CURVED CONCRETE WALLS FOR STORAGE RESERVOIRS. LIII. 



safe average. Taking the concrete in place at one and a half times 

 stronger than the unsupported test cubes gives an average crush- 

 ing strength of 120 tons per square foot at least six months old, 

 the factor of safety of the work at that age would therefore be 6 

 for a limit of resistance of 20 tons, and 8 for a resistance of 15 

 tons. Mr. Bruce in his paper on the strength of concrete (Proc. 

 Inst. C.E., Vol. cxiii.) considers that the modulus of rupture, 

 found experimentally, may be adopted as the working load in com- 

 pression. The average modulus of rupture of 14 transverse tests 

 of sandstone, and whinstone concrete made by him =16 tons per 

 square foot. 



A limit of resistance so high as 20 tons per square foot should 

 be used only for curved dams in cases where the foundation, abut- 

 ments, and metal for concrete consist of sound, hard, igneous rock. 



To provide against the green concrete being subjected to a 

 greater head of water than it can safely bear during construction, 

 the work is carried up in nearly level courses not more than 3 ft. 

 in height, the depth of water being controlled by the scour and 

 outlet valves. Should a flood occur during construction the walls 

 may be submerged without much risk. This has happened in one 

 or two cases, but no injury was done as the lower portion of dam 

 is so much thicker in proportion- There would be more risk of 

 course from flood waters crossing the top of a newly completed 

 wall, but in all probability the extra thickness provided in the 

 upper portion would make the work quite safe. In all calculations 

 the weight of the wall has been disregarded although it must 

 materially assist in its strength. 



Details of construction of curved concrete dams. — It has been 

 found advisable to construct all our concrete dams by labourers 

 employed directly under the officers of the department without 

 the intervention of a contractor. The reasons for this course are 

 as follows : — Information as to the nature of the foundations can 

 only be obtained from trial shafts, and this is often unreliable. 

 The depths as shewn on the section for a contractor's guidance 

 have to be exceeded in many instances to get down to strata 



