Before commencing the excavation for the foundations of a dam, it is most
essential that the character of the soil or rock should be examined
carefully, by sinking a succession of small shafts, not mere borings,
along the site, so that the depth to which the trench will have to be
carried, and the amount of ground water likely to be encountered, can be
reliably ascertained, as this portion of the work cannot be otherwise
estimated, and as it may bear a very large proportion of the total
expense of construction, and in certain cases may demonstrate that the
site is altogether unsuitable for the proposed purpose.
The depth to which puddle trenches have been carried, for the purpose of
penetrating water-bearing strata, and reaching impenetrable ground, in
some cases, has been as much as 160 ft. below the natural surface of the
ground, and the expense of timbering, pumping, and excavation in such an
instance can be easily imagined. This may be realized by referring to
Fig. 4, giving a cross-section of the Yarrow dam, in which the bottom of
the trench is there only 85 ft. below the ground surface. In the Dale
Dyke dam, Fig. 2, the bottom of the trench was about 50 ft. below the
ground surface.
There is one other point which should be mentioned in connection with the
form of the base of the puddle trench--that instead of cutting the bottom
of the trench at the sides of the valley in steps, it should be merely
sloped, so that the puddle, in setting, tends to slide down each inclined
plane toward the bottom of valley, thereby becoming further compressed;
whereas, should the natural ground be cut in steps, the puddle in setting
tends to bulge at the side of each riser, as it may be termed, and so
cause fissures.
Pages:
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34