A coefficient of friction, even when derived from careful experiments on faces prepared to represent the normal working conditions, is, however, likely to become uncertain in its amount, if any alteration in the condition of the two faces in contact should take place in course of time under the actual working condition of immersion in sea water, and instances are not wanting where the frictional resistance has been entirely overcome by the uplifting tendency of the caisson.

It is prudent, therefore, in the designing of these structures to ignore the additional safeguard due to frictional resistance, and to rely upon the ascertained proportion between weight and buoyancy, supplemented by the holding-down power of the anchorages above alluded to.

The coefficient of friction between planed greenheart and patent axed granite has been ascertained by experiments carried out on a full size scale to be about 0.27 for dry timber upon dry masonry, and about 0.39 for wet timber upon wet masonry, these values representing the coefficients in each case for surfaces prepared to represent those actually occurring in practice when new, but not exposed to long-continued immersion.

The calculations necessary for the complete design of both floating and sliding caissons, apart from those required in the determination of the transverse strength against water pressure, are apt to be lengthy and tedious, as there is no royal road to the accurate determination of the centres of gravity and buoyancy, and average dimensions derived from existing examples should be used with caution, and for preliminary investigations only. The whole of the items of riveted steelwork and other materials should be carefully calculated for weight, and the lever arms of each separate item above some convenient datum plane ascertained, and worked out to obtain the sum total of moments about the given plane. From these the centre of gravity of the whole will be obtainable, and a similar course is necessary for all buoyancies, including immersed material and ballast.

The percentage of rivet heads and points is an important item, and for the heavy class of work customary in caissons with close-pitched watertight riveting should be taken as not less than 4½ per cent., while all immersed timber should be calculated at its saturated weight, if the gross bulk of the timber be reckoned for buoyancy. Recent experiments on the weight of greenheart, American elm, and Dantzic oak, saturated and dry, have yielded the following results:-

Table No. 36. Weights Of Timber, Wet And Dry. Greenheart Timber

Subsidiary items, such as paint, protective composition, tar, asphalte, cement, and the like, must also be allowed for, as well as their displacements.

A table is given at the end of this section, on p. 403, which gives the actual weights of recently constructed floating caissons of the type shown in Fig. 398, and of other types differing only in the arrangement of internal bracing and in the external contours. The examples given are of varying depths over the sills of the docks, and the weights given are the totals for riveted steelwork and other materials and machinery, but are exclusive of ballast of every kind, the amount of which in the examples selected ranged in round numbers between 150 and 300 tons, according to the conditions imposed by tidal levels and the position of the air chamber.

In order to obtain the measure of stability and length of pendulum previously alluded to, it will be found necessary to stow all ballast in the bilge at as low a level as possible.

The conditions of stowage, however, are not favourable to a great density per cubic foot, if pig-iron is used, owing to the breaking up of the space by reason of the numerous frames, diaphragms, etc., found necessary at this point, unless special castings to fit the spaces are employed, and it is convenient to use a material better adapted to give the greatest possible density per cubic foot when in position. The material known as burr concrete fulfils these conditions, the principal objection to its use being the difficulty of removal when once set hard. This is met by not using more than suffices to fill up awkward spaces in the constructional work, and to provide a level platform upon which the ordinary pig ballast can be stowed without excessive loss of space in the interstices.

The following table gives the results of experiments to ascertain the stowage value of pig-iron and other ballast under various methods of treatment: -

Table No. 37

Burr concrete ballast is composed of the "burrs" or punchings from steel plates or bars,1 and when grouted together with Portland cement mortar, in the proportion of one of Portland cement to one of sand, becomes a dense conglomerate, which with care and sufficient ramming can be made to weigh 350 lbs. per cubic foot, and is capable of being packed into spaces too confined to admit of very close stowage of ordinary pig.iron ballast.

Although in itself this material forms a protective coating to steel, it is usual to prepare the faces of steelwork to receive the ballast by a coating of asphalte cement laid on all horizontal or vertical surfaces about ⅜ inch thick. The composition of this cement is further described in Chapter VII (The Protection Of Steel Surfaces From Corrosion).

1 See Chapter III (Upon Certain Applications Of Riveted Girderwork, With Some Remarks Upon Rivets And Rivet-Holes). p. 109.

Table No. 38. The Weights Of Material And Machinery In Floating Caissons

Sliding Caissons.. Much of that which has already been remarked previously in connection with floating caissons may be taken to apply also to sliding caissons, with respect to such items as the calculations for centres of gravity and buoyancy, the mode of ballasting, the percentage of weight for rivet heads, the transverse strength, the necessity of carefully providing against an excess of buoyancy under all possible conditions, and the method of making a watertight joint.