L. C. W. DARLEY. 



When carrying out gravitation schemes for some of the large 

 towns such as Orange, Armidale, and Junee, concrete reservoir 

 walls were erected with gravity sections. The section of wall 

 adopted closely follows that recommended by Professor Rankin in 

 his report on the Tansa Dam for the Bombay Water Works. This 

 wall has a slightly battered inner or water face, the profile of the 

 outer face being a logarithmic curve ; this section has of late years 

 been adopted for almost all large reservoir walls. Such a wall 

 resists the pressure of water against it wholly by its own weight, 

 and is consequently termed a gravity wall. Its principle as laid 

 down by Professor Rankine is such that the centre of resistance of 

 any horizontal plane ought not to deviate from the middle of the 

 thickness by more than about one sixth of the thickness — inwards 

 when the reservoir is empty, outwards when it is full — in order 

 that there may be no appreciable tension at the outer edge of the 

 given plane when the reservoir is empty, nor at the inner edge 

 when it is full. 



This is about the most economical section to which a dam can 

 be constructed to resist the pressure by gravity alone ; but in the 

 case of many small towns the cost of such a dam would be pro- 

 hibitive ; the Department therefore had to resort to a more 

 economical section of dam, and this could only be done by building 

 it in a curved form and treating it as an arch, thus putting the 

 concrete wholly in compression. Several large dams have been 

 successfully constructed on this system in America, but it is only 

 within the last few years that they have been introduced into 

 this State. 



A short description of some that have been erected may be 

 interesting. Curved or arched dams can only be constructed when 

 the valley is comparatively narrow and where sound rock can be 

 obtained the whole way across and up each slope, to form good 

 abutments. In some cases where the configuration of the country 

 did not admit of a curve being fitted in from end to end, a short 

 piece of gravity dam had to be constructed at one end from which 

 to spring the arch. This was done at Tamworth and answered 



