slope is 3-1/2 (or,
and the outer slope is 3 to 1. In both cases the puddle walls have been
carried down respectively forty-six and forty-seven feet deeper than
the base.
The
materials selected for the embankment play a very important part. The
best combination consists of gravel, sharp sand, and clay, properly
proportioned, which give weight, cohesiveness, stability, and
impcr-viousness.* The weight of the wall must be opposed to the thrust,
the height and length are determined quantities, and the thickness is
the only remaining factor for adjustment.
Puddle Walls.—Engineers
differ in opinion as to the value of puddle walls. They are designed to
prevent leakage through or beneath the embankment and reach from the
top to below the base. They should be from six to eight feet thick on
top, increasing downwards by offsets at the rate of about one foot for
every three or four in depth.
Where
the embankment is composed of loose material and the water comes in
contact with the clay puddle, it is advisable to enclose the puddle in
concrete, or a watertight wall should intervene between the puddle and
the reservoir.
A
properly constructed embankment, with the inner slope and the bottom of
the reservoir, especially near the toe, securely protected by means of
puddle, concrete, or stone facing laid in cement, is considered by some
engineers preferable to a puddle wall in the centre of the dam.
Shrinkage of Embankments.—The following are the approximate averages of the shrinkage of embankments according to Trautwine (1882, p. 630):
Gravel or sand..............................8 per cent.
Clay.......................................10 per cent.
Loam.....................................12 per cent.
Loose vegetable surface soil.................15 per cent.
Puddle clay................................20 per cent.
* See Fanning, " Water-Supply Engineering," pp. ,53^-342.
