A BIG ENGINEERING PROBLEM.

Otago Witness, Issue 2677, 5 July 1905, Page 78

A BIG ENGINEERING PROBLEM.

By J. T. Noble Anderson', 8.A., C.E.

(For the Witness.)

(See Pictures elsewhere in this issue.) Scientific society in London has been so interestedly the following important problem that I am sure no apology is needed for introducing it to the notice of your readers, many of whom in the country, especially in the mining fields, are often concerned in works where it must be regarded. This problem is : how and to what extent should provision be made against the upward pressure of the water which finds its way by percolation or fissures

in rocks underneath reservoirs?

The extraordinary feature of the discussion which has gone on in London, and one which may well make the public uneasy, is that, though the greater number of the reservoirs which have failed (whether of masonry or earthen) have failed from neglect to provide sufficiently in this direction, yet most engineers still persist in overlooking or imperfectly providing against this great danger. And, further, I cannot find one single text-book which calls attention to this matter, while the text-bo_oks which deal with the mathematical formulae for tho design of raasomy dams notoriously neglect it.

That it is only the very best workmanship which will secure a perfectly watertight foundation or bed for any reservoir dam to sit upon is generally conceded. In fact, out of some nine large and carefullyconstructed reservoirs which I have examined in Australia, only one- was quite staunch. Consequently, to my mind, the correct engineering practice- is to recognise that some leakage or upward pressure will inevitably occur, and provide for it by the cheap and practical expedient oi making masonry drains in the bed of the* reservoir of such a nature as will relieve the pressure without either endangering the structure or robbing the storage of the reservoir of too much water.

The first engineer in whose practice 1 found such a provision was Mr Luke Livingstone Macassey, engineer to the- Belfast Water Commissioners, Ireland, in 1886. And I have always copied him in this respect in any of my work. That, however, the problem is not a new one, and that it may be seen how the upward pressure was duly recognised by leading hydraulic engineers before this, I would mention that the late Mr Thomas Hawksley, when more than 30 years ago consulted by the Liverpool Corporation as to the proposed masonry dam for Lake Vyrnwy, carefully worked out the shape of each layer in depth from regard to what he called the "overturning, uplifting pressure."' And this, which was until last year the highest masonry dam in Great Britain, was designed of cyclopean dimensions in consequence.

The usual practice has been to neglect this factor, and it is not one out of ten of the large dams which have theoretically sufficient dimensions. And this it is which has exercised so much discussion and uneasiness. The question may well be asked, Are all these dams destined to follow their predecessors which failed, some of a few months' life and some after decades of years ?

The starting point of this inouiry, so far as the London newspapers were concerned, was a paper on "Some Disregarded Points in the Stability of Masonry Dams," issued by University College. The paper was due to a Mr L. W. Atcherley, who was supported by Professor Karl Pearson. One immediate effect of this paper was to cause a reference of the question of the stability of the great Assouan Barrage.

The following, from Engineering, March 31, 1905, -will indicate pretty clearly what the result of this inquiry has been : —

In any case, in view of the doubts

raised by Mr Atcherle3''s paper, the engineers to the structure must congratulate themselves on the fact that the dam, in prospect of having its height subsequently raised, was made much stronger than the old theory indicated as necessary. Sir Benjamin Baker has consequently been able to entirely reassure the Egyptian authorities as legards the existing state of affairs.

Translated into plain English, this means that a scheme to raise the dam 22ft, and 60 double its capacity, is now treated as if abandoned. And there is little likelihood of any other site being found where an equally large storage can be secured as cheaply as at Assouan. The Assouan dam, originally estimated to cost £1,500,000, actually cost £2,450,000.

Since then, for an extra small sum. this dam ought to have been made capable of holding double its capacity, the works may now be said to be worth at least two million pounds less than they were assumed to be worth.

Personally, I do not believe that the project to raise the dam will be abandoned, though the public opinion favours that view. On reading the vaiious paperswhich have been published about it, fiom regard to the provision I mentioned at the beginning of this letter for draining the foundation so that the upward hydraulio pressure should be reduced to the minimum, I find that Sir FitzMaurice, who was the resident engineer, in the excess of his zeal to make the work as sound as possible, actually took the very steps moft calculated to increase the upward pressure from the foundations by carefully sealing and corking it clown. Thus he wrote in 1903: —

It had originally b^en intended 1.0 lead all springs through the masonry to the <2awn stream wle of the dam : but ill

the end it was considered that, owing to the water coming in only through these fine vertical cracks, only a email upward pressure could, at the worst, be exerted at the base of the dam.

And consequently he adopted elaborate pains to close every crevice, by "grouting them up one by one." Speaking on the paper from which this is extracted, Sir Benjamin Baker said : The design of a dam was a simple thing. A student at an examination might design the cross section of a dam, but it was quite a different thing to have to carry it out. And (speaking of the engineers in charge) : They watched every detail, and varied the design according to the altered con-

ditions — matters that did not bother an engineer when he was working with a

sheet of paper.

I am afraid that I cannot agree with Sir Benjamin Baker's commendation of the action of the resident and executive engineers in varying the design. Wnile the practice dictated by experience is that all the designs for works should alvays be varied to meet any fresh knowledgeas to the actual conditions to be met, such variations can only safely be made after the matter has been completely referred to the engineer responsible for the original design, and in view of the alteration which led to the Tay bridge disaster, ll;e subordinate officer should be very i-?.ivful that his chief thoroughly understands and approves exactly what is the variation he proposes.

However, to return to the Assouan Barrage question. I anticipate that seme means may be found to reduce the t:pv ard -lift so that at such times when the barrage is required to impound wa^r, the full supply level may be raised lo something near the level originally proposed. This kind of relief, however, cannot always be obtained. For example, in the case of the masonry dam which was proposed at the Lee Stream, where, in place of passing "through" floodgates in the bottom of the dam (aa is done at Assouan), high ficeds are intended to flow "over"' the dam, the important question to regard is the iprooable level of the river on the down stream side, because wTien this is considerable, no matter how the~foundation may have been constructed, the -whole mass below tlis level is submerged, and therefore its waight is reduced by the same weight as the Gnantity of water it displaces ; in other words, it is affected by buoyancy, so that a large part of its bulk is practically reduced to little more than half the weight usually taken when th© design of the dam is calculated.

This frequent, source of ittot tv. cftdgn will now, it is to be hoped, di&appear from general ' hydraulic engineering practice, as a similar erroi was long since corrected in marine engineering practice. But it is noteworthy that such notorious failures as the Bousey dam failed to gain fo** this problem recognition in eaginaer'ng icrtbooks; and it is doubtful 'l pubi'o attention -would have been sufficiently v'veted to tins question to give it permanency if the great Assouan Barrage had 'lot Leen involved. This barrage is admittedly the most striking and impressive engineeringwork of our time, both from its b«>"d construction and from the fact that it has already brought about an increase o-' ever one million to the population of iStfypi.

As a result of the prominence now given to this new problem, it may be e?'peoled that for some time engineers will err in the opposite direction, and pay moie heed to this danger than to the factors in the problem which tend to minimise t. In any case, the incident is note-vorthy, and should help the general public tc ?.ane knowledge of how the science of engiucering is being gradually built up.