Cost of Conorets

COST OF CONORETS RsTA/21 11W WALLS. Plain Concrete. For a discussion of the various items in the cost of concrete, see 1 412-19; and for an example of the cost of a plain concrete retaining wall, see 1 423. Table 78 shows the cost (exclusive of excavation) of a plain concrete retain ing wall containing 427 cubic yards, built by the Delaware, Lackawanna and Western R. R. at Scranton, Pa., in 1907.

1064. Reinforced Concrete.

Table 79 gives the average cost of the reinforced-concrete retaining walls of the type shown in Fig. 121, page 531, built in Chicago by a railroad in connection with track elevation during the year 1907. The work was done by company force.

Goat of

va. Retaining Wall. Of course, to make a fair comparison both forms of wall should be equally well designed and both should be equally stable. The concrete in the reinforced wall will cost more per cubic yard because of the greater cost of the forms (particularly for the counterforted wall) and also because of the interference of the steel in placing the concrete. Further, the cost of the excavation per cubic yard of concrete is considerably more for the reinforced wall than for the plain wall, because of the smaller number of cubic yards in the former and also because the base of the former is usually considerably longer than that of the latter and consequently the excavation is pro portionally greater. A reinforced wall will, of course, cost more per cubic yard, because steel costs more than an equivalent volume of concrete. On the other hand, a reinforced wall will contain con

siderably less concrete than a plain wall.

Not infrequently, the costs of the concrete in these two forms of walls are compared as being proportional to the areas of the two cross sections, and the claim is made that the reinforced-concrete wall is 40 to 60 per cent the cheaper, varying with the height; but such a method of comparison is greatly in error. Again, the cost of the concrete in these two forms of walls is sometimes compared by adding from 10 to 50 cents per cubic yard to cover the extra cost of the rein forced concrete, and the claim is made that the reinforced wall is from 35 to 45 per cent the cheaper; but such an allowance for extra • cost is entirely too small. Because of one or the other of the above errors, many of the estimates of the relative cost of these two forms of walls are misleading.

On a leading railroad the cost of a large amount of work showed that, exclusive of excavation and of company haul on materials, the cantilever reinforced-concrete retaining wall was 19 per cent cheaper than the plain concrete wall; and two other prominent railroads estimate that high counnerforted reinforced retaining walls are 25 per cent cheaper than plain concrete ones.