A Complete History of The Great Flood at Sheffield
by Samuel Harrison
Web Page 31

(120 contd.)
(THE INQUEST. Contd)

By the JURY: The quantity of water flowing in that week was not greater than the average; it caused nothing like a flood. I did not observe, because it was dark, if there was any sinking of the embankment when the crack was observed.

By Mr. THOMPSON: I have acted as engineer for this company for nearly thirty three years. From my experience there was nothing left undone that could have been done as to safety of construction. After the lesson we have now learned, I should be prepared to let off the water in larger quantities, though I believe that in this case any practicable means of the sort would not have been of the slightest service in avoiding this accident. Up to the present time, I do not know any instance of the extraordinary means now suggested having been adopted. I superintended the work carefully the whole of the time, and it was well constructed. The altering the site of the embankment entailed additional expense upon the Company, besides the loss of storage to the extent of several millions of cubic feet. To that extent it was a sacrifice of the pecuniary interests of the Company for the sake of safety. The puddle trench was carried into the solid rock, so that I cannot see that any leakage in the bed of the embankment would have any effect upon it. The material taken off from the interior of the reservoir was pervious to water. The pipes being enveloped in puddle would be saved from local pressure, as the puddle would to some extent give way.

By Mr. RAWLINSON: There were two valves upon the pipes, an outer and an inner one. I have seen a vertical water tower in a reservoir, connected with the pipes, which would enable one to stop the pipes from the inside; but have never heard that it is the invariable mode with the Hindoos. We have talked over such a project. When the flood took place in June, I thought we could afterwards let off the water when we had completed the Agden reservoir, and do any small work that was necessary.

By Mr. THOMPSON: The water I saw coming out behind the embankment was five or six yards from the embankment, perhaps more, and about six feet above the bed of the river. The soil of the embankment was not disturbed there at all. The water that came out there was of an entirely different quality from that in the reservoir, and that led me to believe it was not the same water. that water could have nothing to do with causing the breach in the embankment, as it is a long way from it, and the embankment at that part is still quite sound.

By Mr. RAWLINSON: I don't know that pipes have been ruptured under embankments. I have seen the Manchester reservoirs.

By the JURY: The puddle trench never was dry from the beginning to the finish. Water flowed in at the sides, but it was kept dry by the pumps.

By Mr. RAWLINSON: I might have believed that the water which came out behind the embankment was from the reservoir, and had cleared itself by filtration, only that it was so heavily charged with iron. Water came from below the embankment in three several places; that was in the solid before the embankment was constructed. We laid no drains to carry that water away. Drains are to prevent slips; but we placed the stone inside the embankment for that purpose. There was no necessity to make any special provision for drainage. I have never ascertained in the embankment the least signs of leakage. What water I have seen outside was colourless. I have no doubt if there had been water at the back of the puddle trench, it would have got through. I have always considered that water will get up to the face of the puddle, I have never experimented as to the degree of pressure a puddle wall will bear. The puddle wall was in proportion to the other reservoirs of the Sheffield Water Works Company, and I am not aware that some engineers put a thicker wall of puddle. If puddle had a small proportion of gravel in it, it would not be bad. The puddle used was good, first rate, both in point of quality and workmanship.

This closed the examination of Mr. Gunson.

THE VELOCITY OF THE FLOOD.

Mr. RAWLINSON then said he thought it would be interesting to the public if he were to read over some calculations made by Mr. S. F. Holmes. The total fall from the dam head to Owlerton was 450 feet, or 72 feet per mile; and he had calculated the velocity of the flood between those two points at 18 miles per hour, showing that the water travelled at the rate of 26½ feet per second. The average area of the cross section of the flood between the same points was 3,780 feet, showing that 40,170 cubic feet of water passed per second --a rate which would empty the reservoir in 47 minutes. That was a velocity that he could form no conception of, and it accounted for the destruction. The public ought to have this information given them, that after the dam broke a Derby horse could not have carried the warning down the valley.

The CORONER then announced that he should adjourn the inquiry till ten o'clock. on Thursday.

THURSDAY, MARCH 24th, 1864.

The inquest was resumed at the Town Hall, Sheffield, on Thursday, before J. Webster, Esq., the Coroner. The same gentlemen were on the Bench as on the previous day, but the Water Company were represented by Mr. B. Smith only, Mr. P. Thompson being unable to be present.

EVIDENCE OF MR. M. B. JACKSON.

