It may not unfrequently be necessary to determine, as the work proceeds, experimentally, the best proportion of pitch and diameter of blade to suit any given condition, and where this is likely to be the case, the provision of a separate casting for the blade, attached to the main body of the pile, but separable from it, obviously offers some advantages.

Cast steel is sometimes used for screw-pile blades. In this connection, as in many others, it will not be unwise to remember what does and what does not constitute cast steel.

The combination of the water-jet method, found so successful in sandy soils, associated either with the screw or the disc pile, will cause a variation in design in the arrangement, diameter, and number of the jet holes to be provided for, while the attachment of the pump hose to the top of the pile will also receive attention.

As regards the sinking of cylinders of sufficient size for workmen inside to handle and remove the excavated material, nothing need be said here. The usual methods of applying the compressed air system, the use of the air lock, and the arrangements for the removal of spoil are well known and understood, while in certain cases where the soil entered is tenacious and impervious to water, the sinking of such cylinders by excavation and loading by dead weight can be carried out without the use of compressed air, though this system is often preferred, even in such cases, as affording a safeguard against "blows" caused by sudden and unexpected changes in the nature of the strata passed through.

Whatever may be the type of pile or cylinder adopted, whether it be a solid steel or wrought-iron column of a few inches diameter, or whether it be a cast-iron or riveted cylinder several feet in diameter, the difficulty of ensuring that the upper surfaces of a row of such piles or cylinders shall be in one true level plane after the operations of sinking and testing have been completed, will generally present itself for solution. The operation of pile or cylinder sinking is not in itself one which affords much room for fine adjustments, while the uncertainties which frequently attend the depth to which a pile must be sunk, either in order to reach a firm stratum or to afford such frictional resistances as will carry the load in soft soils, will often render the actual depth to be reached a doubtful quantity.

Where the superstructure of a jetty or wharf is of timber, a certain amount of cutting or packing to get over small differences of finished level may be permissible, but where a decking is of the more rigid type of riveted steelwork, a higher standard of accuracy in level is necessary, and this is frequently attained by the use of make-up lengths, the precise dimensions of these lengths being obtained from the work itself as soon as the operations of sinking, test loading, or concreting have been completed, and the pile or cylinder has reached its final depth and level. By this means the upper surfaces of piles and cylinders, with the seatings of the girderwork which they support, are maintained at their true levels.

An objection which may make itself felt, especially where the work is being carried out abroad, lies in the possibility of undue delay in the time taken in the transmission home of the necessary dimensions, and in the manufacture and delivery of the special lengths of pile or cylinder. As regards certain classes of piles, this objection has been met, where the dimensions admit of it, by the mechanical sawing off on the spot of the redundant length of piles in iron or steel.

Thus, to take an example, solid mild steel or wrought-iron piles, say 6 inches in diameter, in long lengths, arranged for driving through more or less compact strata until a hard or rocky bearing stratum is reached, are sometimes provided with a pointed end formed in steel of somewhat higher temper than the body of the pile, though capable of being welded on to the softer material, while the provision for cutting off, after driving and test loading, may consist of a cutting tool arranged to travel circumferentially round the body of the pile, and driven by worm-gearing, impelled either by manual labour or other power.

Such a tool would, however, in the case of the so-called Phoenix column pile (shown in Fig. 174), be less advantageous than some form of circular saw, which would first attack the projecting flanges before arriving at the body of the metal.

In short, the cutting-off apparatus must be suited to the form of cross-section to be dealt with.

In the case of large cast-iron cylinders, however, the system of making-up lengths appears the only practicable one to adopt.

An example of a make-up length of this type is given in Figs. 358, 364, and also in Fig. 34.

Of the many types of jetty construction which might have been illustrated, space can only here be found for a few figures exemplifying a class of jetty which is composite in construction, that is to say, having an ironwork substructure and timber superstructure. .

A few illustrations will also be given further on showing details of that type of jetty referred to on p. 339, where a rubble mound, or a wharf wall, or a combination of both, is faced with heavy cast-iron cylinders, spaced a considerable distance apart, and carrying a superstructure of riveted steel girders and decking.

The comparatively short life of timber in sea-water, and its exposure to the ravages of the teredo, frequently leads to the adoption of composite structures of this class, where the pile work is of cast or wrought iron up to and somewhat above low water, the remainder of the construction above this level being mainly of timber. The latter portion of the jetty, although still exposed to the inevitable decay which takes place between "wind and water," is, at all events, accessible for repairs or renewal, the life of the iron piles below water-line being assumed to be considerably longer than that of the superstructure which they support.