A Complete History of The Great Flood at Sheffield
by Samuel Harrison
Web Page 29

(120 contd.)
(THE INQUEST. Contd)

Mr. RAWLINSON called the attention of the witness to a report published by the Society of civil Engineers, in which it was stated that pipes laid under an entrenchment occasionally broke from pressure upon them not being equal; and mentioning an instance at Melbourne where the fracture had to be repaired by inserting a line of boiler plating inside. The pipes in this case were excellent castings, and had borne a very severe pressure both externally and internally, but by the weight of the embankment were flattened and distorted, more in the centre than at the sides, because the pressure was unequal. There was, therefore, a possibility of pipes subjected to an unequal pressure being injured ......... Mr. Leather replied that the pipes in the case cited by Mr. Rawlinson were laid in the embankment, not in a trough. Those of the Bradfield reservoir were, however, laid in a trench under the embankment. The cases, therefore, were quite different. The Coroner: But though the trough was made in the natural ground, the pipes themselves were laid in an artificial ground, which might offer more resistance in one part than another ........ Mr. Leather: There were 18 inches of puddle round them, but they would adjust themselves in the puddle.

By the JURY: How do you determine the strength of the embankment ? -- It was ten times that of the pressure against it. I cannot tell how much water would run into the reservoir in 24 hours on a wet day. The pipes have not been examined since the flood. But as there was no leakage through or about the pipes before the accident, and as there was a full flow of water through the pipes when the valves were opened, it is fair to presume there was no leakage. I have seen the valves work under half the full pressure.

Mr. LEATHER, examined by Mr. Perronet Thompson: I have been the consulting engineer of the Company in respect of all their reservoirs. I have not previously had an accident, though some of the reservoirs are of greater extent than the Bradfield reservoir. The embankment of the upper dam at Redmires is much larger. The plans and sections for this and for the reservoirs that have stood for years are precisely the same in principle, and there is a great similarity in situation and materials of some of the dams. I used all the means known to me as a practical engineer in the construction of these dams for ensuring the security of the work. So far as my observation went, the quality of the work was good throughout. The fact of the puddle trench being sunk to the depth of 60 feet, although the plans required only ten feet, is an illustration of the care exercised in providing for the safety of the dam. The specification provided for the trench being made deeper than ten feet if necessary. The weight of the water is measured by the depth, not the quantity of the water on the superficial area. One fourth the water might have given quite as great a pressure if the depth had been the same. I have not known an instance in my experience of pipes carried under an embankment having given way. If the valves of the pipes had in this instance been unable to sustain the weight of water, the only result would have been that the water would have run away; no damage would have resulted. I have taken all reasonable means to provide against danger. The object of an embankment is to confine the water, and the business of an engineer is to make his embankment sufficient to resist the pressure of the water against it, not to provide any other means of letting the water off on the supposition that the embankment is not strong enough. This embankment was sufficient .......... You have told us the width of the puddle wall.--Yes.

The CORONER: We have no fault to find with the puddle wall.

Examination by Mr. THOMPSON resumed: The puddle wall is the real security of the water. The only object of the embankment is to support the puddle wall, not itself to keep out the water. It is not, therefore, specially important that the earth of the embankment itself should be such as will keep out water. This puddle wall was 60 feet deep. The pressure of water in the pipes in Sheffield is much greater than in those at Bradfield, and there is scarcely ever a leakage from that pressure.

The CORONER: If an embankment is to support a puddle wall, and the embankment is insufficiently strong, the puddle bank will, of course, fall ? -- Mr. Leather: Yes ...... Have you examined the embankment since the flood ? ---Yes ....... Do you think it is properly made ?--Yes ......... Is the higher side made in the same way as the lower ?--Under the lower side there was a footing of stone, to prevent the embankment slipping.

By the Jury: What was the thickness of the 18 inch pipes ?---- Mr. Gunson will tell you that..... The pipes would be carried through the puddle trench if not through the embankment ? Yes ....... Is it likely there would be any settling there ? You will find that proper precautions have been taken against that ...... If there was any settling there, is it not likely that the pipes would be broken by it ? ---- They might, but I have no doubt proper precautions would be taken against that.