Matthew B. Jackson, of Sheffield, civil and mechanical engineer said: I have been engaged in the construction of reservoirs in Australia. I was chief engineer of the Melbourne Water Works, consulting engineer of the city of Adelaide Works, Ballarat Corporation Water Works, and first engineer for the Bendigo Water Works. I was on the embankment at Bradfield a few hours after the accident, and three times since I have examined it, in special reference to this inquiry. I am of opinion there is no fault to be found with the quantity of material or the slopes. The material was ample, and the slope sufficient. Perhaps in my own practice I should have preferred a slope of three to one inside. The puddle is good and sufficient. I have put in a greater proportion of puddle; but do not think at all that any accident would have arisen from that. I have no fault to find with the by wash. I have examined the embankment. I should not have formed an embankment in the same way. I should distinctly prohibit the use of railway wagons on a bank, more especially on the inside slope; because, in the first place, I should make the bank in layers of not more than two feet thick. Railway wagons always travel in the same line, and tend to consolidate the embankment unequally. Dubbins and carts travel in different parts, and tend to consolidate the embankment equally. On the inside slope I should have insisted that layers, not exceeding two feet each, should have been carried over and through, each being perfectly finished before another was commenced. On the outside slope I would not object to a tip of three feet, because it is not so necessary to have the outer slope so impervious to wet. It is better open to let water have free exit. I mean by open, that it should be composed of more porous material. The embankment has not been erected in that manner. There are railway wagons on it now, showing that they have been used, and it is obvious the layers have been put on of a greater thickness than two feet. At the embankment at Agden the layers are not being put down and completed consecutively, and if the one at Bradfield has been constructed in the same way, I should apprehend that there would be an unequal settlement. I did not measure the tips of the Agden; I saw they very much exceeded the measurement I have given. Unequal settlement is dangerous, because it may cause a slip. It is peculiarly dangerous if there are pipes passing under the bank in trenches. It might be possible to pass them safely through in a culvert. The danger is that the unequal settlement may break the pipes or spring the joints, so as to produce leakage. The depth of the tips I have mentioned is rather an extreme. In constructing another bank I should take the specifications of Mr. Leslie, of Edinburgh, which are the best I ever read. He restricts the tips to six inches, one layer to be finished before another is begun, and wagons to be prohibited. They are safer than mine, but more expensive. I approve of Leslie's stipulation that the puddle wall should be brought up to and kept on a level of six inches above the adjoining portion of the embankment. At my works at Melbourne I had two fractured pipes. They were under the embankment in the solid ground. They were laid on flags to the edge of the puddle trench, and in passing through the puddle each pipe of each main, which was only 6ft. 6in. in length was supported on an ashlar pillar. The pipes were puddled all round. The fracture in that case I have no doubt arose from an unequal settlement of the embankment, though made with the limited tips I have mentioned. (The witness explained that an embankment settled more in the centre from its length than at the sides, and consequently that if the pipes were laid straight through the embankment, the pipes must bend and break by the unequal settling.) My plan of putting the pipes at Melbourne in was a bad one, but not so bad as the plan at Bradfield. There was no puddle between the flags and pipes at Melbourne; it would not have made any difference if there had been. I have not the least doubt that the structure of the Bradfield dam caused an unequal pressure and fractured the pipes, and caused a leakage. There is this difference. The way my pipes were laid at Melbourne was more liable to cause fracture of the pipes; Mr. Gunson's plan was most likely to cause a drawn joint. I heard the mode described yesterday of keeping down the water in the puddle trench. The evidence of Mr. Gunson to a great extent dispelled a doubt I previously entertained. The doubt was as to whether the water had got under the puddle trench and risen immediately under the outside slope, inducing a settlement and slip immediately preceding the burst. I am inclined to think that such was not the case, and that the puddle trench was a good job, though I scarcely think the evidence of Mr. Leather and Mr. Gunson satisfactory that the puddle trench was dry, as that would depend upon the time of the year and the weather. I am inclined to think that, on the reservoir being filled, the unequal settlement of the embankment has occasioned a springing, or starting, of the pipe joints. If a joint were once sprung, the internal pressure of the water itself would be sufficient to blow the lead out. Making the pipes cast narrower at the entrance to the socket is a good precaution; they are, in fact, dove tailed. It is, however, possible for the pressure of the water on the valve which was outside the reservoir to have started a joint between the valves and the puddle wall. The opening or closing of the valves would increase the probability of this.

Mr. LEATHER was appealed to, and he said he did not think there was any such probability.

Mr. JACKSON resumed: The probability of this would be diminished in proportion to the number of pipes below or outside the valve. There are four or five pipes below the valve at Bradfield. It is quite possible for the water to have crept along outside the outlet pipes between the pipes and the puddle with fatal effect.

The CORONER here said it was the business of the jury to find out every possible fault in the construction of the dam. There had evidently been faults. According to the description of Mr. Leather and Mr. Gunson, the work was so perfect that it was almost impossible to improve it. His object was to show that there was something fatal in the design of the work, or the reservoir would not have burst.

Mr. B. SMITH begged to call the attention of the Coroner to the fact that the evidence on the previous day was that the bursting of the reservoir was caused, not by a fault in the embankment, but from a slip in the ground.

The CORONER: I am trying to find a fault in the embankment Your evidence yesterday was that the work was perfect.

Mr. B. SMITH: As perfect as human work can be.

The CORONER: Your witnesses went for rather more than human perfection, I think, and I am now trying to test their work.

Mr. JACKSON resumed: The outer surface of the pipe being cast-iron, would not unite with the puddle. At Melbourne I put shields round the pipes to prevent water creeping along them. They would have the same effect as the collars spoken of by Mr. Rawlinson. The shields have to be put on in two parts and bolted or otherwise fastened. I put four shields on each main. The shield was seven feet six inches in diameter. I have examined the stratification of the rock, both inside and outside the Bradfield Reservoir. I do not see any great objection to baring the rocks to the embankment; I see nothing to lead me to conclude that that would be fatal to the bank, provided proper means were taken to drain away all water percolating under the outside slope of the bank. The question of baring the rocks inside the reservoir is more a possible cause of waste of water to the Company, to be weighed against a certain economy in the construction of the bank, than anything else. No damage from such a cause would result to the embankment, by any water passing into the fissures of the rock, unless it flows up under or immediately adjacent to the seat of the outside slope. That it would necessarily do; it might come up half a mile lower down the valley If it flowed up under the seat of the embankment it would do no harm if drained away.


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