floods in the autumn or winter. No rafting can be carried on when the river is in flood; the current would be too strong to permit of the raft being kept under control.
[Fig. 87] is a sketch of a similar bridge where the river is confined to a regular channel between two sloping banks of strong clay.
[Fig. 88] shows a bridge erected over a narrow rocky pass in the river. The channel is hemmed in by the almost perpendicular sides of mountain granite, there are no banks to overflow, the flood waters cannot spread laterally, however much they may increase in depth, and with building-stone at hand in abundance, and foundations formed in the solid rock, the situation is one of the most favourable for a strong permanent bridge. The cast-iron arch of 150-feet span has a graceful appearance, and harmonizes well with the surrounding scenery. A small masonry arch at each end of the bridge provides for communication along the banks of the river.
With rivers which are neither under the control of navigation authorities nor used for rafts of timber, there is much greater freedom for the designing and carrying out of bridges or viaducts suitable for the actual physical conditions of the locality. The headway will be guided only by the height of the railway to be carried across, and by any flood-water levels which may affect the work. The size of the spans will be regulated by the width of the river, the depth of the water, and the nature of the ground into which the piers have to be built. For broad, shallow rivers with good firm river-beds, piers may be built at moderate cost, and comparatively small spans adopted; on the other hand, with a broad deep river it will be better, as previously explained, to reduce the number of piers and increase the span. In the one case, for example, a river 150 feet wide may be crossed with three spans and two piers in the shallow water, as in [Fig. 89]; in the other it may be more prudent and economical to cross in one span, without any intermediate pier, as shown in [Fig. 90].
Next in importance to the large bridges and viaducts over rivers are the viaducts which have to be constructed for the crossing of deep inland valleys. The occurrence of one of these deep valleys between long lengths of average table-land renders necessary either a series of cuttings and falling gradients to get down to a low level, or the erection of high-level works
to continue onward the rail-level at the height already attained. A decision to adopt the latter course brings forward the consideration as to the method of carrying out the work. To form a high embankment across such a valley would entail an enormous expenditure for earthwork, and several openings, or bridges, would have to be made in the embankment for streams, rivers, and roadways. Instead, therefore, of making this part of the line entirely of embankment, it is usual to carry the earthwork forward until the height is about 25 or 30 feet, and to form the remainder of the opening of arching, as shown in [Fig. 91].
This arrangement is not only less costly than an embankment of such height, but has also the great advantage that any or all of the arches are available for the passage of streams, rivers, roads, and accommodation works.
The character of the work to be carried out in the construction of bridges or viaducts over rivers or valleys must greatly depend upon the description of materials at command. Where good building-stone is plentiful, and the price of labour moderate, works of masonry should be adopted as far as practicable. Brickwork is an excellent substitute for masonry, provided that specially selected bricks are used for all facework, or parts exposed to the weather. For water-washed piers and abutments, the lower portion should be faced with good hard stone.