Mr. Inglis. one of the divisional engineers, it was a tunnel about 1 mile in length, and as the workings progressed the settlement in the tunnel took place. He could not now tell the relative position of the workings and the settlement, but at any rate it took place in waves and progressed from one end of the tunnel to the other, except at points where wide faults occurred. The greatest difficulty arose from the fact that while the rest of the tunnel settled-the total settlement had been about 10 feetthe portion over the faults did not settle and had to be cut down and lowered so as to make it join with the lower portion which was due to the settlement arising from the working of the coal. He should have liked to have seen the data, as an Appendix to the Paper, on which the Author's formula was built. It might have the effect of stimulating other members to add to it. The only value the proposed formula had was the practical area it covered in the way of examples which were found to be more or less efficient, coupled with the circumstances of each case. The sums involved were so large that it was to railway companies' and canal companies' interests to watch each case and deal with it on its merits. He did not know whether it was asking too much of the Author, but the formula was stated to be founded on certain definite cases, and it would be worthy of the volumes of the Institution if those cases were given.

Mr. Cooper.

Mr. R. ELLIOTT COOPER remarked that his own experience of the effects of subsidence caused by coal workings had now extended continuously over about 6 years. In 1892 a main line of railway with a number of branches had been commenced through a district that had not been worked except in a slight degree. This was a coalfield about 150 square miles in extent, a large part of which was now being worked, and the effect of the subsidence could be seen, he might almost say, from day to day. When the railway was first laid out, the Chairman of the Company, who was himself a large colliery owner and an eminent engineer and a member of the Institution, expressed his opinion that it was a mistake to buy any coal at all; and that view entirely agreed with Mr. Wright's opinion, who had given that very good advice. With the exception of a single case the opinion of the Chairman was followed; and although several out of fifty or sixty bridges on the railway which would be undermined, only in one or two cases had there been slight signs of damage caused by the subsidence, and practically speaking no injury whatever had resulted; whereas if any attempt had been made to buy the coal and so protect the works, the amount of money that would have been so expended would have

greatly exceeded what it would have cost to even rebuild Mr. Cooper. some of the bridges. The amount that was being asked in the Derbyshire district for each of the several seams of coal for the purpose of pillars was so great that it became a matter of serious consideration as to what extent, even in the one case where a pillar was bought, it should be done. That single case was a viaduct 80 feet high from the foundations to the rail level, and if he had known then as much as he did now, if he had been able to foresee how very little damage the subsidence created, he would certainly have recommended that even in that particular instance the coal should not have been bought. There was a point that perhaps was not sufficiently considered even apart from the question of the safety of the structures. Supposing, for instance, a number of detached pillars of coal were bought, in a district like that he had mentioned-where no coal had been worked and where in years to come the whole of the surface subsided to an entirely new level varying between 1 foot and 3 feet 6 inches-if detached pillars of coal were left, or a number of fixed points were created, which might be very inconvenient. For instance, if there was a viaduct or tunnel immediately adjoining a station, and the level of those two points was fixed, and the coal was not bought under the station, a gradient of considerable steepness would be formed between the two points. The station could not be raised, unless it was pulled down and the sidings ballasted up, and the result would be a very awkward position, which might have been altogether avoided. He had taken great interest in watching the effect of the subsidences, and he had found that any little damage that was done was almost invariably caused by the "pull" rather than by the subsidence itself. He had observed in one case that whereas a parapet of a bridge had slightly opened, caused by the pull, when the bridge had become undermined completely the cracks had been entirely closed, because the surface had returned to its level instead of being, as it were, on a curve. That had occurred not only in the case of one of his bridges, but in the offices of the colliery company itself, where a crack appeared from top to bottom; but now that the whole of the coal had been cleared away from under the building there was not a crack to be seen. Another point that had to be considered in all questions of subsidence was the strata that overlay the coal. In the particular line that he referred to for about 12 miles the strata immediately below the surface, for a great depth was a strong blue bind, but in another district on the same railway there was about 50 feet or 60 feet of solid limestone. The difference between the effect of the

on

those two portions of the line

was very remarkable. Where the

blue bind existed there was invariably a considerable "pull,"

the effect was

quite noticeable. Where the lime

LANCASHIRE,

DERBYSHIRE AND EAST COAST RAILWAY, COAL-WORKINGS AND

stone was found there was no effect whatever of the

"pull." There it
simply went as if

the surface were
a piece of india-
rubber. At one
place he had in
his mind where
there was a long
piece of level line
perfectlystraight,
it could be seen
year after year
how
a slight
curve was formed,
but without the
slightest injury

