Railway Construction

by William Hemingway Mills

The plenum process is based on the principle of the diving-bell, the water being prevented from entering at the bottom by keeping the cylinder full of compressed air. An air-chamber, or air-lock, with perfectly air-tight joints, is securely fixed to the

top or upper working length of the cylinder, and no access can be obtained to the interior of the cylinder without passing through this air-lock, which has one lower door or valve opening into the cylinder, and an upper door opening out into the open air. Temporary inside staging is formed by putting planks across from flange to flange, and placing short ladders on these landings for the use of workmen descending or ascending. The excavated material is hoisted by a winch, generally placed on the landing just under the air-lock. The air-pump is placed in some convenient position outside, near at hand, the pressure-pipe passing through the air-lock into the interior of the cylinder. Air is forced into the cylinder to a pressure sufficient to drive out and keep out the water from the interior, and allow the workmen free access for excavating the material round the cutting edge and base of cylinder. The amount of pressure required will depend upon the depth of the working below the level of the water alongside. Men accustomed to the process can work without much inconvenience under a pressure of 20 to 22 pounds per square inch, equal to a depth of 45 to 50 feet; but when the pressure exceeds 25 pounds, the duty becomes very trying, and is attended with considerable risk. Instances are recorded of men working at depths of 105 and 110 feet, necessitating a pressure of over 45 pounds per square inch; but it is very questionable whether the men exposed to such a severe ordeal were not permanently affected, if some of them did not actually succumb.

It will sometimes occur that, after sinking through soft porous strata to a considerable depth, a layer of clayey material is penetrated sufficiently retentive to keep out the water and permit of the removal of the air-lock and the completion of the sinking as an open-top cylinder.

When working on the plenum system everything must pass through the air-lock, both materials and men. The excavated material is hoisted up to the level of the air-lock, the upper and lower doors of which must be closed, and the pressure inside the air-lock brought to the same as that inside the cylinder by means of a regulating valve. The lower door is then opened to admit the excavated material, and then closed again to cut off all communication with the interior of the cylinder. The upper door is then opened, and the material hoisted out into the open air. The same process has to be adopted for the egress of the workmen,

and the reverse arrangement for the ingress of men and materials. The shape and dimensions of the air-lock may be varied according to circumstances, but the principle will remain the same.

When the cylinder has been lowered to what is considered a sufficient depth, it is usually loaded with a certain amount of dead weight in the shape of old iron or other convenient material, and allowed to remain loaded for some days to ascertain if it will sink any further. Should this test be found satisfactory, the dead weight is removed, and the interior of the cylinder pumped dry and carefully filled with good cement concrete.

Cylinders for foundations are generally made circular in section, that form being the most convenient for turning and facing the flange-joints. They can, however, be made oval in section, or of any section that may be found most suitable for the work required.[ Figs. 116] and [117] give the particulars of a double-line railway bridge carried on cylinder piers across a river. The detail sketches explain the form of cutting edge, flange joint, and method of bracing. This bridge is one that was reconstructed and widened from a single-line to a double-line bridge. Traffic was carried over on one line while the second line was being erected, hence the reason why one strong central girder was not adopted.

Cylinders of 7 feet diameter and upwards are sometimes filled with concrete in the lower portion, on which is built either a circular lining or a solid mass of masonry or brickwork up to the level of the girder-blocks. In some cases the cylinders proper, together with their concrete filling, terminate a little above the water-level, and upon these foundations are erected strong cast-iron columns, plain or ornamented in design, to carry the girders and roadway. The cylinder itself is generally considered merely as a casing or medium for obtaining a foundation, the weight of the superstructure being carried on the internal filling or lining.

Caissons constructed of plates of wrought-iron or steel are much used for the foundations of large piers in deep water. Practically they may be considered as cylinders on a large scale, with the difference that whereas cylinders are generally continued up to the under side of the girders of the superstructure, caissons are only carried up to a short distance above the water-level. A caisson forms a strong water-tight iron cofferdam, from

which the water can be excluded, and a masonry or brickwork pier constructed inside. It may be made all in one piece to correspond to the form of the pier, or in separate pieces to form one whole, each being sunk independent of the other, and connected together afterwards. Being built up of plates cut to the proper size and shape, it is a very simple matter to rivet on additional tiers of plates as the caisson is lowered deeper and deeper into the bed of the river. The lower length is made with a cutting edge to penetrate the ground; the exterior is made without any projection larger than the rivet heads, and the interior is strengthened with T-irons or double L-irons at the joints, and strong cross-bracing to resist the pressure of the water. About 7 or 8 feet above the cutting edge a strongly framed iron floor is riveted to the vertical sides, and strengthened by plate-iron under-brackets placed at short distances. The excavators work in the space below the floor, and the excavated material is passed up through openings formed in the floor at convenient points to suit the working. The methods of lowering a caisson are the same as for lowering a cylinder. If the pneumatic system has to be adopted, then two or more air-tight tubes of liberal dimensions (say 5 to 8 feet diameter), according to the size of the caisson, must be attached to the floor, and on the top of each of these tubes air-locks must be secured for the removal of men and materials. The masonry or brickwork of the pier is built upon the iron floor, and a portion of this building work is usually carried on during the sinking of the caisson to obtain weight to assist in the lowering. When down to the proper depth, the space below the floor is properly cleared of débris and water, and then carefully filled in with cement concrete.