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Rh could be used only for removing the superfluous water, and picks and heavy spades had to be resorted to and the material charged into the buckets and hoisted out by the locks. Lines of narrow-gauge railway were laid down to run small bogies, on which the skips were carried in all directions within the chamber. Presently the hard boulder clay was reached, which was nearly dry, and which proved tougher and harder than anything the existing tools could work in. Various means were tried, and even blasting by powder or dynamite resorted to, but with little result. The rate of progress became exceedingly slow, and the men's energies became quite exhausted. Here Mr. Arrol's ingenuity fortunately came to the rescue, and he devised a most simple, yet a most efficient, tool to grapple with this difficulty namely, a hydraulic spade shown in Figs. 49 and 50. It consisted in the main of a ram, to which a spade is attached, the ram fitting into a cylinder, which represents the handle of the spade. On the top of the cylinder was fitted a head-piece, which could be set against a piece of batten or any other convenient object. The ceiling of the air-chamber furnished the resistance, and the projecting rivet heads were useful in preventing side slip of the head-piece. The spade was lifted by two men, a third attending to the fixing of the head and the turning of the cock admitting water pressure. This was of the ordinary amount, namely, 1000 lb. per square inch, less the amount of air pressure inside the caisson.

The spade was set on the ground, the pressure turned on, and as soon as the head-piece had been firmly set against the ceiling the full pressure was given, and the spade forced down into the clay. The water was then exhausted and the spade brought forward, thereby detaching a slab of clay from 16 in. to 18 in. deep, and from 2 in. to 4 in. thick. The spade was then set up again, and the clay thus cut away in long ridges all over the area. Many of these slabs of clay had to be cut to allow them to be charged into the skips. Any boulders too large to pass through the air-shaft were either drilled and blasted, or left in the chamber to be hereafter incorporated with the concrete filling. The water discharged by the spades had to be collected in a sump and discharged through an ejector.

In digging or undercutting the edge of the caisson all round, the spades had to be worked at an angle varying from 30 deg. to 60 deg., the rivet-heads coming in usefully hero to provide a good hold for the headpiece of the spade.

In undercutting round the sides, portions were left standing to carry the weight of the caisson while excavation was still going on, and these portions were by degrees removed until the bearing surface became too small to sustain the weight, and the caisson settled down into them. In proportion as the caisson descended, more weight was added on the top; but this was also required to be done with care, for fear of displacements taking place. After entering into the hard clay, the caisson edge was nearly sealed by the pressure of the water upon the clay outside, and instead of the pressure of air corresponding to the head of water outside, a few pounds sufficed to keep the caisson dry.

Thus, in the case of the Queensferry north-east caisson, founded at 89 ft. below high water, the outside head is equal to fully 42 lb. of air pressure inside, but it was actually worked and finished with a pressure of from 15 lb. to 18 lb. per square inch, after once the hard clay had been entered. This made it, of course, much easier for the men to work in, apart from a considerable saving of wear and tear of the machinery.

It will be readily understood that it was of paramount importance that the caissons, when sunk to their final depth, should be in the correct positions laid down for them; yet it is equally easy to understand that in sinking such a mass it would not take a great deal to cause a displacement in one way or other, since there is nothing to hold it in its proper centre.

The position of each caisson was therefore checked nearly every day, and if it deviated in any way from its right course, steps were at once taken to remedy this. To do this it was only necessary to undercut the caisson edge on the side to which it had moved, and to gradually tilt it to a small degree in the direction of that side. When this had been done the caisson was sunk further down for a distance with this tilt on it, and was then righted by undercutting on the high side. This was repeated until the centre of the caisson had got back to its proper position. The slight upward taper in the cylindrical portion of the caisson added some little help in that direction, and the writer thinks that a little more taper than that given to the Forth Bridge caissons would be found very advantageous in future works of this kind. Another plan is to set up inside the air-chamber and against the top of the sloping plates of the shoe, a number of strong timber struts at an angle to the perpendicular, and well secured against some timber or stone in the solid ground. This should be done previous to the caisson being allowed to descend. When it does so all these struts have a tendency in descending to become longer and to force thereby the caisson over in the direction in which it is intended that it should go. This should also be done at high tide, when the buoyancy of the caisson is largest. As in all things, so here, prevention is better than cure, and it is best to keep a sharp eye upon the workmen in the chamber, to insist that the edge be undercut on the outside to the extent of 6 in. to 8 in. all round before it is allowed to descend, and to have the position checked as frequently as possible, in order to find out a movement to any side at once, and not allow it to become too great to be remedied. In working on a sloping face, whether of rock, silt, or clay, it is good practice to keep the caisson tilted very slightly to the lower side, which has the effect of