By the CORONER: What is the cause of the embankment bursting I really do not know ...... What do you conjecture to have been the cause ?-- I have very great difficulty indeed in forming any opinion -- exceedingly great. I have no opinion worth relying upon. I can form conjectures, and so can anybody else, but they are not worth much ...... Your conjecture is perhaps worth as much as those we have heard in the town; what is it ? --There is a possibility of a landslip under the seat of the embankment having produced it, but that I cannot tell. I do not believe the embankment itself has slipped, but the stratification beneath it may have slipped.

By Mr. RAWLINSON: YOU mean that you do not think the embankment was the first to slip ? That is the more correct way of Putting it.

By the CORONER: Have you any other suggestion to offer ? --- A fracture of the pipes has been suggested. If the pipes had broken, that might have caused it; but we have no indication of any such breakage.

By Mr. PAWSON (foreman): Mr. Leather said the puddle trench was carried down to a point where the ground was impervious.... Was any length of time allowed to elapse after sinking to that depth to see whether water would well up, or was the puddle filled in immediately ? --- Mr. Gunson can tell you better about that; but I know some time elapsed, because they kept it open for me to see.

By the CORONER: Why do you hazard the conjecture that there has been a land slip beneath the embankment ? ---Because we know they do take place; a land slip has taken place in this valley below the dam, and in many other valleys...... It would be a slip of the surface, not of the rock. You tell us that the puddle bank was based upon the solid rock at a depth of 60 feet. How could there be any slip there ? --- I am speaking of a slip under the embankment; not under the puddle bank. Then you do not ascribe the bursting of the reservoir to unsound principles of engineering or to bad workmanship ? --- Certainly not.

By Mr. Rawlinson: Was the embankment formed by wagons and tips, by barrows, or by both ? ---- You had better ask Mr. Gunson on that point ........ Would a blown joint cause equal injury with a crack in a pipe if such took place ? ---- Certainly not ....... Do you know the maximum volume of water ? --- I do not, but it has been carefully recorded ....... A land slip may arise in this way. The substratum may be surcharged with water. The rocks might not under ordinary circumstances receive such a volume of water as would disturb the superincumbent material. On the reservoir being filled with water, however, the pressure through the natural fissures might create a landslip that would not have taken place without it ....... Is that anything like your view ?-- No, I do not think that is it. My view is, the water that would produce the land slip is the water that naturally percolates into the strata getting between the face of the rock and the bed of clay resting upon it, and causing the superincumbent mass to slip off ...... That is constantly seen in railway cuttings ten or twelve years old ? --- In that case there is not only the water but the concussion of the trains ....... You say the water through the fissures would be cut off by the puddle trench ? --- Yes ....... Have you examined the bare stratification on the by wash side of the reservoir since the flood ? --- No ....... You don't know whether it may be taken for granted that it is pervious or impervious; whether it would retain water, or whether a potion would percolate into it ? ......... I have not examined, because I believe if the rock would allow the water to percolate such water would he cut off by the puddle trench.

By the CORONER: Suppose the percolating water was not stopped by the puddle, but went through the rocks at a lower level than the bottom of the puddle trench, might it not have the effect of washing down the embankment on the low side of the puddle bank ? --- No. The probability is that it would find its way to a spring lower down the valley. Have you seen the embankment of the Rivington Water Works at Liverpool ? ---No ....... Are you aware the reservoir bottom there was quarried the same as yours; and that subsequently a large quantity of water found its way below the stratification of the puddle bank and leaked out a considerable distance below the embankment ? ----That is so probably; but I do not know of it.. Are you aware that any special arrangements were made to prevent the water creeping alongside your two outlet pipes by putting collars around them ? ----- No ....... Did it come to your knowledge or not that one of the Birmingham reservoirs six or seven years ago was destroyed by the water creeping along the cast iron pipe and blowing a hole through the embankment like a tunnel ? ---- I am not aware of it. I should think in any such case it would be detected by a previous leakage ...... I believe there is evidence that a considerable body of water was issuing under your side wall below the embankment before the bursting of the embankment. You are perhaps not aware of it ? --- I am not ....... What is the extreme life of a pipe, such as your outlet pipes--I mean how long would they last,--they would come to destruction some time of course ? --- I do not know. Everything will come to destruction sooner or later no doubt. I do not know how far the two pipes were placed apart.

