; and in case of the
earthwork dam at the back settling, as would certainly be the case,
unless its construction extended over a very lengthened period, the
puddle would be almost certain to become fissured and leaky; in addition,
the comparative amounts of puddle used in this manner, as compared with
the vertical wall, would be so much increased. With the puddle wall in
the position usually adopted, unequal settlement of the bank on either
side is less liable to affect the puddle, being vertical.
It would be interesting to refer to the embankment of the Bann, or Lough
Island Reavy reservoir, Fig. 8, designed by Mr. Bateman, now nearly fifty
years ago, where a layer of peat was adopted both on the slope, 15 in.
thick, and in front or on the up stream side of the puddle wall, 3 ft.
thick. The object was, that should the puddle become fissured and leaky,
the draught so created would carry with it particles of peat, which would
choke up the cracks and so reduce the leakage that the alluvial matter
would gradually settle over it and close it up. On the same diagram will
be noticed curved lines, which are intended to delineate the way in which
the earthwork of the embankment was made up. The layers were 3 ft. in
thickness, laid in the curved layers as indicated.
It is a moot question whether, in making an earthwork embankment,
dependence, as far as stanchness is concerned, should be placed upon the
puddle wall alone or upon the embankments on either side, and especially
upon the up-stream side in addition.
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