behind in the crevice. The clay roof, rather than the walls of this
crevice of sand, gave way and pressed down to fill the vacancy, and
the leakage worked up along the weakened plane of tangential strain
bd. On the appearance of serious leakage the overflow level of the
water originally at ef was lowered for safety to gh; and for many
years the reservoir was worked with its general level much below gh.
The sand-filled vein, several inches in width, was found, on taking
out the puddle, to have terminated near the highest level to which
the water was allowed to rise, but not to have worked downwards.
There can be little doubt that the puddle at the right-hand angle j
was also strained, but not to the point of rupture, as owing to the
rise of the sandstone base there was comparatively little room for
settlement on that side. In repairing this work the perfectly safe
Fig. 9.—Overhanging Rock Leakage
form shown by the dotted
lines ka, kj was substituted
for the flat surface aj, and
this alone, if originally
adopted, would have prevented
dangerous shearing
strains. As an additional
precaution, however, deep
tongues of concrete like
those in fig. 7 were built
in the rock throughout the
length of the trench, and
carried up the sides and
over the top of the pedestal.
The puddle was
then replaced, and remains
sensibly watertight. The
lesson taught by fig. 8 applies also to the ends of puddle walls
where they abut against steep faces of rock. Unless such faces
are so far below the surface of the puddle, and so related to
the lower parts of the trench, that no tension, and consequent
tendency to separation of the puddle from the rock, can possibly
take place, and unless abundant time is given, before the
reservoir is charged, for the settlement and compression of the
puddle to be completed, leakage with disastrous results may occur.
In other cases leakage and failure have arisen from allowing a part of the rock bottom or end of a puddle trench to overhang, as in fig. 9. Here the straining of the original horizontal puddle in settling down is indicated in a purposely exaggerated way by the curved lines. There is considerable distortion of the clay, resulting from combined shearing and tensile stress, above each of the steps of rock, and reaching its maximum at and above the highest rise ab, where it has proved sufficient to produce a dangerous line of weakness ac, the tension at a either causing actual rupture, or such increased porosity as to permit of percolation capable of keeping open the wound. In such cases as are shown in figs. 8 and 9 the growth of the sand vein is not vertical, but inclined towards the plane of maximum shearing strain. Fig. 9 also illustrates a weak place at b where the clay either never pressed hard against the overhanging rock or has actually drawn away therefrom in the process of settling towards the lower part to the left. When it is considered that a parting of the clay, sufficient to allow the thinnest film of water to pass, may start the formation of a vein of porous sand in the manner above explained, it will be readily seen how great must be the attention to details, in unpleasant places below ground, and below the water level of the surrounding area, if safety is to be secured. In cases like fig. 9 the rock should always be cut away to a slope, such as that shown in fig. 10.
If no considerable difference of water-pressure had been allowed
between the two sides of the puddle trench in figs. 8 or 9 until the clay
Fig. 10.—Proper figure for Rock Slopehad ceased to settle down,
it is probable that the
interstices, at first formed
between the puddle and
the concrete or rock,
would have been sufficiently
filled to prevent injurious
percolation at any
future time. Hence it is
always a safe precaution
to afford plenty of time
for such settlement before
a reservoir is charged
with water. But to all
such precautions should be added the use of concrete or brickwork
tongues running longitudinally at the bottom of the trench, such as
those shown at a higher level in fig. 7.
In addition to defects arising out of the condition or figure of the rock or of artificial work upon which the puddle clay rests, the puddle wall itself is often defective. The original material may have been perfectly satisfactory, but if, for example, in the progress of the work a stream of water is allowed to Defects in puddle wall flow across it, fine clay is sometimes washed away, and the gravel or sand associated with it left to a sufficient extent to permit of future percolation. Unless such places are carefully dug out or re-puddled before the work of filling is resumed, the percolation may increase along the vertical plane where it is greatest, by the erosion and falling in of the clay roof, as in the other cases cited. Two instances probably originating in some such cause are shown in fig. 11 in the relative positions in which they were found, and carefully measured, as the puddle was removed from a crippled reservoir dam. These fissures are in vertical planes stretching entirely across the puddle trench, and reaching in one case, aa, nearly to the highest level at which the reservoir had been worked for seventeen years after the leakage had been discovered. The larger and older of these veins was 4412 ft. high, of which 14 ft. was above the original ground level, and it is interesting to note that this portion, owing probably to easier access for the water from the reservoir and reduced compression of the puddle, was much wider than below. The little vein to the left marked bb. about 312 ft. deep, is curious. It looks like the beginning of success of an effort made by a slight percolation during the whole life of the reservoir to increase itself materially by erosion.
Fig. 11.—Vertical Vein of Leakage
There is no reason to believe that the initial cause of such a leakage could be developed except during construction, and it is certain that once begun it must increase. Only a knowledge of the great loss of capital that has resulted from abortive reservoir construction justifies this notice of defects which can always be avoided, and are too often the direct result, not of design, but of parsimony in providing during the execution of such works, and especially below ground, a sufficiency of intelligent, experienced and conscientious supervision.
In some cases, as, for example, when a high earthen embankment crosses a gorge, and there is plenty of stone to be had, it is desirable to place the outer bank upon a toe or platform of rubble stonework, as in fig. 7, by which means the height of the earthen portion is reduced and complete drainage secured. But here again great care must be exercised in the packing and consolidation of the stones, which will otherwise crack and settle.
As with many other engineering works, the tendency to slipping either of the sides of the valley or of the reservoir embankment itself has often given trouble, and has sometimes led to serious disaster.
