[Fig. 106] shows a similar under-line bridge, but with curved instead of straight wing walls.
[Fig. 107] is an example of an under-line arch bridge in a rather high embankment, and where side arches have been adopted instead of long wing walls.
The above six types are equally applicable for private roads crossing the railway, but, as previously mentioned, a lesser width and headway will be accepted for under-line bridges for private or occupation roads, than for public roads. For the over-line bridges, however, the width and headway will be regulated by the number of lines and standard height of the railway.
When these arch bridges have to be built on the skew to suit an oblique crossing of the road, extra care will be necessary in setting out the work, and marking on the centering the spiral courses of the arching.
Arch bridges may be built of masonwork or brickwork, or a combination of the two. If the available quarries do not yield good flat bedded stones readily worked, it is better, where possible, to use strong hard bricks for the arching, and utilize the stone for the remainder of the work.
Although arching undoubtedly forms the most durable type of bridgework, numbers of cases occur where the available height or space between rail-level and road-level is too small, or the cost of masonry and brickwork too great, to admit of anything but girder-work. Detailed sketches of some of the many forms of girder bridges are given in [Figs. 132] to [153], illustrating various systems of roadways and parapets. In some instances the main girders are made sufficiently deep to serve as parapets, while in others a shallower girder has been adopted, on top of which has been placed a light cast-iron parapet composed either of close plate-work or of ornamental open railings. The open ironwork parapet has a good appearance, but as a screen is not so efficient as the close cast-iron plates.
In addition to the bridges required for the regular public roads, it is usually necessary to construct a certain number of occupation or private road bridges over and under the line to accommodate portions of estates and large properties intersected or severed by the railway, and which would be inadequately provided for by ordinary gate crossings on the level. The position and description of these occupation bridges is generally matter of private arrangement. The bridges will be somewhat
similar in character to the public road bridges, but of much less width for the roadway. Those over the railway must have the standard span and height adopted as a minimum for the other over-line bridges, and those under the railway must have the full width on the top for the lines of rails, but will have less width between the abutments for the roadway.
Foundations.—So much depends upon the soundness and security of the foundations of any bridge, viaduct, or large building, that it would be almost impossible to devote too much care to the selection and treatment. Unless the foundation be firm, the entire structure will be exposed to the risk of failure, either in subsidence of masonry, giving way of arches, or depression of girders. A small matter overlooked during the construction of this part of the work will be most difficult to correct or adjust afterwards.
The insistent weight of all structures built of masonry or brickwork will cause the mass to settle to a certain extent, according as the joints of mortar or cement become compressed by the number of superincumbent courses. In a similar manner the gravel and clay of a foundation will compress more or less according to its compactness and the weight of the structure. No inconvenience will, however, arise if the settlement or compression be uniform throughout the entire area.