daring example in existence of the employment of the arch principle.
Its height from the rock bed is 64 ft., and it is subject during floods
to a head of water not much less. The length of the chord of the arc
across the valley is about 250 ft. and the radius 335 ft. The dam was
begun in 1883, with a base 20 ft. thick, narrowing to 13 ft. at a height
of 16 ft. The cost of this thickness being regarded as too great, it
was abruptly reduced to 8 ft. 6 in., and for the remaining 48 ft. it
was tapered up to a final width of about 3 ft. The masonry is described
by Mr Schuyler as “a rough uncut granite ashlar, with a hearting of rough rubble all laid in cement mortar and gravel.”
This dam has been in satisfactory use since 1885, and the slight
filtration through the masonry which occurred at first is said to have
almost entirely ceased.
In New South Wales thirteen thin concrete dams, dependent upon horizontal curvature for their resistance to water pressure, have been constructed in narrow gorges at comparatively small cost to impound water for the use of villages. The depth of water varies from 18 ft. to 76 ft. and five of them have cracked vertically, owing apparently to the impossibility of the base of the dam partaking of the changes of curvature induced by changes of temperature and of moisture in the upper parts. It is stated, however, that these cracks close up and become practically water-tight as the water rises.
Fig. 18.—Section of Bouzey Dam.
Something has been said of the failures of earthen dams. Many
masonry dams have also failed, but, speaking generally, we know
less of the causes which have led to such failures. The
examination of one case, however, namely, the bursting
in 1895 of the Bouzey dam, near Épinal, in France, by which many
Failures.
lives were lost, has brought out several points of great interest. It
is probably the only instance in which a masonry dam has slipped
upon its foundations, and also the only case in which a masonry
dam has actually overturned, while curiously enough there is every
probability that the two circumstances had no connexion with each
other. A short time after the occurrence of the catastrophe the dam
was visited by Dr W. C. Unwin, F.R.S., and the writer, and a very
careful examination of the work was made by them. Some of the blocks
of rubble masonry carried down the stream weighed several hundred
tons. The original section of the dam is shown by the continuous
thick line in fig. 18, from which it appears that the work was subject
to a pressure of only about 65 ft. of water. In the year 1884 a length
of 450 ft. of the dam, out of a total length of 1706 ft., slipped upon
its foundation of soft sandstone, and became slightly curved in plan
as shown at a, b, fig. 19, the maximum movement from the original
straight line being about 1 ft. Further sliding on the base was prevented
by the construction of the cross-lined portions in the section
(fig. 18). These precautions were perfectly effective in securing the
safety of the dam up to the height to which the counter fort was
carried. As a consequence of this horizontal bending of the dam the
vertical cracks shown in fig. 19 appeared and were repaired. Eleven[1]
years after this, and about fifteen years after the dam was first
brought into use, it overturned on its outer edge, at about the level
indicated by the dotted line just above the counterfort; and there is
no good reason to attribute to the movement of 1884, or to the
vertical cracks it caused, any influence in the overturning of 1895.
Fig. 19.—Elevation and Plan of Bouzey Dam.
Some of the worst
cracks were, indeed,
entirely beyond
the portion
overturned,
which consisted
of the mass 570
ft. long by 37
ft. in depth, and
weighing about
20,000 tons,
shown in elevation
in fig. 19.
The line of pressures
as generally
given for
this
dam with the
reservoir full, on
the hypothesis
that the density
of the masonry
was a little over
2, is shown by
long and short
dots in fig. 18.
Materials actually
collected from the dam indicate that the mean density
did not exceed 1⋅85 when dry and 2⋅07 when saturated, which
would bring the line of pressures even closer to the outer
face at the top of the counter fort. In any event it must have
approached well within 312 ft. of the outer face, and was more
nearly five-sixths than two-thirds of the width of the dam
distant from the water face; there must, therefore, have been
considerable vertical tension at the water face, variously computed
according to the density assumed at from 114 to 134
ton per square foot. This, if the dam had been thoroughly
well constructed, either with hydraulic lime or Portland
cement mortar, would have been easily borne. The
materials, however, were poor, and it is probable that
rupture by tension in a roughly horizontal plane took place.
Directly this occurred, the front part of the wall was subject
to an additional overturning pressure of about 35 ft.
of water acting upwards, equivalent to about a ton per
square foot, which would certainly, if it occurred throughout
any considerable length of the dam, have immediately
overturned it. But, as a matter of fact, the dam actually
stood for about fifteen years. Of this circumstance there
are two possible explanations. It is known that more or
less leakage took place through the dam, and to moderate
this the water face was from time to time coated and
repaired with cement. Any cracks were thus, no doubt,
temporarily closed; and as the structure of the rest of the
dam was porous, no opportunity was given for the percolating
water to accumulate in the horizontal fissures
to anything like the head in the reservoir. But in
reservoir work such coatings are not to be trusted, and a
single horizontal crack might admit sufficient water to
cause an uplift. Then, again, it must be remembered that
although the full consequences of the facts described might
arise in a section of the dam 1 ft. thick (if that section
were entirely isolated), they could not arise throughout the
length unless the adjoining sections were subject to like
conditions. Any horizontal fissure in a weak place would,
in the nature of things, strike somewhere a stronger place,
and the final failure would be deferred. Time would then
become an element. By reason of the constantly changing
temperatures and the frequent filling and emptying of the
reservoir, expansion and contraction, which are always at
work tending to produce relative movements wherever
one portion of a structure is weaker than another, must
have assisted the water-pressure in the extension of the
horizontal cracks, which, growing slowly, during the
fifteen years, provided at last the area required to enable
the intrusive water to overbalance the little remaining stability of
the dam.
Reservoirs
From very ancient times in India, Ceylon and elsewhere, reservoirs of great area, but generally of small depth, have been built and used for the purposes of irrigation; and in modern times, especially in India and America, comparatively shallow reservoirs have been constructed of much greater area, and in some cases of greater capacity, than any in the United Kingdom.
- ↑ See Proc. Inst. C.E. vol. cxxvi. pp. 91-95.
