These remarks equally apply to earthwork dams, as regards sufficient
provision of by-wash, careful execution of work, and security of
foundation, but their area of cross section, supposing them to be
water-tight, on account of the flatness of their slopes and consequent
breadth of base, is, of course, far in excess of that merely required for
stability; but in these latter, the method adopted for the water supply
discharge is of the very greatest importance, and will be again referred
to.

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