VITRIFIED PIPE CULVERTS. Vitrified sewer pipes are extensively employed for small culverts under highways, although in the Northern States, monolithic concrete seems to be displacing the larger sizes. Formerly, such culverts were quite common under steam railroads; but in recent years many of the railroads, at least in the Northern States, have discontinued their use because of frequent breakages due either to the pipe's being laid too near the track or to the action of frost in the soil around the culvert, or to unexplainable causes, or because of 'the disjointing of the pipes due to the settlement and the consequent spreading of the earth embank ment. Vitrified pipes are extensively employed for culverts by highways and railways in the Southern and Southwestern States, where stone suitable for either masonry or concrete is scarce and not to be had at reasonable prices.
The pipe employed for culverts is that known to the trade as culvert pipe or "extra heavy" or "double strength" sewer pipe, which is 20 to 40 per cent (varying with the maker and the size) heavier than the quality ordinarily employed for sewers. When double-strength sewer pipe was first made, it was generally believed that such pipe would be abundantly strong for culverts for either highways or railways, provided a culvert under a highway had at least 1 foot of earth over it and under a railway 3 feet of earth and ballast; but experience has shown that under present loads this is not enough in either case. Part of the disrepute of vitrified pipe for highway culverts is due to the fact that the ends of shallow cul verts have not been protected by a masonry head-wall, and conse quently the pipes have been broken progressively from the end of the culvert by the wheels of passing vehicles. With proper methods of construction, vitrified pipe of less diameter than 30 inches will give satisfactory results for highway culverts, provided in all cases there is at least 18 inches of earth over the tile, and provided where 8- or 10-ton traction engines are in common use there is at least 24 inches.
Construction. In laying the pipe, the bottom of the trench should be rounded out to fit the lower half of the body of the pipe, with proper depressions for the sockets. The earth should be rammed
carefully, but solidly, around the lower part-of the pipe. Appar ently the pipe is sometimes broken by too vigorous ramming over the pipe with a too heavy rammer; and therefore care should be taken to determine the effect of any particular tamping. If it is desired that the culvert shall discharge under much of a head, the joints should be calked with cement mortar to prevent the possibility of the water's being forced out at the joints and washing away the soil from around the pipe.
The end of the culvert should be protected with a timber or masonry or concrete bulkhead. Of course, a head wall of masonry or of concrete is better than a timber one. The foundation of the bulkhead should be deep enough not to be disturbed by frost. In constructing the end wall, it is well to increase the fall near the outlet to allow for a possible settlement of the interior sections. The upper end of the culvert should be so protected that the water will not readily find its way along the outside of the pipes, in case the mouth of the culvert should become submerged. When concrete and brick bulkheads are too expensive, a fair substitute can be made by setting posts in the ground and spiking on plank. When planks are used, it is best to set them with considerable inclination towards the road-bed to prevent their being crowded outward by the pressure of the embankment.
The freezing of water in the pipe, particularly if more than half full, is liable to burst it; consequently the pipe should have a suffi cient fall to drain itself, and the outlet should be so low that there is no danger of back-water's reaching the pipe.
When the capacity of one pipe is not sufficient, two or more may be laid side by side. Although two small pipes do not have as much discharging capacity as a single large one of an equal cross section, yet there is an advantage in laying two small ones side by side, since then the water need not rise so high to utilize the full capacity of the two pipes as would be necessary to discharge itself through a single one of larger size.