lateral displacement to the high side due to the inclination, at a depth d, is d tan cos 0. This lateral displacement may be graphically determined as shown in the vertical section on the line of dip.

Let A represent the bridge to be supported by a pillar to be left in the coal D D at a depth of d yards. Calculate the size of the pillar from the formula given, and mark it upon the horizontal line at B and C drawn through the ground-level at A. Draw BJ and CK, making an angle of 90°-0 with the horizon from" B and C respectively. Then JE and KF represent the lateral displacement due to the dip in the coal, and J E' and K F" the displacement in plan. This is shown in the plan, where the dotted. circle represents the position of the pillar when the coal is horizontal, and the circle in a full line its position corrected for dip.

Fig. 5.

The purchasing of pillars of coal for the support of bridges, viaducts, tunnels, &c., always involves heavy outlay, and it is therefore necessary to determine the minimum size of pillar that may be used with safety. The foregoing rules will enable engineers to do this with comparative accuracy, in the absence of special local factors, which must be dealt with upon their merits, the pillar being altered to meet them. In the case of large pillars at a depth of more than, say, 300 yards, it is frequently found possible to rib them across, or to work half and leave half by means of banks about 20 yards wide driven through the pillar, as shown in Fig. 5. In this manner a considerable saving is effected in the purchase price. The strata appear to arch themselves over the worked portion, and there is practically no subsidence.

PLAN OF PILLAR PARTIALLY WORKED BY
MEANS OF 20-YARD BANKS.

Where several workable seams of coal occur, one below the other, it is necessary to consider the cost of possible repairs, and probably of a new bridge, before going to the great expense of purchasing support in an increasing descending ratio in all beds. In most cases where the coal is more than, say, 300 yards deep, and where the superstructure of the bridge is of steel or of wrought iron, the coal may be extracted in a regular moving face and yet perhaps do little damage, save the lowering of the general level of, the line and its attendant inconveniences. The decision as to this, however, must be left to the discretion of the engineer, who will,

no doubt, have access to the colliery plan showing the workings approaching the statutory limit, and will be able to form an opinion accordingly, so far as that aspect of the pillar is concerned.

Suiting the Design to the Supposition that the Coal may afterwards be worked out.-Tunnels and aqueducts should have support, and therefore do not come within the scope of this section. In the case of bridges, to which it chiefly applies, the remarks previously made as to steel superstructure, and well-tied abutments and wings, are again applicable. The goaf should be packed as solid as possible when the coal is extracted. The subsidence that may be expected, amounting to about two-thirds of the thickness excavated, will, if necessary, have to be provided for in the first height of the bridge; as, for instance, if it be over a waterway where the water-level is maintained, by raising and puddling the banks as subsidence proceeds. Lofty viaducts should be protected by pillars; lower structures may be worked under as described. The girders should have a good bearing upon the piers, which should be solid and not pierced by an arch. Provision as to height is necessary as in the preceding paragraph, and the goaf should be tightly packed, as before.

The Paper is accompanied by a drawing, from which the Figures in the text have been prepared.

Mr. Mansergh.

Mr. Saner.

Discussion.

Mr. JAMES MANSERGH, Vice-President, was sure the members would agree that a vote of thanks be presented to the Author for his useful and practical Paper. Any engineer was to be commended, especially a young engineer, who gave to his fellows the benefit of such observations as the Author had made. The Paper dealt merely with subsidence due to coal-workings; but the members were all aware that subsidence took place in many other workings, especially in the district where brine was pumped. The discussion would therefore be open on the subject of subsidence not only from coal-workings but from brine-pumping, and any other operations of that nature.

Mr. J. A. SANER exhibited upon the screen a series of photographs illustrating the effects of subsidence due to brine pumping in the district of Northwich. The question of subsidence, as brought forward by the Author, was very interesting; and although, fortunately, he was not frequently called upon to design permanent works over such places as the photographs had illustrated, it was instructive to learn what was done in the coal districts and in other districts where such subsidences took place A rule was laid down by the Author for finding the area of subsidence caused by working the coal; but he was afraid that in Cheshire, in the Northwich district, such rules would be entirely useless. There, instead of having the coal seams 4 feet to 6 feet in thickness, there were two superimposed beds of rocksalt 84 feet and 82 feet in thickness, and a layer of marl about 30 feet thick between them. The effects that had been seen on the screen were caused by the fact that the salt was obtained in two ways. In one case the rock-salt was mined as in an ordinary mine, the workable portion being about 20 feet thick in each bed, so that the mines when left were about 20 feet high. The remainder of the salt-bearing strata was worked by pumping water which was converted into brine, either natural springs— although he was afraid they had now been eclipsed by the artificial springs-or water which had been allowed to run down into the mines, thus the pillars which supported the roof were eaten away, and the subsidence took place. He thought that in some places the ground had subsided to the extent of 60 feet or 70 feet, and

