Construction of Earthen Dams. 



249 



of pervious stratum, and through this stratum well into impervious 

 k yers of either potclay or rock below it. The section of this trench is 

 as shown in the sketch below. 



This trench is now filled with clay. The filling must be con- 

 structed as a compact mass filling the whole section of the trench, 

 care being taken to avoid badly consolidated joints or porous layers. 

 This can be prevented by constructing the different layers as rapidly 

 as possible and by keeping wet the surface attained whenever the 

 work is stopped. The clay must be damp and plastic, but not wet. 

 This puddle filling is generally carried up to 2 feet above the surface 

 of the ground. 



If good retentive clay cannot be pj'ocured, a core wall of concrete 

 is generally substituted, having a section as shown above. 



The concrete used for the core is made in the following propor- 

 tions : 1 part cement, 2 parts sand, and 4 parts broken stone. 



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(e) Outlet. — For our requirements, a 6-inch pipe with a valve 

 on the down-stream side will suffice. In very rare cases a delivery 

 exceeding that of a 6-incli pipe will be needed. The position of the 

 outlet pipe should be so chosen that settlement in the dam wall will 

 ha«/e no injurious effects on it, and the best place will be where it can 

 be laid in rock or in hard sound insoluble material. If these are 

 not available, the outlet pipe should be laid in a trench well carried 

 down into the ground, and the pipe embedded in a casing of concrete 

 with a wide base. Whether laid in rock or otherwise the pipe should 

 be encased in at least 6-inch of concrete. When deciding where the 

 outlet pipe is to be put, it should be borne in mind that space should 

 be provided below the outlet for the accumulation of silt in the 

 reservoir; the capacity of the reservoir below the outlet is smaller 

 relatively than the capacity above the outlet; it is, therefore, not 

 worth much extra expense to utilize the former, the lower the sill 

 the greater the cost of the outlet and the danger of damage through 

 settlement of the dam ; generally the canal will gain command of 

 the ground quicker and will be shorter for a higher sill than for a 

 lower one. At the inlet the pipe should have a head wall and if the 

 pipe is level with the ground a channel should be excavated from the 

 pipe into the reservoir basin to ensure the tapping of the reservoir 

 down to the level of the intake. A staunching ring round the outlet 

 pipe, situated in the centre of the dam will prevent any percolation 

 of water along the sides of the pipe. A head wall at the outlet of the 

 pipe is also required. A basin 5 feet wide and 12 feet long excavated in 

 front of the outlet and paved with a 9-inch grouted pitching will 

 pheck the rush of the water from thp outlet pipe into the canal, and 



