688 Mr, James Manser gli [March 18, 



This strain Professor Unwin has calculated at a ton and a quarter per 

 square foot, which was sufficient to make a horizontal tear or rent 

 along the back of the wall. Once this was made the water would enter 

 it, and, acting upwards as a wedge, widen the rent and ultimately 

 overturn the part of the wall above, cutting it right across vertically 

 at each end of the disturbed part, a length of about 190 yards. 



The structure of all the walls in the Elan Valley will be identical 

 in character ; they are being formed of blocks of stone (plums as the 

 men call them) practically unhewn, varying from 5 or 6 cwt. to as 

 many tons in weight, built so as to avoid horizontal bedding planes 

 but with good vertical bonding, and embedded in and surrounded by 

 a matrix of high-class Portland cement concrete. Both the up and 

 down stream faces are being finished with heavy broken-coursed and 

 rock-faced grit or conglomerate blocks closely jointed. The stone 

 weighs about 170 lbs. per cube foot and the concrete about 146, and 

 we are aiming at getting a little more than half the mass oi plums, so 

 that the finished weight of the dams shall be as nearly as possible 

 160 lbs. per cube foot. The design of the walls is such that no 

 effective tensile strain can ever come upon their water faces, but if it 

 did, the structures as put together will resist a tensile strain of at 

 least 12 tons per square foot. When the Caban reservoir is full the 

 total water pressure against the exposed face of the dam will be about 

 60,000 tons. The work is being so built that there shall be no 

 interstices in it, and that each dam when finished shall be to all 

 intents and purposes a monolith, only removable by some great con- 

 vulsion of nature. Without reckoning anything for the cohesibility 

 of the structure, but only considering the weight, the factor of safety 

 against overturning (as did the Bouzey) is from 3^ to 4. 



The drainage area above Caban Coch is by far the largest that 

 has been hitherto dealt with in this country in constructing works of 

 this character. Deducting the reservoirs, the Manchester Thirlmere 

 area is 11,000 acres, the Liverpool Yyrnwy 22,000, and this is 44,000; 

 The provision to be made for passing flood waters during the execu- 

 tion of the works is consequently a very important matter. At the 

 Caban it is quite within the range of probability that at the very 

 height of a flood 700,000 cube feet a minute may have to be dealt 

 with. 



Diagram No. 7 is an outline drawing showing the way in which 

 we are arranging for the passage of such a flood during constructioDj 

 and how it will be disposed of when it comes afterwards with all 

 reservoirs full. 



First of all we cleared out of the bed of the river o?i and for some 

 distance heloiv the base of the wall a very great number of large 

 boulders and some rocks in sHii in order to enable the water to run 

 freely away. We then erected a concrete and timber stank on the 

 Breconshire side of the river to elclude the water, and thus allow of 

 the excavation for the foundation of that end of the wall being got 

 out and the base of the wall and the ferecon culvert built. This has 



