Construction op Earthen Bams. 247 



{d) Suitability of Foundations. — The foundations at the selected 

 site should be carefully inspected. Very often when the surface of 

 tlie selected site seems suitable, by digging a few trial holes in the 

 site it is found that the soil o.verlays a very deep stratum of gravel and 

 sand, making the work impossible. Trial pits should always be sunk 

 in order to find the impervious foundation, such as rock or potclay. 

 If such foundations are not obtainable at a reasonable depth, the site 

 mist be abandoned. 



(e) Waste Weir (Oversjyill). — The site available for the waste 

 weir often determines the feasibility of the storage work. The 

 requirements for the waste are : Sufficient length, a flat longitudinal 

 pnd gently sloping cross-section, hard foundation, proper elevation, 

 and good outfall. The length is necessary to get the discharging 

 capacity, the flat section and hard foundations to do away with costly 

 works in the channel, the elevation of the crest is determined by the 

 full-supply level of the reservoir, and a good outfall is required to 

 pi event damage by floods. The ideal site for a waste weir is generally 

 a low " nek " near to the embankment, but separated from it by a 

 -kopje." 



It is necessary that the waste weir should discharge the maximum 

 flood expected. If it cannot do this, the safety of the works is 

 endangered. In determining the maximum discharged, it is unwise 

 to assume or guess the amount, and farmers wanting to construct a 

 storage work are urged to obtain the required information from the 

 Hydrographic Surveyor, Irrigation Department, who makes it his 

 duty to gather this information all over the Union of South Africa 

 by taking actual measurements. It is far better to construct a waste 

 weir that is several feet too long than one I foot too short. 



If it is impossible to have a waste weir over a " nek " away from 

 the embankment it will be necessary to construct a wingwall up- 

 stream and a lining wall on the down-stream side to prevent the 

 flood water coming over the waste weir scouring away the toe of the 

 embankment. 



(/) Froxiniity of Lands to he Irrigated. — The loss of water in 

 the canals conveying the water from the storage works to the lands is 

 considerable, and very often this loss reaches 60 per cent. The further 

 the lands are away from the reservoir, the greater will be the loss, 

 and in determining the site for the works, this matter is of much 

 importance, as far as expensive works may be required, in order to 

 compensate for the losses in the canals, and these losses may show the 

 supply to be inadequate. 



(2) Davi Einhankinent. 



The construction of the earthen embankment here described, 

 refers only to such as will retain water to a maximum depth of 15 

 feet against the wall. If it is desired to construct works for storage 

 of a greater depth, expert technical advice should be obtained. 



In constructing an earthen embankment the following points 

 must be observed : — 



{a) Section of dam, {h) setting out of dam wall, (c) cleaning 

 seat of dam wall, (d) puddle core, (e) outlet, (/) earthen 

 embankment, (g) pitching, (/i) waste weir or overspill. 



