Plate 14

THE CALCULATED RISK

 

   The calculated risk in providing farm water supply by building dams in drought times with earths too dry for effective cohesion was taken when we built forty farm dams in the latter stages of the recent drought.

   Plate 14 illustrates three such dams.

   A large lower valley dam, with a maximum wall height over 30 feet and built of fine dry clays near the end of the drought of 1957, failed when it filled rapidly. The cost of the earth works was £1,200, but if the earth of the wall had been wetted and completely compacted, the cost would have exceeded £3,200. Effective repair will be £300 and is covered by our insurance. Plate 14 (upper) shows the size of this dam (capacity 40 million gallons) and the broken wall.

   Upper middle shows the water line with the grass above, which had commenced to grow before the sudden filling. Lower middle, the shape and finish of the back of the wall, and lower left shows part of the broken wall section standing almost vertical. The floor of the break was a filled large erosion gully which, being slightly moist, did not wash out again. The design and site preparation, including cut-off trench and the cleaning and cultivation of the foundation area for bonding, were all good. The rise in the water level in the dam was much faster than the rising moist zone in the wall, so the topmost dry earth first carried away.

   Plate 14 (lower right) shows the site preparation of another dam, but built of dry granitic earths at "Pakby". In this instance, the cost of adequate wetting of the wall material would have doubled the cost of the dam, which is stable to date but has not yet overflowed through the spillway.

   A third dam, built of dry, fine clay, which failed above the first water line, is illustrated in Plate 14 (bottom). On the left is seen the two holes inside the wall where water channelled through when the dam filled quickly, and at right is seen the condition of the back wall after failure. The opening of the lockpipe valve prevented further damage. When this dam emptied there was an earth bridge over the hole which later broke when a beast walked on it. (The pictures of the two clay-wall dams were taken after the text of this book was printed.) Many dam walls constructed with fine, dry clays are stable, although this material is the most vulnerable to this class of dam failure. No instability has yet developed in walls which we built during the drought with other earths such as the dry granitics, earths containing sand or clays mixed with sand, shale or stone particles.

   The calculated risk is part of the problem of farm water supply and this risk is accepted when considered as a worthwhile business proposition.