during construction. The tunnels will be carried out at an approximately 
level grade to the point of entrance up-stream, which will thus put the upper 
end of the tunnel partially below water-level, owing to the grade of the 
river. 
Several methods have been considered for connecting the diverting- 
tunnels to the river, and for forming the temporary dams required to divert 
the river into these tunnels, and to keep it from coming back into the excava¬ 
tions for the dam-foundations. The steep sides of the gorge, and the exist¬ 
ence of an old natural dam formed by a slip centuries ago farther up the 
river, suggest that it would be possible to mine the sides of the gorge above 
the proposed dam site and bring down sufficient material to form an efficient 
temporary diversion. Provision is made for the closing and sealing of 
these tunnels when the dam is completed. 
The driving of the headings of these diversion tunnels as prospecting- 
drives is part of the present scheme of investigation, and this work is now 
about to be started. Drives Nos. 3 and 4 are to be prolonged to the point 
where the diversion tunnels crosscut them, and headings will be driven 
up and down stream from that point to meet headings driven from entrances 
and exits, thus attacking the tunnels from eight separate faces. This work 
and the completion of the under-river drive—now about 30 ft. under way— 
is practically what remains to complete the work of investigation. 
Numerous specimens from the completed drives and shafts have been 
submitted to compression tests. These on preliminary designs are required 
to stand a constant compressive load of 12*5 tons to the square foot. 
This can be reduced where required by spreading abutments. The erosion 
under water-pressure and porosity tests are now under way at the gorge. 
Reservoirs are placed on top of the cliff, and water-pipes are led from them 
down into the gorge, and either cemented into a block of stone taken from 
the tunnels, or turned into the tunnels themselves and cemented into the 
sides of the drives, into holes that have been drilled to receive them. The 
reservoirs are kept full of water, which thus puts the specimen block or 
side of the tunnel under the same water-pressure that it will be called on to 
withstand when the dam is built and holding back the river. Each morning 
the reservoirs are filled up, the exact leakage being carefully noted and 
recorded. 
The amount of evaporation is allowed for by providing one pipe as an 
evaporation test, the end being securely blocked, so that evaporation alone 
is responsible for the amount of daily replenishing. A corresponding 
evaporation deduction is then made from each of the other records. 
Comparisons can then be made with test-pipes let into specimen blocks of 
proved stability and watertightness. 
Some further test-holes had to be sunk on the eastern side some distance 
back from the cliff-edge, as there is a surface depression in the terrace that 
