Concrete retaining walls may for construction purposes be divided into two classes: Plain concrete walls of gravity section and reinforced concrete walls consisting of a thin slab taking the thrust of the earth as a cantilever anchored to a base slab or as a flat beam between counterforts. The reinforced wall requires much less concrete for a given height than does the plain, gravity wall, but the concrete is more expensive owing to the reinforcement and to the more complex form of construction, and, in some measure, to the greater cost of placing the mixture in narrow forms and around reinforcement. It is common, too, to require a richer concrete for the reinforced than for the plain wall.
Fig. 98.—Comparison of Plain and Reinforced Sections for Retaining Walls (C. E. Graff).
COMPARATIVE ECONOMY OF PLAIN AND REINFORCED CONCRETE WALLS.—Prior to the construction of some 2,000 ft. of retaining wall ranging in height from 2 ft. to 38 ft., at Seattle, Wash., calculation was made by the engineers of the Great Northern Ry. to determine the comparative economy of plain concrete and reinforced concrete sections. The sections assumed were those shown by Fig. 98, and comparisons were made at heights of 10, 20, 30 and 40 ft., with the following results:
| Height in | Plain. | Reinforced. | Per cent. |
| feet. | Cu. yds. per ft. | Cu. yds. per ft. | Saving. |
| 10 | 1.63 | 1.29 | 20.4 |
| 20 | 4.08 | 2.59 | 36.4 |
| 30 | 8.40 | 4.73 | 43.3 |
| 40 | 14.70 | 8.07 | 45.0 |
The saving in concrete increased as the height of the wall increased; for a 40-ft. wall reinforced concrete at nearly double the cost per cubic yard in place would be as cheap as plain concrete.
Fig. 99.—Comparison of Plain and Reinforced Sections for Retaining Wall (F. F. Sinks).
Taking substantially the section of reinforced wall being used on the Chicago track elevation work of the Chicago, Burlington & Quincy R. R., and comparing it with a plain wall as shown by Fig. 99, Mr. F. F. Sinks obtained the following results:
| Plain Wall, Cost per Lineal Foot— | |
| 4.8 cu. yds. concrete at $4 | $19.20 |
| 115 ft. B. M. of forms at $31 | 3.56 |
| ——— | |
| Total 4.8 cu. yds. at $4.74 | $22.76 |