It was not Mr. Saner.

he knew of much deeper holes in special cases.
known whether the subsidence had stopped, neither could the
exact area or the exact extent of the possible damage be
ascertained. Further, although plans of the old mines were
kept to a certain extent, the water had done away with very
much larger areas of the salt than were mined. It was impos-
sible to predict where the subsidence would take place next.
The terrible looking holes which had been shown on the screen
occurred chiefly on the edge, somewhat beyond the edge, of
what was known to be the extent of the rock-salt; and he
thought in that case it bore out the Author's statement, that
the greatest effects of the subsidence were not exactly over the
rock or over the coal, but some distance outside the edge of
the working. The subsidences in Castle Northwich and Left-
wich were about 300 yards south of the rock as at present
known; and it appeared as if the draw of the subsidence
assumed an angle approximately equal to the angle of repose
of the overlying strata, and that those holes might be caused
by the overlying marls (because all the rock-salt lay in the Keuper
marls) cracking, and the surface-water drawing down the sand
and glacial drift into the caverns beneath, forming as it were an
hour glass. He thought the cone-shaped cavity which was
formed was merely filling a similar cavity below, and that the
earth that fell from the cone was deposited in a heap in the
cavern below. These conical cavities were sometimes as much as
120 feet in diameter at the surface. That appeared a feasible
theory of the extraordinary subsidences which took place, not only
in Castle Northwich and Leftwich, but also, as it were, in a ring
round the known area of the rock salt. As to the buildings suit-
able for such places, he would prefer there were none at all; for
his experience had shown that however solid masonry might be
made, or whatever the work was on the top, it was more or less
damaged by such subsidences. One of the bridges over the river
Dane, a tributary of the Weaver, for instance, had masonry piers
that were exceedingly solid so far as could be ascertained. They
had been built for some years, but they were cracked down the
centre in halves, not through the joints of the masonry, but straight
down the middle. That bridge had, from ascertained figures,
subsided to the extent of 7 feet since 1882, so that it could not be
wondered that the masonry of the abutments was somewhat
cracked. It appeared, so far as ordinary buildings were concerned,
that the form of structure which seemed to be a survival of the
fittest in Northwich was either wooden or iron framework filled

Mr. Saner, with stucco or brickwork. The house which had been shown on the screen on its beam end was built in that form, and it seemed very little damaged by being turned over through an angle of nearly 45°. If there had been a foundation upon which to rest hydraulic jacks, that house would never have been pulled to pieces as it had to be. In the subsidence which took place on the 15th November last, a house of similar construction moved bodily over into the street; the lower sills moved about 1 foot, and the upper sills overhung 4 feet. But within 48 hours the owner of the house had borrowed hydraulic jacks and had raised it into a horizontal position again. The Weaver Works, of which he had charge, with the exception of the town bridge at Northwich, which he was now engaged in removing, and replacing with a swing bridge of a special construction he hoped at some future date to explain to the Institution, had been entirely removed from the subsiding district, but up to 1859 the effect on the locks and the weirs was of a disastrous nature. He believed that either two or three locks had to be rebuilt within very short periods close to the town of Northwich. He had, however, raised one part of the towing path 12 feet in less than 10 years.

Mr. Wright.

Mr. TYLDEN WRIGHT remarked that the damages caused by salt subsidences were very great, but he was afraid they were beyond all rules, and would not be met by any mathematical formula. The matter was so serious with regard to the coal measures, that he was surprised the subject had not earlier been brought before the Institution. Last year more than 200,000,000 tons of coal were raised, and that represented almost 50,000 acres of coal of a thickness of 3 feet. According to the Author, with whose results he thoroughly agreed, the subsidence over that 50,000 acres would be no less than 2 feet. When that took place in a district permeated with canals and railways, it was a most serious matter for the proprietors of those railways and canals, for the engineers who advised them, and he might also add for the engineers who advised the owners of private mansions as to the effect of that subsidence. He agreed, in general, with the Author; and allowed that the depression would take place in the lines that the Author represented, that if the seams were horizontal, the line of fracture would be vertical; if it was inclined, most mining engineers would allow that it would be half-way between the angle of the seam at the surface and the surface itself. But he did not agree that the wave of subsidence followed the excavation of the coal. He had found from most careful experiments extending over 7 years, month by month, that the subsidence