reconstructed in a more substantial manner. It is a simple and comparatively inexpensive matter to rebuild a drain before the earth filling is brought forward, but it is a costly work to open out an embankment, and rebuild a culvert afterwards. Unless the seat of an embankment is well drained and kept free from the accumulation of running water, the earthwork will be exposed to washing away of the lower layers, and consequent subsidence. Each watercourse or open drain must be provided for either by a separate culvert of suitable size or, as may be done in some cases, by leading two or more watercourses into one, and thus passing all through one culvert of ample capacity. When fixing the sizes of the culverts they must not be limited to the normal flow of water, but a large margin must be allowed sufficient to meet extraordinary floods. The depth of the bed or invert of a culvert is a very important point. If laid too high, and the stream above should at any time deepen, the high invert would check the flow of the water, and would also incur the risk of being undermined and gradually carried away. If, on the other hand, the invert be laid too low, it will gradually silt up to the level of the stream-bed alongside, and there will be so much of the culvert space lost for all practical purposes. In cases when the invert of a culvert has to be laid at a special low depth to allow for future improvements in drainage, it is advisable to give extra height from the invert to the crown, or top, so as to provide ample waterway in the event of any silting up in the mean time. Particular care should be taken when building the foundation of a culvert. It has to be laid on the site of the watercourse, or on a new channel which will ultimately form the watercourse, and it should be built sufficiently deep into the ground to avert as far as possible the chance of water finding a course through below the foundation.
The invert may be of stone pitching or brick if the current is not rapid, or liable to bring down stone boulders from its gravelly bed.
With a stream-course having considerable fall, and which carries with it large stones, roots of trees, and other débris, the invert should consist of strong pitching, composed of large-sized, rough-dressed stones of hard, durable quality, capable of withstanding the pounding of the boulders brought down during floods. A soft description of stone would be quite unsuitable for the invert of such a stream; the pitching would wear away
quickly, break, and become detached, leaving the foundation and side walls exposed to the cutting inroads of the water.
Where large flat bedded stones or flags of tough quality can be obtained, they form good covers, or tops, for culverts up to two feet in width. They should have not less than nine inches bearing on the side walls, and their contact edges should be fairly dressed, so as to fit sufficiently close to prevent the embankment filling from falling through.
Where the stream, or run of water, is very small, strong earthenware pipes, 9 inches or 12 inches in diameter, well bedded, may be sufficient to carry away all the water likely to arise. For small springs in low swampy ground, dry stone drains may in many cases be used with advantage. These are made by cutting a trench, say two feet deep by twelve or eighteen inches wide, in the seat of the embankment from side to side, and filling it up with dry rubble stones, not boulders, hand-laid, the upper layer placed on the flat to keep the earthwork as much as possible from filling in between the stones.
In soft boggy ground, where the depth to a hard bottom is very considerable, wooden culverts are frequently adopted. Although these cannot be classed as permanent structures, still, when they are made of sound well-creosoted timber, and substantially put together, they last for a number of years. Sometimes they are made cylindrical in section—a species of elongated cask with strong iron hoops every few feet. Others are rectangular in section, made with two strongly trussed side frames connected and covered with cross-planking and longitudinal tie-planking on the top and bottom.
Wooden culverts are seldom made of very large size, rarely exceeding an opening of 3 feet, and it is considered preferable to use two of these culverts of moderate dimensions than one of large size. [Figs. 67] and 68 give sketches of wooden culverts of cylindrical and rectangular section, and [Fig. 69] of flag top culverts of 12-inch, 18-inch, and 2-foot openings. In masonry culverts the side walls are shown to be of rubble stonework, but brickwork can be used instead, provided the bricks are well burnt, hard, and capable of withstanding the action of the water.
