On mountain roads or roads subjected to heavy rainfall culverts of ample dimensions should be provided wherever required, and it will be more economical to construct them of masonry. In localities where boulders and other debris are likely to be washed down during wet weather, it will be a good precaution to construct catch pools at the entrance of all culverts and cross drains for the reception of such matter. In hard soil or rock these catch pools will be simple well-like excavations, with their bottom two or three feet below the entrance sill or floor of the culvert or drain. Where the soil is soft they should be lined with stone laid dry; if very soft, with masonry. The size of the catch pools will depend upon the width of the drainage works. They should be wide enough to prevent the drains from being injured by falling rocks and stones of a not inordinate size.
The use of catch pools obviates the necessity of building culverts and drains at an angle to the axis of the road. Oblique structures are objectionable, as being longer than if set at right angles and by reason of the acute- and obtuse-angled terminations to their piers, abutments, and coverings.
Materials for Culverts. Culverts may be of stone, brick, vitri fied earthenware, or iron pipe. Wood should be absolutely avoided.
For small streams and a limited surface of rainfall either class of pipes, in sizes varying from 12 to 24 inches in diameter, will serve excellently. They are easily laid, and if properly bedded, with the earth tamped about them, are very permanent. Their upper surface should be at least 18 inches below the road surface, and the upper end should be protected with stone paving so arranged that the water can in no case work in around the pipe.
When the flow of water is estimated to be too great for two lines of 24-inch pipes, a culvert is required. If stone abounds, it may be built of large roughly squared stones laid either dry or in mortar. When the span required is more than 5 feet, arch culverts either of stone or brick masonry may he employed. For spans above 15 feet the structure required becomes a bridge.
Earthenware Pipe Culverts. 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. If the ground is soft or sandy, the earth should be rammed carefully, but solidly in and around the lower part of the pipe. The top surface of the pipe should, as a rule, never be less than 18 inches below the surface of the roadway, but there are many cases where pipes have stood for several years under heavy loads with only 8 to 12 inches of earth over them. No danger from frost, need be apprehended, pro vided the culverts are so constructed that the water is carried away from the level end. Ordinary soft drain tiles are not in the least affected by the expansion of frost in the earth around them.
The freezing of water in the pipe, particularly if more than half full, is liable to burst it; consequently the pipe should have a sufhi cient fall to drain itself, and the outside should be so low that there is no danger of back waters reaching the pipe. If properly drained,
there is no danger from frost.
Jointing. In many cases, perhaps in most, the joints are not calked. If this is not done, there is liability of the water being forced out of the joints and washing away the soil from around the pipe. Even if the danger is not very imminent, the joints of the larger pipes, at least, should be calked with hydraulic cement, since the cost is very small compared with the insurance against damage thereby secured. Sometimes the joints are calked with clay. Every culvert should be built so that it can discharge water under a head without damage to itself.
Although often omitted, the end sections should be protected with a masonry or timber bulkhead. The foundation of the bulk head should be deep enough not to be disturbed by frost. In con structing the end wall, it is well to increase the fall near the outlet to allow for a possible settlement of the interior sections. When stone and brick abutments are too expensive, a fair substitute can be made by setting posts in the ground and spiking plank to them. When planks are used, it is best to set them with considerable inclina tion towards the roadbed to prevent their being crowded outward by the pressure of the embankment. The upper end of the culvert should be so protected that the water will not readily find its along the outside of the pipes, in case the mouth of the culvert should become submerged.
When the capacity of one pipe is not sufficient, two or more may be laid side by side as shown in Fig. 19. Although the two small pipes do not have as much discharging capacity as a single large one of equal cross-section, yet there is an advantage in laying two small ones side by side, since 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 large size.
Iron Pipe Culverts. During recent years iron pipe has been used for culverts on many prominent railroads, and may be used on roads in sections where other materials are unavailable.
In constructing a culvert with cast-iron pipe the points requiring particular attention are (1) tamping the soil tightly around the pipe to prevent the water from forming a channel along the outside, and (2) protecting the ends by suitable head walls and, when necessary, laying riprap at the lower end. The amount of masonry required for the end walls depends upon the relative width of the embankment and the number of sections of pipe used. For example, if the em bankment is, say, 40 feet wide at the base, the culvert may consist of three 12-foot lengths of pipe and a light end wall near the toe of the bank; but if the embankment is, say, 32 feet wide, the culvert may consist of two 12-foot lengths of pipe and a comparatively heavy end wall well back from the toe of the bank. The smaller sizes of pipe usually come in 12-foot lengths, but sometimes a few 6-foot lengths are included for use in adjusting the length of the culvert to the width of the bank. The larger sizes are generally 6 feet long.