A box culvert with a stone top is generally limited by practice to a span of 4 feet, although it would, of course, be possible to obtain thicker stones which would safely carry the load over a considerably greater span. Therefore, when the required culvert area demands a greater width of opening than 4 feet, and when this type of culvert is to be used, the culvert may be made as illustrated in Fig. 74, by constructing an intermediate wall which supports the ends of the two sets of cover-stones forming the top. A section and elevation of a double box culvert of 3 feet span and a net height of 3 feet, is shown in Fig. 74. This figure also gives details of the wing walls and end walls. The double box culvert illustrated in Fig. 75 has two spans, each of 4 feet. The stone used was a good quality of limestone. The cover-stones were made 15 inches thick.

Fig. 74. Double Box Culvert.