THE WATER SUPPLY OF KINGWILLIAMSTOWN. I97 



77/ r Dam. 



The dam was built some nine years ag'o. It is somewhat 

 unusual in plan, consisting of a central portion, which is curvi- 

 linear, and a straight portion on either side, which is the ordinary 

 gravity section. These also act as abutments for the curved 

 portion, the sloping walls on either side of the cushion acting 

 as buttresses. The ends of these are joined by a low concrete 

 wall, into which the measuring weirs are built ; this originally 

 formed a water cushion four feet deep for the overflow of the 

 dam, but, as will be referred to later, has since been raised. The 

 objects sought to be obtained by this mode of construction were: 

 First, economy, as it enabled the section of the dam to be reduced 

 at the point where it was the greatest height, the thickness of 

 the curved portion at the base being only 12 feet, as compared 

 with 27 feet, which would have been required for the ordinary 

 gravity section. Secondly, it would allow for a certain amount 

 of expansion and contraction to take place in the dam throtigh 

 changes of temperature, instead of setting up unknown stresses 

 in the dam itself, which must be produced more or less in long, 

 straight dams built in between solid rocks, in a country where 

 there are such extreme variations of temperature. The central 

 curved portion of the dam is used for the overflow, the actual 

 spillway being 220 feet in length and 3 feet deep. This form 

 of construction has proved quite satisfactory, there being no 

 signs of cracks visible in any part of the structure. The 

 upstream face of the dam was built of plain concrete blocks, and 

 the downstream side of rock-faced blocks. These rock-faced 

 blocks have not proved satisfactory, for although they looked 

 very pretty at first in breaking up the water flowing over the 

 dam, the rock face was rapidly worn oiif by the action of the 

 water. It would, no doubt, have been better if both upstream 

 and downstream faces had been built with smooth concrete 

 blocks. 



The curved portion of the dam was further strengthened 

 by a double row of old railway metals, the railway metals on the 

 upstream, side being continued to the top of the dam ; those 

 on the downstream side were only carried up to a height of about 

 15 feet from the foundation. To those vertical railway metals 

 were bolted horizontal rails throughout the whole length of the 

 curved portion of the dam, and were continued well into the 

 straight portion. Both vertical and horizontal rails were 10 feet 

 apart. These old railway metals were not intended to form 

 reinforced concrete, in the ordinary sense of the word, but to 

 thoroughly tie the work together horizontally and vertically ; also 

 to form a good tie between the curved portion and the straight 

 portion forming the abutments. They were also let some 

 distance into the solid rock below the foundations of the dam. so 

 as to form an additional safeguard against any tendency to slide. 

 These rails were fixed into position before the construction of 



