A Complete History of The Great Flood at Sheffield
by Samuel Harrison
Web Page 34

(120 contd.)
(THE INQUEST. Contd)

MR. LEATHER RE CALLED.

Mr. LEATHER recalled: I did not disapprove of the contract taken by the Company for the construction of the work. Seven tenders were sent in; two were below my estimate; that of Mr. Craven, which was taken, was £900 more than my estimate. He is carrying out the work under the same contract, and has received a large sum for extras.

Mr. B. SMITH said the original contract was £27,469. About £1,000 was added on the change of situation of the embankment. The extras were upwards of £6,000, and they were going on still. Mr. Leather certifies for the extras on the reports of Mr. Gunson, occasionally coming over to look at the work himself.

EVIDENCE OF MR. RAWLINSON.

Mr. R. RAWLINSON, Government engineer, was sworn, and said: I have had experience in the construction of waterworks. I have executed water works at Wigan, Berwick on Tweed, Swansea, and other places. As inspector, I have made myself acquainted with the great works erected during the last twenty years, and have personally inspected many of them, including those at Liverpool, designed and executed by Mr. Hawksley, and those of Manchester, designed and executed by Mr. Bateman. I have inspected carefully the Bradfield reservoir, which has failed, and other reservoirs of the company, and the reservoir at Agden now being constructed. I have also carefully looked over the specifications. As a practical engineer, I think no water works embankment ought to have pipes laid through it so as to Prevent repair or renewal when necessary. The engineer should be master of his work; not let it be master of him. I know of many instances of failure when pipes and culverts have been laid through the main embankment. Cast iron pipes having plain socket joints, although previously tested in four times the head of water they may have to carry, are occasionally found fractured and defective when the water is turned on, although in the street trenches in which they are laid they have only four feet of earth to bear. With the most careful jointing, the joints are sometimes found to be blown or defective. Pipes laid in the streets can be repaired without much damage. A defective pipe in this embankment may have worked the destruction we have witnessed, and no human ingenuity could prevent it under the circumstances in which those pipes exist. I do not say that it did do so; I wish that to be clearly understood. That, therefore, is not a legitimate way in which to lay pipes for the making of a reservoir. In my own practice, I have formed a tunnel or culvert through the solid stratification on one side of the valley, and perfectly free from the loose earth of the embankment. That culvert is executed in the very best possible manner with hydraulic lime, and puddled to make it perfectly water tight over the top. Within that culvert or tunnel the outlet pipes are ultimately laid. Provision is made for closing the pipes inside the reservoir. The centre portion of the tunnel -- namely, that under the heaviest portion of the embankment -- is closed by brickwork set in cement, so as to make a perfectly watertight plug at that point. Valves are placed at the outer end of the pipes in the usual way, to work the pipes. In the inner shaft arrangements are made to draw the water at various heights in the reservoir, or to exclude it from the shaft, so as to enable the pipes to be examined. Such an arrangement renders any accident to the pipe for mischief absolutely impossible. Where I have laid cast iron pipes through puddle under small heads of water, as pipes leading to a valley syphon, I have found it necessary, in order to prevent the water creeping along the outer surface of the pipe, to put on collar shields, as described by Mr. Jackson. I have done this because I found I could not make puddle adhere to the cast iron pipe so as to be water tight, even under only five feet of pressure. Unless those precautions are taken the water does creep along the surface of the pipe. In land draining, where ordinary drain pipes are laid in a clay sub soil, the best drainers make no provision for an open substratum. They know that water will find its way to the pipes; and I believe the pipes drain as much by their external surface as by their inner capacity. I believe there is an outer creep along every pipe. The result of that experience teaches me that a smooth line of cast iron pipes, with joints such as described by Mr. Gunson, are not to be trusted as certain to be water tight in such an embankment, although lined the whole length with puddle. I heard Mr. Gunson's evidence as to the mode in which he had laid the pipes through the embankment. I expected to learn that the crossing of the puddle trench had been provided for, so as to give a bearing to the pipes uniform with that of the solid ground on either side. I was surprised to learn that in place of this an artificial trench had been excavated, at a flat slope from the bottom of the puddle trench until at the surface line it exceeded 200 feet in length; that this had been filled with puddle to the depth of 20 feet in the puddle trench, thinning itself to 18 inches of puddle in the solid at either end, leaving upwards of 200 feet of pipes, in 9 feet lengths, with no bottom support other than the puddle beneath them. This would be in that portion of the embankment where there would be the greatest possible weight, namely, under the apex. I have had considerable experience on railway work. I have seen 60 feet embankments -- and this was 90 feet -- carried apparently over solid ground; I have executed bridges and culverts on such ground beneath those embankments. Serious fractures always, total destruction occasionally, resulted. I have known it settle so much as to move the grass surface 300 feet away on either side. That occurring in a naturally compressible stratum, I should dread placing jointed pipes in an artificially formed compressible sub stratum. If that line of pipes has been depressed into the puddle, in all human probability it has not gone down equally in the puddle trench and under the loose embankment. But if it can be proved it has gone down equally as whalebone would bend, and not drawn a joint, in my opinion it has left a cavity in the puddle trench above it, because the puddle is an artificially and carefully formed material, made so solid that if the puddle is as perfect as I believe, it could not follow the compression of the pipe equally on either side of the puddle wall. We have been told that the bank was formed on the same plan as the Agden reservoir in course of construction; the material not being water tight on each side. The inference is, that the water as it rose would penetrate the bank, and search out its weakest point. The reason the embankment did not show signs of failure on the first admission of the first 50 feet of water would arise from the fact that the lower half, being upwards of 200 feet wide, may have been sufficiently tight to prevent any access of water to the puddle wall. As the water rose foot by foot in the reservoir it narrowed the intervening space between the water and the puddle wall 2½ feet; and from my inspection of that bank, and from its state as it exists now, to be seen by any one, it is obvious that the upper half of the bank is not made of water tight material, but contains a very large proportion of rubble stone. I measured one not on the surface more than four feet long, two feet wide, and nine inches thick. The water would thus penetrate to the puddle wall, gradually creep vertically down the face of the puddle wall, and inevitably find out the weakest point. On the opposite side of the puddle wall, there is the same defective arrangement of rubble stone dangerously close to the puddle wall. That this was so through the deepest part of the bank is evidenced by the description of the ultimate breaking down of the top of the bank. The first top water has been described as coming over in sheets and waves of foam. That water did not flow down the slope of the embankment, but was absorbed vertically into it. We have been told this reservoir embankment was made exactly as the Agden embankment. From the mode of tipping the wagons and the material tipped, I have no hesitation in coming to the conclusion that the substance of that bank is as porous as a sieve. The specification limits the tips to three feet each in thickness the tips at present in work are at least double that height. This method of working rolls the largest stones continually to the foot of the tips, and makes, in fact, a rubble embankment open and porous in layers. With regard to the mode of obtaining material to make the embankment, I hold it is most objectionable to take the material for making your embankment from within the reservoir and below the water mark, excepting a trial shaft has shown that that material is in its whole substance water tight. In the Bradfield reservoir several acres of surface have been bared by excavation. Many square yards of fissured rock have been bared. Into that rock I found by examination that surface water readily flows without pressure. With pressure the flow of water into them would be greater. No engineer can tell what is to become of that water. It may waste itself harmlessly below, but there is a possibility of its communicating with fissures beneath the external slope of the embankment. In such a case the engineer has no right to run the risk of letting water into the substrata, because the water will be beyond his control, and it will be impossible for him to tell what will become of it, except by experience. I do not undertake to say that water has done any injury. I have no evidence, neither can anybody say it has not done injury. I have examined the by wash, and do not think it adequate to convey away the flood water. I would have made a very much larger by wash. In this case the by wash has not been the cause of any injury, as the water never rose to it. I do not approve of a sloping by wash such as the whole of the Sheffield Water Works possess. I think it is always better to break your by wash channel by a series of steps. I think for such a capacity of water and such a drainage area the two 18 inch pipes totally inadequate to give safety to the bank in the case of anticipated danger. To take away the incoming water of a flood from the reservoir, the by wash and pipes ought always to be equal to safely removing the greatest possible flood on a full reservoir. I have no wish to make statements away from this special question, but shall be glad to answer questions.

