STRENGTH OF PIp CULVERTS. The data in the preceding section seem to show that the breaking load of the cast-iron pipe is the equivalent of the pressure of a bank of earth 130 feet high, and of the reinforced concrete pipe is the equivalent of a bank 70 feet high; but it should be remembered that these are the results when dry sand was packed around the pipes as carefully as possible, and that any variation from the conditions of the experiment may materially affect the load the pipe will support. For example, if the bedding is such as to give as much thrust at the sides of the pipe as vertical load at the crown, there will be no bending moment in an annular cross section, and hence the pipe will carry a maximum load. Again, if the pressure is uniform over the horizontal projection, the pipe will carry twice as much as if the pressure is concentrated along a line at the top and the bottom. The last relation suggests that in bedding the pipe in firm ground the trench should be so shaped that the pipe will surely be free at the bottom, even after settlement occurs. The nature of the filling and the method of depositing it have a great influence upon the strength of the pipe; but in every case it is wise to attend carefully to the bedding of the pipe, particularly to securing (1) a uniform support under the pipe, (2) a considerable horizontal thrust at the sides, and (3) a uniform pressure on top.* Box CULVERTS.
Box culverts, i.e., culverts having a rectangular waterway, were formerly often built of timber or stone, and are at present often made of plain or reinforced concrete.
There are two forms of box culverts: one having only roof and side walls, and one having floor, roof, and side walls. Strictly speak ing, the first should be called an open box, and the second a closed box; but ordinarily, the first is referred to as an open box, and the second simply as a box culvert. In the open box culvert the side walls have independent footings which carry the load; and in the closed box the floor carries the load, although the so-called floor may project outside of the side walls.
Timber is not much used now for culverts owing to its high price and perishable nature; and stone-box culverts are not much used now owing to the difficulty of obtaining suitable stone within a reasonable distance. Wood culverts should be considered
only temporary, and the area of the waterway should be so much larger than actually required that a permanent culvert can be con structed inside of the timber one before the latter decays.
Stone-box culverts were made by resting slabs of stone upon side walls which were sometimes laid up dry and sometimes with mortar. The span of the cover stones varied from 2 to 4 feet, and the thick ness from 10 to 16 inches. The former editions of this volume contain a full discussion of box culvert and also illustrations of the standard stone-box culvert employed on several railroads.
Plain concrete is not economical for use in box culverts, since both the roof and the floor, and probably also the sides, must resist a force tending to produce tension in the concrete—a stress which the material can not economically carry.
Reinforced concrete is, therefore, the only material that is economical and durable for use in box culverts. The reinforced concrete box section is used also for cattle passes, for undergrade highway crossings, etc.
For spans of less than 15 or 20 feet, the box culvert is usually superior to an arch culvert for four reasons, viz.: 1. The space occupied is only a few feet more than the clear span, and hence the box culvert can be put in with less excavation and less disturbance to the embankment than is necessary for an arch culvert. 2. The box does not concentrate the load upon the foundation as does the arch, and hence the box culvert can be founded directly upon a soft soil where an arch culvert would require piling. 3. The foundation is immediately under the span, and hence it is much easier to drive piles for the foundation of a box culvert than for an arch, particularly if the culvert is to occupy the major portion of a panel of a wood-pile trestle—as is often the case. 4. The form work is more simple for the box than for the arch culvert.