JOHN GUNSON said: I am the acting engineer for the Sheffield Water Works Company, and have had the construction of the Bradfield reservoir from the plans of Mr. Leather. The embankment was made in the usual way, but it was found necessary to dig the puddle trench deeper than we originally intended, the reason being that there was so much water. We went to a depth of 60 feet. There was a very large flow of water, which was pumped out by two engines, one of them working a 12 inch and a 13 inch pipe. The other pumped two 12 inch pipes. Together they would be about 20 horse power, and they were kept constantly working for nearly two years. This shows that a large body of water comes through the rocks to the puddle. We got rid of the water by pumping it out. The water originally came out of the reservoir, about one hundred yards north of the embankment. It was caused by a fault or throw of the rock, and came from the tops of the hill, perhaps miles away. When we got the puddle trench in that was an effectual barrier, and the water from that spring was thrown back into the reservoir. The puddle trench was cut and worked in the ordinary way. When we got above the level of the spring, the consequence was of course that the water had to make its way back again. The trench laid for the pipes was nine or ten feet wide, and perhaps the depth was equal to the width, but ran up considerably less. The pipes were laid in puddle, at a distance from each other of two feet six inches. The depth of the puddle above and below the pipes was eighteen inches It was the same kind of puddle as the puddle wall--the best. The trench was filled up with the best material, thin layers of gravel, and very solid. We put in the materials of the embankment in layers or tips. There were barrows used, three wheeled carts, two wheeled carts and wagons. We did not take the material for the embankment indiscriminately from the banks of the reservoir, and tip it on the embankment. First, 25,000 cubic yards entirely of stone was placed on the outward slope of the bank, the stone forming an embankment itself up to within fifty feet of the top of the embankment. The object was to prevent the embankment slipping on the surface it was placed on; and to get that done effectually, we paid 3d. per yard additional for the work. The next process was so to lay the material that the finer sort came up to the puddle wall. The work was somewhat similar to that which is going on at present at Agden reservoir. The material is also generally the same, fit I consider for the work which it has to do. [Plans showing the course of the pipes were here produced, and handed for the inspection of the jury.] Great pre cautions were taken in laying the pipes in the puddle trench. The puddle trench was 40 or 50 feet below where the pipes went. The pipe trench was about 9 feet below the surface. About a couple of lengths of pipe went through the puddle trench proper. My fear was, if nothing was provided, the puddle in the puddle trench being deeper than in the pipe trench, it might sink; and so we made a special arrangement for the protection of the pipes, by an extension of the width of the puddle wall. This was done to the extent of something like 100 feet on either side. The pipes being socket pipes, they would admit of some degree of inflection, without breaking. The pipes were inserted about six inches into each other, lead being used for from three to four inches. The pipes were laid in good strong shale, not compressible by pressure. No water could possibly creep along the pipes. The sockets of the pipes were cast a little larger back than they were in the front, so that the internal pressure of the water should not force the lead out. To do so it would have had to act as against a wedge. There were no collars put upon the pipes to prevent the water creeping along--the sockets of the pipes themselves formed collars, and they were very strong ones. The lines of pipes were 2ft. 6in. apart. The sockets were so large that had they been placed opposite it would have reduced the puddle to that extent--so I advanced one line of pipes a yard above the other.

The Court at this stage of the inquiry adjourned for half an hour.


   Web Page 28 Web Page 29 Web Page 30   
RETURN:
   Sheffield Flood - 1864   (Main page)    Web Page / Book Contents List
26  -  June  -  99   
Mick Armitage (e-mail)