This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
In respect of the last item, the details of the timber keels and facings of the masonry will be similar in kind (see Fig. 397), while the arrangements of the roadway over the upper deck, the strength of rail girders and bearers for the given rolling load, and the switch arrangements for providing an automatic continuity in the rails when the caisson fleets over (about 3 inches in the case of sliders), will be found generally similar in both types.
As regards other details of the upper deck, some variation in practice is found to occur, due to the various methods employed in providing for the hauling in of the caisson underneath the camber deck.
The camber, or recess in the dock side, into which the caisson is drawn, is of considerable dimensions, and occupies a space which must be decked over so as not to diminish valuable wharfing space, and to provide for the continuity of the road or railway across the upper deck of the caisson.
This camber deck is sometimes made a fixed structure, sometimes partly fixed and partly capable of being lifted, and latterly, in recent examples, has been made to lift for its whole length, being hinged at the inner end, and lifted to the required amount at a convenient point near its outer end by hydraulic rams or other mechanical appliance, in accordance with the nature of the power used.
In the first case, where the camber deck is fixed, the upper deck of the caisson, when designed for foot passenger traffic only, is sometimes kept low enough to pass under the camber deck when hauled in. Where railway and wheeled traffic have to be provided for, this method is not applicable, and the caisson deck is made to fall and rise by means of special apparatus, deriving its power usually from the hauling engines, and acting automatically. In these cases the sliding ways or rollers upon which the caisson travels are kept level, and the caisson maintains a level course throughout its travel.
Such means of raising or lowering the upper deck of the caisson are found to necessitate a considerable amount of top weight, if the rolling loads to be carried are considerable, and the scantlings of the upper deck correspondingly heavy.
Another method is therefore sometimes adopted, whereby the necessary clearance between the under surface of the camber deck and the upper surface of the caisson deck, when in camber, is obtained by lifting the camber deck either in part or entirely (as previously described), at the same time causing the caisson to travel on a descending plane into the recess.
When the travel of the caisson is completed, and the dock entrance completely opened, the camber deck is then lowered on to its bearings, and the wharfage space is left entirely unencumbered.
On the return journey, the camber deck being again lifted, the caisson rises up the incline to its normal position across the dock entrance, the camber deck is let down, and the line of rails and roadway are continuous and horizontal for the entire combined length of the caisson and its camber.
It may be said that the provision of such an incline throws greater duty upon the hauling engine, and theoretically this is true; but numerous observations of the actual hauling power required show that the additional work done is but a small proportion of the whole, and will influence the design of the hauling gear in only a very minor degree.
It is, of course, to be understood that in the type of sliding caisson now under consideration, the greater portion of the weight of the caisson on the sliding ways or rollers is counterbalanced by the buoyancy of the air chamber and that of the immersed material.
The actual surplus weight on the ways or rollers can therefore be adjusted by ballasting so as to give the desired amount, which should be the least, compatible with general steadiness in working, and stability as against overturning moments produced by currents on a falling or rising tide, or temporary small difference of water level.
The frictional resistances caused by this surplus weight on the ways, together with the power required to move the entire mass at the given speed and to overcome all the resistances of the gearing employed, are the measure of the hauling power to be employed.
The functions of the air chamber, then, in sliding caissons are the same as that for floating caissons, and for docking purposes the slider is designed to float out of its position at the level of the upper deck of the air chamber (the hauling gear having been disconnected), and the same measure of stability or length of pendulum between centres of buoyancy and gravity is desirable.
As in the floating caisson so also in the sliding caisson, it is sometimes considered advisable to provide in the air chamber tanks for use in emergencies, by whose means the weight of the caisson can be increased by letting water in without flooding the entire air chamber. This also implies the provision of pumps worked by convenient power, such as steam, compressed air, or electricity, to get rid of the surplus water and restore the caisson to its normal working condition.
Such arrangements are more usual in cases where a large range of spring tides occurs, accompanied by exceptionally high tides above the normal high water spring tide level.
One of the most important features of the sliding caisson is the mechanism by which the caisson is hauled into and out of its camber or recess.
This is most usually accomplished by a pair of endless chains working over sprocket wheels (or drums, in accordance with the type of chain adopted) and actuated by powerful engines placed at the further end of the caisson camber, in a chamber below coping level specially prepared for its reception.
The motive power of such engines is usually either hydraulic, compressed air, or electric, supplied to the engines from some central station equipped with hydraulic pumps and accumulators, compressed air pumps and receivers, or electrical generating apparatus.