By the JURY: I would not have put pipes in the embankment. Wherever put, the pipes should have been larger. It is a fatal objection to the scheme that there have not been valves to shut the water off from the pipes inside the dam. The pipes and by wash would not carry away a flood coming when the reservoir was full. There ought to be a goit to carry away the flood water when the dam was full. I am surprised that the engineers destroyed the one they used while the embankment was being made.

Mr. PAWSON: Mr. Gunson had told us .hat the puddle trench was sunk down to an impervious stratification of rock, and that the trench was dry at the bottom after that time. But a juryman supposes that water may have afterwards welled up through the stratification of the rock into the trench so made; and the removal of the soil from the bottom of the reservoir may have let into the rocks the water that under pressure has so welled up at the base of the puddle itself. Ought not some provision to have been made against such a contingency ?

Mr. RAWLINSON: I have not seen the puddle trench, and cannot form an idea. The only answer I could give would be to say what it has been considered necessary to do in other cases. I know instances in which water has been found in a puddle trench which deep sinking would not get rid of In such cases drains were laid so as to cover the orifices from which the water issued, and collect the water to one point, provision being made for carrying it safely to the outer part of the embankment.

By the CORONER: Several causes may have led to the catastrophe. A fractured pipe, a blown or drawn joint, a creep along the pipes, a pressing down of the pipes in the puddle trench by the heavy material on both sides of it; or a washing away of the outer slope, as suggested by Mr. Leather, by a land slip, caused by undiscovered fissures and springs, in communication with the interior of the reservoir, which fissures and springs, had they existed and had such communication, would become active for mischief as the water rose in the reservoir. Those are the methods which occur to my mind as agents which may have caused the destruction of the bank--one of them, or more of them combined, may have done it. My opinions that it was the most fatal mistake to lay the pipes in the centre of the embankment upon an artificially formed compressible material. I think also that in the formation of the embankment the stones which are being tipped into the Agden embankment should be kept away from the puddle wall; the three feet layers are much too thick; and that six inch layers, as suggested by Mr. Leslie, are the only safe way of making the embankment.

By Mr. SMITH: There are many instances, especially of late, in which pipes are not laid through the embankment. At Dublin, the culvert plan round the embankment is being made; I think also at the Rivington reservoirs.

Mr. SMITH: It is a mere opinion of yours, I suppose, that the pipes have caused the accident ? -- It is humanly impossible to speak to that without an examination of the pipes. I almost fear that with an examination a positive conclusion could not be come to.

Mr. SMITH: Reservoirs built on the same plan as this have existed in Sheffield for twenty five years, without the slightest accident before. Is not that a fair reason for considering the plan a tolerably safe one ?

Mr. RAWLINSON: You have never had such a head of water in Sheffield before as in this reservoir. In this case, if the inner part of the embankment could have kept the water from the puddle, we should probably never have heard of the accident. To a certain extent this embankment was safe, for it had stood with the water at fifty feet since June. When, however, the water mounted up the bank, something took place which altered all the conditions. I think the water found its way between the puddle wall and the embankment.


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