that the ordinary discharge will not occupy more than a few inches of its depth.

Notwithstanding all the elements of safety which I have claimed for this embankment, there is one danger to which it and all other earthen embankments are liable, which remains to be provided against-that of the waters of the reservoir overflowing the embankment. This may be caused either by incessant rains or by a sinking of the embankment. The latter is due, as I have shown in a former paper, to its being soaked with water which may come by leakage from the reservoir, by rain, or, lastly, as in the case of the Sheffield reservoir, by the spray blown over from a full reservoir. It is obvious that no embankment could withstand a torrent flowing down its steep slope of "made-up" earth, and that it would be torn up piecemeal in a few minutes. The only remedy hitherto applied to avert this fearful danger is a waste-weir, placed a few feet below the level of the top of the embankment, so as to carry off the flood-waters, and prevent them rising so high as the top of the embankment. Certainly there could be no better remedy contrived if the relative levels of the weir and the embankment were sure to continue. Unfortunately this is not the case; for the embankment, like all earth- · works, is liable to subside, while the waste-weir, which is necessarily built on solid ground, for the sake of its masonry, is immoveable. The consequence is, that the waste-weir is sometimes found higher than the embankment, and therefore useless at the very time when its services are required. This was the case in the Holmfirth and Sheffield tragedies, the embankments in both these cases having subsided to a lower level than the waste-weirs. Engineers are aware of this, but they content themselves with recommending that the embankments be watched and raised to their original height as soon as subsidence has taken place. I am not so charitable as to suppose that hydraulic engineers are not aware of the risk to which they expose their constituents by such advice. What right have they to conclude that flood-waters won't fill the reservoir before their advice can be acted on? or that the subsidence won't take

VOL. VII.

E

place, as at Sheffield, when the reservoir is filled, in either of which cases its destruction is certain? Why is it that they recommend the construction of reservoirs without warning their employers that they contain an element of danger which no prudence can avert, and which has already proved so disastrous? And why is it that six of the most eminent hydraulic engineers of England, who have been engaged to report on the cause of failure of the Sheffield reservoir, tacitly ignore the possibility of the overflow, and consequent destruction of the embankment, being caused by subsidence, to which, however, they acknowledge that all embankments are subject? Is it because they cannot assign subsidence as the cause of the failure of the Sheffield reservoir, without denouncing at the same time every one of the reservoirs of the present day as being liable to the same casualty?

Engineers know perfectly well that there is only one way of saving a reservoir situated as the Sheffield reservoir was -that of drawing off from below the flood-waters and others, so as to rapidly lower the surface to a level of safety. But they also know that it is impracticable, with the present form of embankment, having the puddle wall in the middle, to have discharge pipes or conduits of dimensions capable of doing this, as the danger of leakage, breakage, and deformation rapidly increases with these dimensions. In the safety-dam which I propose, with the puddle on the inner slope, no such difficulty occurs. A stone conduit of large dimensions, of the best form to stand pressure, and not necessarily water-tight, is to be conducted through the whole thickness of the intended embankment, till it approaches the position which the puddle slope will occupy. There it must be constructed of the best and strongest ashlar, laid in cement. It will then be finished off to the inner slope of the embankment, with a sluice frame of metal, in which the valve, nearly of the full dimensions of the conduit, slides up and down under the action of a screw. screw will be worked from the top of the embankment by means of a rod laid along the slope, and properly supported (see woodcut).

The

The sluice frame may be divided longitudinally into several openings, each of which will be provided with a valve, a screw, and a rod for working. These will be worked more

easily than a single large valve.

With such a conduit and sluice, an embankment could be rapidly lowered, even in floods. If the Sheffield and Holmfirth reservoirs had had them, those terrible catastrophes would not have taken place. Without them, no earthen embankment to a reservoir is safe. It may be thought that an exception should be made in favour of those earthen embankments constructed in thin layers well beaten. down. But as even these will subside in some degree, when soaked with water, they should have ample means of drawing off the water from below.

The same sluice serves for regulating the ordinary discharge as for drawing off the flood-waters. It requires no tower in front, as in use at present, when the valves are at the front of the embankment; nor is the water, in its passage through the embankment, subjected to pressure, and consequent danger of leakage, as now, when the valves are placed at the rear of the embankment.

Report of Committee.

The leading feature of the author's plan is the adoption of a sloping puddle stratum on the inner face of the embankment, instead of the central perpendicular puddletrench usually adopted.

Your committee believe there is much truth in Mr Aytoun's arguments as to the superior resistance which would be offered by an embankment constructed on this plan. They are of opinion that, in the ordinary construction, cases might occur in which the central puddle wall would hold back the water draining through the upper half of the bank, and that a film of water being thus collected against the wall, the full, or nearly the full, static pressure of the water would be exercised against the wall and its backing of earth.

The question may be considered under a very simple form, as follows:-In fig. 1 let us assume h to represent alike the

Fig. 1.

height of the water and of the embankment, and b the horizontal width at the base of the lower half of the bank. Putting g for the specific gravity of the material, ƒ for the co-efficient of friction of the layers of the embankment on one another, or of the bank on the foundation, and s for the co-efficient of safety. Assuming the layers to be horizontal, we have

8 × 1h2 = 1h × b × 9 × f.

Taking b as usual at 21h, and for a minimum value g=1.6, we get

sh2=21h2gf.. 8 =

=

Now, f is sometimes very high for earth, but taking it for a minimum as only equal to that for some varieties of stone on stone, or 0.6, we get the co-efficient of safety s=24. On the one hand we have neglected the pressure required to uphold the materials of the upper half of the embankment; and on the other, the excess of the mass of the other half of the embankment over the triangle represented by hxb. These may be regarded as balancing one another. Now, there is evidently no great excess in the value of the co-efficient of safety, and it therefore becomes necessary to pay attention to the details of the construction.

In the very common case shown by fig. 2, where the

Fig. 2.

foundation slopes away from the puddle wall, supposing that all proper precautions have been taken to cut foundations, steps, &c., yet it is evident that, were the earth laid

down originally in horizontal layers, from the greater depth of the embankment down stream, the mass, in settling, would cause the layers eventually to assume a sloping position.

The greatly reduced resistance that would be presented by such a position is evident. To obviate the danger that might arise from this, our best retaining embankments are now formed or built up, with the earth laid in layers sloping inwards, so as abundantly to make up for any excess of settlement on the down stream side. This excellent plan seems to have been first put into practice by Mr Thomas. Stevenson in 1844, in making the reservoir at Hynish Harbour, where a slope of 1 in 7 was adopted. Again, in making the North Esk Reservoir, the Messrs D. and T. Stevenson pursued the same method, except in using a slope of 1 in 10, which was considered sufficient, and caused less inconvenience in carting the earth.

With a central puddle wall, an embankment formed by tipping the earth, as in railway work, would evidently produce a dangerously weak structure. But such a method of embanking could be adopted if the face of the inner slope be that chosen for the puddle, and under this view the proposal in the paper might offer the cheapest and possibly the

Fig. 3.

strongest mode of construction.* The positions of the layers in the lower half of the bank become a matter of indiffer

*Note added after adoption of report.

To prevent misconception here, it may be explained that the committee do not contemplate omitting any of the means usually taken to consolidate the material of the embankment, at least in the upper half of it. The bank would be carried up in layers, but as these layers slope outwards, much less expense would be incurred in depositing the material in its proper place.

R. H. B.