CURVED CONCRETE WALLS FOR STORAGE RESERVOIRS. LVII. 



suitable tool between the concrete and the mould-board. It is 

 best obtained by placing the concrete in position in a fairly wet 

 condition. But wet concrete contracts by the gradual escape of 

 excess moisture and vertical cracks would appear in the wall, and 

 therefore to guard against this it becomes necessary to use the 

 concrete as dry as possible and ram thoroughly. To obtain the 

 impervious skin the concrete face should be floated with neat 

 cement immediately the mould-boards are removed and while it is 

 in a green and moist condition. 



The whole of the concrete is run in on a tramway of a gauge 

 narrow enough for the skips to pass between the mould-boards on 

 the thinnest part of the wall. If the site is suitable, the stone- 

 crusher, mixing boards, etc., are placed so that the material 

 gravitates through all the processes to the work. Where the 

 pipes pass through the dam they are carefully cleaned, washed 

 over with cement grout and bedded in, and surrounded by one to 

 one cement mortar. 



Outlet Works. — The outlet works generally consist of a large 

 cast-iron scour pipe, usually 24 inch diameter, controlled on the 

 outside by an ordinary double faced stop valve operated from the 

 top of the dam. At the inner end of the pipe a bell mouth is 

 formed in the concrete to facilitate the insertion of a wooden ball 

 should the necessity arise to remove or repair the stop valve. A 

 cast iron offtake pipe about twice the capacity of the proposed 

 main is built into the wall at a somewhat higher level than the 

 scour pipe and fitted on the outside with a stop valve, and on the 

 inside with a trunnion joint and moveable wrought iron galvanised 

 pipe with a galvanised wire netting screen at the end. By means 

 of an ordinary crab winch fixed to a platform on the top of the 

 dam, the offtake pipe can be lowered or raised as required, so as 

 to draw off near the surface, it can also be hauled up to the vertical 

 position and the screen cleaned when necessary. In small reservoirs 

 where there is little wave action, the moveable pipe is buoyed by 

 a float. The crest of the dam forms the waste weir, which is 

 made as long as possible where the catchment is large. 



