LVIII. C. W. DARLEY. 



As far as possible it is desirable to avoid constructing concrete 

 dams, certainly the upper or thin portions during very hot weather. 

 It has been found by experience that walls so constructed are far 

 more liable to crack during succeeding cold weather, especially in 

 the case of reservoirs that have not been filled for some time after 

 completion, thus allowing them to thoroughly dry out and contract. 

 However, although the cracks, which in some cases may extend 

 almost the whole way down the wall, are very unsightly and 

 alarming to the uninitiated, they need really cause no anxiety to the 

 engineer, for soon after the dam fills with water, thus moistening 

 and expanding the concrete, and partly no doubt pressing home 

 the arch, the cracks close up and fine particles of matter in the 

 water render them water-tight. 



In the case of the Tamworth dam, the top portion of which was 

 built during warm weather and remained dry till after the very 

 cold winter had set in, some apparently large cracks appeared, 

 but they all closed soon after the reservoir was tilled, so much so 

 that it is now quite impossible to detect where they were, like- 

 wise in the Mudgee dam constructed under similar circumstances 

 as regards heat, etc., which also remained unfilled for over a year, 

 several large cracks appeared, some open as much as ^ in. on top, 

 but they all closed up soon after the first filling. In this respect 

 curved dams have a decided advantage over straight gravity dams, 

 for when the latter form of wall cracks, which all long walls of 

 concrete are liable to do when subjected to change of temperature 

 and stand dry for any length of time, they do not so readily close 

 up again. 



Cost of curved concrete dams. — The cost of material of course 

 varies at each site, according to length of rail carriage distance, for 

 carting from railway, distance from suitable stone or sand, and 

 cost of labour vary with adaptability of site for economical 

 working. The actual cost is best illustrated by taking a fair 

 average case in practice and giving the cost in detail. In the case 

 referred to, the distance of rail carriage was 253 miles, and road 

 carriage six miles. Sand was obtained from a dry creek three miles 



