766 REPORT— 1903. 



Past President Inst.C.E., who was President of this Section at the Meeting at 

 Nottingham in 1866. 



Earthen embankments are formed of the most suitable materials to be obtained 

 by excavation in their neighbourhood ; the water is retained by a wall of water- 

 tight clay puddle forming the core of the embankment, extending for its whole 

 length and continued at each end into the natural ground forming the hillsides. 

 This puddle core has to be carried down into the ground until watertight strata 

 be met with, occasionally necessitating a puddle trench having a depth of 80 feet 

 or more below the bottom of the valley and 200 feet or more in depth in the hill- 

 sides. Where the strata forming the sides of the valley are not watertight, it is 

 necessary to continue the puddle core along the sides of the reservoir by means of 

 wing trenches. The determination of the depth and extent of the puddle trench 

 in order to secure the watertightness of the reservoir is one of the most difficult 

 and anxious duties of the engineer on whom rests the responsibility of its 

 construction. In forming his judgment he has to rely entirely on his experience 

 for guidance, this being one of those matters which cannot be learnt at an 

 engineering school or even in an engineer's offi.ce. How much depends on the 

 exercise of a wise and trained judgment may be understood when it is realised 

 that an error in this respect may result in very costly works having to be subse- 

 quently undertaken to stop an escape of water which might in the first instance 

 have been prevented by a comparatively small outlay. 



Provision has to be made for the passage of flood- waters during the construction 

 of the embankment. This is ordinarily effected by the construction at about the 

 level of the stream of a tunnel of sufficient diameter to convey with only a slight 

 head the volume of water produced by the greatest flood which experience has 

 taught us to anticipate. This tunnel is sometimes formed beneath the embank- 

 ment, but pi-eferably, where the circumstances are favourable, it is carried through 

 the natural ground near to one end of the embankment. A shaft is built in 

 connection with the tunnel, in which, after the embankment has reached its full 

 height, are placed the outlet valves of the reservoir. 



It is of the utmost importance that ample provision should be made for carrying 

 off the flood and other surplus waters coming from the gathering ground when the 

 reservoir is full, for if this be not done serious consequences may ensue, including 

 the washing away of the embankment with resulting destruction of property and 

 even of life. The surplus waters sometimes fall down a shaft erected within the 

 reservoir, and make their escape by means of the tunnel previously mentioned, 

 but more frequently they flow ov.er a .masonry weir and reach the stream below 

 the embankment by means of a bye wash formed in the hillside. In my opinion 

 the latter method is in most cases to be preferred, as being free from the risk of 

 blockage by ice to which the shaft and tunnel are liable. Engineers are occasion- 

 ally reproached with extravagance in the magnitude of the provision made for the 

 escape of flood waters, but it must always be borne in mind that a maxitnimi flood 

 has to be provided for, such a flood as may occur only once in twenty or thirty 

 years, but which must find a means of escape when it does occur, without danger 

 to life or property. 



Masonry dams are not so frequent in this country as earthen dams, partly by 

 reason of tlieir greater cost and partly because the geological conditions are gene- 

 rally not favourable to their formation, for not only do they require a supply of 

 suitable stone near to hand for their construction, but they also need an incom- 

 pressible foundation, such as rock or very strong shale. Any irregularity in the 

 compression of the foundation occasioned by the weight of the dam would be 

 liable to fracture the masonry of which it was built. 



In the case of masonry dams a tunnel for the passage of flood waters during 

 construction is formed at a suitable level in the masonry of the dam, and after 

 completion of the work they are generally allowed to pass over the top of the dam 

 for the whole or a portion of its length, thus obviating the necessity for and the 

 cost of an independent bye wash. 



Whilst masonry dams have the advantage over earthen dams of not being 

 liable to be breached by a waterspout, I am not aware of any case in which an 



