For most cases in practice the size of waterway may he deter mined from the knowledge which usually exists in the vicinity regard ing the character of a stream,from the sizes of other openings upon the same stream, or from comparison with other streams of like character and extent in the same locality. Where data of this kind do not exist, careful examination of water marks on rocks, the presence of drift, etc., may be made to determine the height to which water has previously risen. The shape of the valley and the slope of the surface is of more importance than the area of country drained. The use of a formula like Talbot's assists the arrangement of the factors which enter into the determination, and is intended only as an aid to judg ment in selecting the size of opening required.
179. Pipe Culverts.—Vitrified clay pipes make satisfactory as well as comparatively cheap culverts when small openings are required, and for openings from 12 to 24 inches in diameter, they may often be used economically. It is not usually desirable to build a culvert less than 12 inches in diameter. For those larger than 2.1 inches concrete will usually he found more suitable, although vitri fied pipes 30 and 36 inches in diameter are sometimes used.
The best quality of double-strength, salt-glazed sewer pipe should he used for culverts. These pipes are made in lengths of 24 and 30 inches and diameter from 12 to 36 inches, with socket joints. They should he sound and well burned, giving a clear ring when lightly struck with a hammer.
The joints should be filled with Portland cement mortar a require ment particularly necessary where the pipe is likely to flow full, or under pressure, as it will prevent the water being forced out and the earth being washed from around the pipe.
Vitrified pipes cannot safely be used where they are directly exposed to the shocks of traffic, and many failures of such culverts have been due to this cause. In highway work they should be pro tected by at least 2 feet of filling, the roadway being graded so that a vehicle may pass smoothly and without shock over the culvert. In railway work a fill of about 5 feet over the culvert is usually neces sary. The use of vitrified pipe for railway culverts is desirable only under favorable conditions, when danger from shocks of traffic can be avoided, and good foundations make breakage from settlement improbable.
The cost of vitrified pipe varies widely with the conditions of trade, and with the expense for freight and haulage to the site of the work. The cost of laying the pipe depends upon local conditions
and the way the work is handled. Table XXVIII gives areas, weights, and rough averages of costs in a number of localities in the Middle West before the War.
The ends of pipe culverts should always be protected by a masonry or concrete wall. Fig. 96 shcws a vitrified pipe culvert with end wall as used in highway work. These walls should extend at least 2 feet below the bottom of the culvert to prevent water passing under the culvert and undermining it, and should also reach above the surface of the roadway, thus serving as a protection both to the cul vert and to the road. When the culvert is under an embankment, the wall should rise high enough to catch the slope of the embankment and form a curb to retain the earth.
Table XXIX gives dimensions that may be used for end walls for highway culverts index ordinary conditions.
Culverts of cast-iron pipe have been used to considerable extent iim railway work for sizes from 1 to 4 feet in diameter. The present tendency, however, is to use concrete for the larger openings, on account of its relative cheapness and the occasional cracking of the large iron pipes. Ordinary water pipe is sometimes used, but heavier pipe made for the purpose is more commonly employed.
For highway work, cast-iron pipe has the advantage of resisting shocks better than vitrified pipe, and may be used for small openings where the service is severe. It is not extensively used on account of its cost. Special culvert pipes in lengths of 3 or 4 feet are now avail able, which are made lighter than ordinary water pipe, some of them being made with a thinner shell reinforced by ribs. They are also made in longitudinal sections to he bolted together.
Corrugated metal culvert pipe is made lighter than cast iron, and does not ordinarily differ greatly in price from clay pipe. It is rather easy to handle and is less likely to break under shocks than vitrified pipe. It should, however, be covered by a thickness of at least 1 foot of road material.
The life of a culvert of this kind depends upon the ability of the metal to resist rust. Wrought iron is much better than steel in this respect, but must be selected with special reference to its resisting qualities. Pipes made of nearly pure iron have given good results, although numerous failures have resulted from the use of improper material.