76 
The N.Z. Journal of Science and Technology. [April 
is likely to prove the correct one is, of course, distinctly satisfactory. The 
shafts disclosed practically no fissures or clefts, but have gone down through 
uniform, firm rock foundation, which has become more solid in a uniformly 
graduated manner in the 50 ft. below water-level that it has been pierced, 
and has proved very “ tight ” and hard to shift with explosives. 
The ordinary water-level is at 182-5 ft., and the deepest river sounding 
obtained was 30 ft., making the river-bottom level 152-5 ft. The bottom of 
the western shaft is sunk to 126 ft. level, the last 5 ft. of this shaft being 
intended for a drainage sump to keep the under river clear of a slight 
soakage from the cliff behind and of any water that may drain in through 
the roof. The floor-level of the drive itself is thus 131 ft., with a slight 
upward grade to 132 ft. at midstream for drainage. The roof of the drive 
will here equal 137-5 ft. level, providing a minimum amount of cover of 
15 ft. between the bottom of the river and the roof of the drive—provided 
no deep, narrow clefts in the river-bottom have been missed in the 
soundings. Bearing the possibility of this in mind, a supply of various¬ 
sized stout wooden plugs and old sacking is kept handy to plug any breach 
that might possibly occur. 
The sounding of the river was one of the most difficult tasks attempted, 
on account of the high velocity of the stream and the roughness of the 
bottom. Finally, satisfactory soundings were obtained by using a sounding¬ 
line of piano-wire with a weight of from 50 lb. to 80 lb. (old truck-wheel's) 
for a sinker. A heavy wire, with a cage on a traveller, was stretched across 
the river at the site ; a small windlass, round which the sounding-line was 
wound, was fitted to the cage, which was allowed to travel along the wire 
so as to get soundings at 5 ft. intervals ; the bottom end of the sounding¬ 
line, with the sinker attached, was then cast into the stream from projecting 
points well up the river, and by the time the current had carried the weight 
down to the line of the dam the leadsman, if lucky, had, with the aid of 
the windlass, got the sounding-line perpendicularly under the cage, while 
the depth was noted by an observer on the river-bank. It was a lengthy 
process, however, occupying two days ; and it can well be imagined that 
for every successful cast there were dozens of unreliable ones that had to 
be rejected. As a matter of fact, practically all the successful casts were 
those where the weight caught on the bottom when under the cage, in 
which case it generally meant that the sinker and most of the sounding¬ 
line had to be sacrificed. Luckily there was a good supply of old iron to 
draw on for the sinkers. 
Although the under-river drive is not yet sufficiently advanced to arrive 
at any definite conclusion as to the watertightness of the rock, it may be 
remarked that it is piercing uniformly firm rock formation, and also that 
it had a partial test about three weeks ago in the following manner : The 
river was in high flood on the 6th October, 1919, rising 10 ft. and flooding 
out Nos. 3 and 4 levels. The shaft and under-river drive, being purposely 
unconnected with the rest of the system in anticipation of such a flood, 
were not affected, and, although the river rose and spread until it was well 
over the end of the under-river drive, there was no leakage and the drive 
remained perfectly dry. Neither was there any lateral leakage from the 
river to the shaft in the 40 ft. exposed to this danger. 
We come now to the discussion of one of the most difficult of the tasks 
that are to be faced before the dam can be constructed, and that is the 
controlling of the river and dealing with such a great volume of water during 
construction—-a minimum of 4,600 cubic feet per second and a probable 
maximum of 20,000. 
