4o6 Report S.A.A. Advancement of Science. 



D = Depth of water to be impounded, which should be 

 calculated from the maximum overflow or floofl 

 level. 



s = Safe crushing strength of material per square foot. 

 This varies from 20 tons for granite ( jr 1 )asalt 

 abutments to from 10 to 12 tons for sandstone 

 and other similar formations. 



For convenience of construction the inner face has been kept 

 vertical, while the outer face has been battered (as shown on the 

 accompanying drawing). The flood water passes over the crest of 

 the dam — no special provision being made for its discharge, except 

 in cases where the catchment area demands it. 



The thickness of the wall at any depth is calculated from a 

 simple diagram in the following manner : — 



P = D X 62*5 = D X 027902. 

 2240. 



T = R^P. 



s. 



T = R.D. X '0014 when s = 20 ton.s. 

 and T — R.D. x '0023 ., s = 12 „ 



Although the theoretical cross section of the dam would form 

 a triangle having its apex at top water level, it is the usual practice 

 to increase the thickness at overflow level from 3 feet to 5 feet, 

 according to circumstances, as in most cases the walls are liable to 

 shock from floating timber during floods. 



In calculating the rross section the weight of the wall itself has 

 not been taken intO' account, although, of course, it is a considerable 

 factor on the safe side. 



Construction. — The methods of construction are of the simplest, 

 and the writer had an opportunity of seeing the work at Wollongong 

 while the foundation courses were in progress. 



The work is carried out departmentally for the reason that it 

 is not always possible to determine the actual depth of foundations 

 from trial pits, and contractors' claims and disputes are a\oided. 

 The workmen employed under the Resident Engineer are used to 

 this class of work, and a more uniform standard of construction is 

 obtained. 



It has also been found that the work can be carried out depart- 

 mentally quite as economically as if done under contract, and so far 

 the Department has had no reason to depart from this course. 



Foiindatioiis. — All loose or shaken rock is removed, and all 

 smooth or inclined surfaces are stepped and roughened. It is not 

 visual to cut a trench for the fijundations where the surface rock is 

 fairly uniform, but the whole of the base is well washed with a jet 

 of water under a pressure of about 20 lbs. per square inch, and all 

 jo::'itc or fissures are raked out and grounted with cement mortar. 



