The first sign of the ground Dean spotted was a rain puddle reflecting the glow from the lights of the plane as the wheels touched the runway—one, two, three times before the tired aircraft glided to the taxiway.
The puddle wall is crossed by a pedestal of concrete carry- - 3 ing the brick discharge cul v ert.
When, as may happen in dry seasons, the puddle wall remains long above the water level, it parts with moisture and contracts.
The puddle at a was originally held up by the flat head of this pedestal; not so the puddle at b, which under the superincumbent weight settled down and produced the fault bc, accompanied with a shearing or tangential strain or, less probably, with actual fracture in the direction bd.
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.
The puddle was then replaced, and remains sensibly watertight.
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.
If the soil is allowed to become dry and pulverized, rain is likely to run off or " puddle the surface without penetrating it more than a very short distance.
In such a structure the whole attention as regards water-tightness should be concentrated upon the puddle wall or core.
The embankments on either side of the puddle wall are merely to support the puddle and to keep it moist above the ground level when the reservoir is low.
Water-tight concrete is a suitable material for the purpose; it need not be made so thick as the puddle core, and is therefore sometimes used with considerable advantage in lieu of the puddle for the whole depth below ground.
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.
8 applies also to the ends of puddle walls where they abut against steep faces of rock.
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.
Above ground, if the water level is to be higher than the natural boundary, the same puddle walls or cores are carried up to the required level, and are supported as they rise by embankments of earth on either side.
They may be quite permeable, but to prevent undue settlement and distortion they must, like the puddle, be well consolidated.
The loss of water by leakage through such joints or fissures below the puddle wall may or may not be a serious matter in itself; but if at any point there is sufficient movement of water across the base of the trench to produce the slightest erosion of the clay above it, that movement almost invariably increases.
Thus the permeable vein grows vertically rather than horizontally, and ultimately assumes the form of a thin vertical sheet traversing the puddle wall, often diagonally in plan, and having a thickness which has varied in different cases from a few inches to a couple of feet or more, of almost clean sand rising to an observed height of 30 or 40 ft., and only arrested in its upward growth by the necessary lowering of the reservoir water to avoid serious danger.
Wherever the base of a puddle wall cannot be worked into a continuous bed of clay or shale, or tied into a groove cut in sound rock free from water-hearing fissures, the safest course is to base it on an artificial material at once impermeable and incapable of erosion, interposed between the rock and the puddled clay.
Obviously, the junction between the puddle and the concrete might have been made at any lower level.
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.
Here the straining of the original horizontal puddle in settling down is indicated in a purposely exaggerated way by the curved lines.
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 had 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.
The original material may have been perfectly satisfactory, but if, for example, in puddle the progress of the work a stream of water is allowed to 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.
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.
It remains rather obscure, though the type species originally " was discovered in great abundance in a roadside puddle subject to desiccation."
There are several cases in Great Britain where it has been necessary to carry down the puddle trench to about zoo ft.
The base of a puddle trench is often found to have been placed upon rock, perfectly sound in itself, but having joints which are not impermeable.
Having passed through the puddle core the leaking water sometimes rises to the surface of the ground, producing a visibly turbid spring.