to any single
structure. As the
subsidence pro-
ceeded, the level
piece of line
would eventually
take its original
form, but it
would be about
18 inches lower.
The mines in

both cases were between 400 yards and 600 yards deep. Where the

limestone came to the surface, and, as he had said, was about 60 feet Mr. Cooper. thick, the subsidence at the most that he had been able to find was only between 1 foot and 18 inches. At the adjoining colliery, and with the same depth to the coal, but where the blue bind was the overlying strata, it had amounted to between 2 feet 6 inches and 3 feet 6 inches. The effect of subsidence amounted to practically nothing, but the "pull" undoubtedly had a slight effect. One seam was 5 feet thick and the other 5 feet 6 inches thick. Several speakers had referred to the probability that even where a supporting pillar of coal had been left slight subsidences will follow when the coal round the pillar had been taken out, and he could corroborate this fact from instances within his own knowledge. He had a station where the coal was being worked along the line of the railway, the face being at right angles. When the workings reached a point about 60 yards from the end of the platform there was not the least subsidence but a distinct " pull," and the gaspipe running under the coping of the platform broke. At the present time there were three or four small cracks of about inch; but now that the workings were under the station apparently the "pull" had entirely ceased, and the platform was gradually settling to its final level. All the experiences that he had had, and they entirely agreed with what Mr. MacDonald and Mr. Wright had said, pointed to the fact that engineers need not be very much afraid of building structures and allowing the coal to be worked out underneath, at any rate if the depth was anything like 400 yards. As to the question of the relative advantages of arches and girders, he did not think there was much difference. He was not speaking of large arches, but small bridges, such as occupation bridges under deep embankments, where it would be foolish to carry up abutments to carry the railway by a steel superstructure merely to avoid running the risk of any possible damage to a small bridge. On the railway under his charge he had a great many small-arch bridges, and in no single case had the least damage occurred.

Mr. H. S. CHILDE desired to suggest that there was an element Mr. Childe. which the Author had probably not considered in laying out the size of a pillar, namely the nature of the coal that was to give the support. Some of the coals-he referred to Yorkshire and North Derbyshire-were hard, and the pillar to be left would. not be so large as the pillar of some of the softer coals, and it might be that the Author's rule would not apply in that case. With regard to Fig. 1, there was another cause of damage sometimes to buildings on the surface when the coal was worked from the

Mr. Childe. inner angle of the fault. There were cases where to support a building coal must be left under the fault to the extent of about 220 yards from the foot of the fault. The lie of that fault was probably an average Yorkshire one, about 23 to 1, or an angle from the vertical of 20°. With regard to the other cases the Author had brought forward in the Figs., he thought that, generally, the members would agree with the Author, excepting that probably he had not considered the question of the line of subsidence going into the solid. His line of subsidence went goaf-ward only, but there were cases when it went into the solid to the extent of 12° to 17° from the vertical, and that required a great deal of watching. If the formula was meant to apply where the whole of the coal was to be left, the question arose when the owner, who was leaving coal, was to be compensated, whether he should not also be paid for consequential damage or extra cost of working. It might be that it would be more economical and better to go further and buy the whole pillar. The attrition and decrepitation set up in some seams in those pillars were in progress from month to month, and it would be better to buy a solid pillar than buy in what was sometimes called gridiron fashion. Then the structure that was to be supported had to be taken into account.. It might be that there were only some light buildings, but if there was almost a village, the question of a pillar or no pillar would have to be considered very carefully, and if a pillar, it would have to be a very large one. The other question that had to be considered very often was where water had to be supported. In supporting railways or buildings there was no fear of damage to the mine. The surface might or might not be damaged. But if it was a reservoir bank or a canal, it sometimes had to be considered whether, although the working of the coal might not do very much damage to the canal, it might not cause fissures in addition to subsidence, and there might be a claim in respect of damage by the water flowing into the mine.

Mr. Ross.

Mr. A. Ross observed that in devising a new railway scheme over an undeveloped field of coal or a field that had probably not been proved, engineers had not very much guide, and it should be recollected that a good number of the railways in England were so constructed. But if an engineer had knowledge that he was on a coal-field, and had probably the mining engineer's report, it was only due to the work that he should take certain precautions. Whether he should avoid tunnels and viaducts was another matter. The physical conditions of the coal-fields of England did not admit of avoiding them altogether, but certain