plan consists in gradually sinking to firm bot tom shafts of brickwork or masonry resting on a strong curb or ring of wood or iron haying a scarfed or sharpened lower edge. This is supported from a suitable scaffold, and is gradually lowered as the masonry is added until the bottom is reached (i5/. 5o, jig. 7). The shaft is further sunk to solid bottom by undermining from the inside by scoops, sand-pumps, etc., and thus the structure, either by the weight of the masonry above, whose walls are carried tip as the shaft descends, or aided by artificial loading, gradually sinks to a firm bottom. Foundations for bridge piers have been sunk in this manner. In the case of the Parnitz Bridge at Stettin each pier was erected upon a single shaft of masonry, which was sunk dry, not by removing the earth from the open top as above described, but by using compressed air, excavating; the soil beneath the foot of the curb and remov ing- it from time to time as the sinking of the shaft progressed. In this case the curb consisted of an air-tight chamber of iron, open below, from beneath which the water was forced out by compressed air, which dis placed it.
kircuum Process.—The idea of utilizing the pressure of the atmosphere in sinking- foundations originated with Dr. Lawrence H. Potts. The plan —which is adapted to soft soils only—is called the vacumn process. Dr. Potts proposed the following method of facilitating, the sinking of large iron cylinders to serve as foundations of bridge piers, etc. He closes the hollow iron cylinder on top by the use of an air-tight trap-door opening upward; then, by means of a suction-pump and suitable connections, lie exhausts the air from the inside of the cylinder, the same having previ ously been placed in proper position and pumped free from water. Under these circumstances, in suitable ground (not too compact), the atmospheric pressure will force the cylinder downward a distance greater or less accord ing to the resistance of the bottom, and at the same time the interior will become filled with soft or semifluid earth forced into it by the atmospheric pressure transmitted throng)] the water to the soil around the open bottom of the cylinder. The trap-door is now opened, the contents of the cylinder are removed, and the same series of operations is repeated until the bottom of the cylinder reaches a solid stratum. Additional lengths of the cylinder are bolted on from time to time as may be necessary while the sinking progresses.
Plenum Process.—In the construction of the Medway Bridg-e at Roches ter, in 1851, it was the intention to sink the tubular cast-iron piles (7 feet in diameter) by the use of the vacuum process above described. But, as old masonry was encountered during the sinking of one of the cylinders, it became evident that the operation by rarefaction would have to be aban doned. Chief-engineer Hughes successfully applied a process that had first been made use of by the French engineer Triger in 1841 for sinking a shaft in the middle of the river Loire, and later (185o) in sinking the foun dation of the Macon Bridge across the SaOne. In the Triger system—
called, in contradistinction to that above described, the plentnn process— the air in the interior of the cylinder, instead of being rarefied, is com pressed to such an extent as to force out the water, compelling it to escape beneath its open end into the surrounding water, leaving the interior of the cylinder dry.
At the top of the cylinder there is provided an air-lock (fi/. 41, fig-. 19) throngh which the workmen can descend to the bottom of the chamber and remove the soil from the foot of the tube by undermining. The soil thus excavated is loaded into suitable receptacles and hoisted to the air-lock. The workmen then ascend, leaving the chamber by way of the air-lock, through which, also, the load of earth is removed. Then the compressed air is permitted to escape by opening- a snitable valve, and the cylinder, being no longer supported by the upward pressure of the air beneath its closed top, sinks by its own weight into the cavity or loosened soil pre pared about its open bottom, while the water from without enters by the same avenue. 'When equilibrium is restored. the air-compressor is again started, the water is forced out, and the above-named series of operations is again repeated. As the cylinder descends, additional sections are bolted on at the top, the air-lock chambers being,- removed froin the old, and replaced on the top of the last, section. The general method here described will be understood from Figure 19 (fii. 41). That the cylinder may not tilt, it is necessary to guide it in its descent by a frame of some kind rest ing on piles. The depth to which cylinders may be sunk by this process will be limited by the friction of the walls, and by the danger of working in a compressed atmosphere, which increases rapidly with the depth. 13y this process large cylinders and caissons for bridge piers have been sunk to a depth of 4o or 5o feet.
Modifications of the I'lenum Process have been introduced, embodying improvements in certain details. Figure 2o shows how by a simple arti fice the volume of air to be compressed is substantially diminished and the same object accomplished as though the whole interior of the cylinder were subjected to compression. In this plan a working chamber of wrought iron, provided with a roof or cover of the same material, is placed in the bottom of the cylinder. The interior of this chamber is placed in com munication with the air-lock at the top by means of a metal tube of suffi cient size to permit the descent and ascent of the workmen and of the buckets for removing the excavated material. By this expedient the vol ume of compressed-air space is greatly reduced, and the annular space between the communicating tube and the cylinder walls may be filled out with masonry or concrete as it is found necessary to assist the sinking by adding to the weight of the structure.