Solid screw piles of wrought-iron or steel, similar to [Fig. 114], are used for some descriptions of work. These are generally made in long lengths, in sizes varying from 4 to 8 inches in diameter, and with screw blades of wrought-iron or cast-iron fixed in the most secure manner to resist the strain produced when screwing into the ground. The couplings for these solid piles must be very carefully made, all contact surfaces truly faced and fitted, bolts turned, and bolt-holes drilled.
[Fig. 115] is a sketch of a hollow cylindrical water-jet pile, which has been used successfully in cases of light sand. The lower end of the pile is made externally in the form of a solid disc, terminating in a conical point, having an aperture in the centre to correspond to the water-jet. To the top of the pile is secured a tight-fitting cover through which a tube passes from a force pump. Water at high pressure is pumped into the tube, and as it forces its way out through the conical point the sand is stirred up and loosened, and thus allows the pile to descend. When the pile has been lowered to a sufficient depth the pumps and tube are removed, and the sand settles down into its former compact condition.
Great care must be used with the first two or three lengths of any screw pile to ensure the pile taking a correct or true vertical position. Each series of screw piles should be properly braced together to obtain stability under moving loads.
Hollow cylinders of cast-iron, wrought-iron, or steel form most efficient foundations or piers for large bridges over soft ground or fresh water of considerable depth. Made open at the bottom, and constructed of complete rings, or, if of large diameter, of rings built up in segments and securely attached together
with water-tight joints, the cylinder is placed in its proper position on the ground or lowered into the water preparatory to sinking. The lower length is made with a sharp cutting edge to facilitate penetration. By excavating and removing the material round the cutting edge and base inside the lower length, the cylinder descends gradually either from its own weight or by assisted weights, and length after length is added until it is sunk to the depth required. The excavated material is filled into buckets and hoisted to the surface by a winch fixed on the top length. When sinking in water the working top of the cylinder is always kept at a suitable height above the water for convenience in removal of the earth or clay from the interior to barges or gangways alongside.
Some strata are more favourable for cylinder sinking than others. Material of a strong clayey nature admits but a small amount of water into the excavation, and a moderate-sized pump will keep the working fairly dry until considerable depth has been reached. Some other materials are so open that the water cannot be kept down with ordinary pumps, and the cylinders can then only be lowered by the pneumatic process. This process has been carried out in two methods, one of them on the vacuum principle, and the other by air pressure, or, as it is termed, the plenum system. With the former method the cylinder is placed in position, and an air-tight cap, through which a pipe passes, is secured on the top. Powerful air-pumps are then set to work, and the partial vacuum thus created in the interior causes the material round the cutting edge and base to be loosened and drawn into the cylinder, the cylinder at the same time going down or sinking by its own weight, or assisted, if necessary, by added weights. The cap is then taken off, and the material removed from the interior, the operation of exhausting and emptying the interior being repeated until the cylinder is sunk to its proper depth. This method has been found to work well in strata which contained a large proportion of clay to assist in excluding the air and water, but was not nearly so successful when applied to material containing stones and large boulders.