River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

by E. S. Bellasis

In cases where, owing to a fissure in the rock below the bottom of the puddle trench, water comes through under the puddle, it is usual to carry it away in a pipe running vertically in a groove up the side of the trench and then horizontally till it emerges from the dam. Such water, and any other leakage, can often in Great Britain be used as part of the compensation water. There is, however, a certain chance, when there are water-bearing fissures in the rock below the bottom of the trench, that some percolating stream of water may wash away the puddle, and it is preferable to use a concrete core-wall in such cases, carrying it up to about ground-level and keying it into a much thicker wall of puddle which is carried up to the water-level.

It has been suggested that the clay puddle or other impervious layer should be placed, not vertically and in the middle of the dam, but lying on the upstream face of the dam, so as to keep out water from the whole dam instead of from only half of it. Objections to this, if clay puddle is used, are that vermin may bore holes in it, and that, with some clays, it would slip. These objections might be overcome to some extent by laying a pitching of concrete blocks over the puddle. Other objections, applying also when masonry or concrete is used, are that the superficial area and cost are increased, and that cracks would occur from settlement of the earth and from changes of temperature when the water in the reservoir was low. A good many cases have occurred in which an impervious layer laid on the slopes has failed from one cause or another. In France it is usual to rely on such a layer—concrete is used—and to dispense with a core-wall. The practice of having a vertical wall appears to be the best, and is the most widely adopted. When puddle is used the weight of the mass above it forces it to completely fill the trench, and when once it is in position and covered up it is not at all likely to be damaged.

Fig. 61.

The outer portion of a high embankment sometimes slips ([fig. 61]), and precautions should be taken against this. A slip may occur if the site of the dam has not been carefully selected as to geological formation, or if there is unequal settlement owing to the work having been done at different times. One cause of slips is sudden and partial changes in the degree of saturation, and another cause is excessive saturation. Some clays when wet require extremely flat side slopes, and will not stand even at 5 to 1. The outer parts of the embankment are not required for stopping percolation (this will be further considered in the following paragraph), and, though they must be carefully laid and consolidated, they should be of porous material, and the part on the downstream side of the dam must be well drained. A series of surface drains may be arranged and filled with loose stone and gravel. There is also a distinct advantage in using heavy material such as small stone for the lowest portion of the outer parts on both sides of the dam. When good material cannot be obtained, the side slope on the downstream side of the dam may be flattened. A side slope starting at the top with 3 to 1 and becoming 4 to 1 lower down, and finally 5 to 1 at the base, is a very good form for prevention of slipping and generally for the safety of a dam. The part on the side next the reservoir is not likely to slip. It becomes soaked, but it has the pressure of the water against it and is pitched. In Madras, where reservoirs are very numerous, the slope on the side next the water is generally only 1½ to 1.

The different parts of an earthen dam fulfil two distinct functions. Some parts, which may be called the staunches, have to stop the percolation of water from the reservoirs. Other parts, which may be called the supports, have merely to hold up the staunches. In the British type of dam the portion nearest the core-wall on either side ([fig. 60]) is generally made of earth specially selected for impermeability. The distance to which it extends from the wall depends partly on the quantity of such earth available. In any case it has to be very carefully made and consolidated, to avoid unequal pressures on the core-wall, or unequal settlement which might cause it to part from the wall. One of its functions is to keep the core-wall moist when the water-level in the reservoir falls. Whether it is also to be considered as a staunch or a support might at first appear to be of no consequence, but it is of importance as affecting the question of drainage. The support on the downstream side of the dam must, as has just been seen, be made of porous material and be well drained; but obviously a staunch must not be porous, nor can it be penetrated by drains. The question must be decided in each case according to judgment. In a discussion which took place on the above-mentioned paper by Strange, at the Institution of Civil Engineers, much diversity of opinion was expressed among eminent engineers as to the desirability of draining the downstream half of the dam, i.e. the part downstream of the core-wall. By some it was urged that drainage is necessary to lessen the chance of the earthwork slipping. Others contended that any drain which penetrates the dam must facilitate the percolation of water from the reservoir. It is clear that some of the speakers regarded the dam downstream of the core-wall as being partly staunch, and some as being wholly support. If for any reason there seems a chance of water leaking through the core-wall, it is desirable to regard the earth-filling next to it as staunch.

In Western India a kind of puddle is made by mixing three parts of “black cotton soil” with two of sand. The object of the puddle wall is only to prevent water from finding its way along the surface of the ground. It is carried down only to a fairly water-tight stratum and is carried up only to 1 foot above the ground. Above that the mass of the dam is made of black cotton soil as a staunch, with more porous material on both sides of it.

In order to afford full protection against waves and their splashes, the pitching on the upstream face of a dam should extend up to a height of 5 feet, measured vertically, above the highest water-level. In the case of a dam in which the “fetch” or distance over which the waves have been in process of formation exceeds two miles, the above height should be slightly increased.[21] The pitching is usually of stones roughly squared at their outer ends and laid on a layer of broken stones.

The water from a reservoir is usually drawn off by means of pipes which are laid inside a masonry culvert built under the dam. The pipes can thus be inspected. The culvert is blocked at its upstream end by a thick masonry wall through which the pipes pass. Accidents which have happened in the past have been due to weakness of the culvert or to water finding its way along the outside of the masonry. The culvert can be made of proper strength, and it should have a thick coating of clay puddle which is worked into the clay-puddle core-wall of the dam. If the core-wall is of masonry or concrete, the masonry of the culvert is properly joined to it. In many cases the culvert and pipes are taken through a cutting or tunnel and not under the dam.

At the upstream end of the culvert there is a masonry tower—access to it is obtained from the top of the dam by a foot-bridge,—and from it valves for opening and closing the pipes are worked. If the reservoir is for the water supply of a town, it is arranged, by means of a vertical pipe, that the draw-off can be at various levels so that the surface-water can always be used. In the case of some of the towers at the reservoirs whence Birmingham is now supplied, the vertical pipe consists of a number of steel cylinders with gun-metal faces which are so accurately made that the joint is water-tight when one cylinder merely stands on another. The draw-off is obtained from a given level by lifting a particular number of cylinders. Sometimes the tower is made of reinforced concrete. When it is lofty it should be strong enough to resist a strong wind, blowing when the reservoir is empty.