CHAPTER XIII

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METHODS AND COST OF CONSTRUCTING RETAINING WALLS.

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.

[Illustration: 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.

[Illustration: 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

Reinforced Wall, Cost per Lineal Foot-- 3.46 cu. yds. concrete at $4.10 $14.18 115 ft. B. M. of forms at $31 3.56 109 lbs. reinforcing steel at 3¼ cts. 3.54 1.34 cu. yds. extra fill at 20 cts. 0.27 0.32 cu. yd. extra excavation at 20 cts. 0.06 ----- Total, 3.46 cu. yds. concrete at $6.25 $21.61

The saving in this case was $1.15 per lineal foot of wall with the unit cost of reinforced concrete in place 24 per cent. greater than the unit cost of plain concrete. It will be noted that there is some 28 per cent. less concrete per lineal foot of wall in the reinforced section and also that this section is so designed that the form work is about as simple for one section as for the other. Another point to be noticed is that there is no saving in excavation by using a reinforced section instead of a gravity section, in fact the excavation runs slightly more for the reinforced section.

[Illustration: Fig. 100.--Forms for Retaining Wall Work, N. Y. C. & H. R. R. R.]

~FORM CONSTRUCTION.~--Retaining wall work often affords an opportunity for constructing the forms in panels and this opportunity should be taken advantage of when possible. Several of the walls described later give examples of form work that may be studied with profit in this respect.

Figure 100 shows a panel form construction employed on the New York Central & Hudson River R. R. The 3×8-in. studs are erected, care being taken to get them in proper line and to true batter and also to brace them rigidly by diagonal props. Generally the studding is erected for a section of wall 50 ft. long at one time. The lagging, made in panels 2½ ft. wide and 10 ft. long, by nailing 2-in. plank to 2×4-in. cleats, is attached to the studding a panel at a time and beginning at the bottom, by means of the straps, wedges and blocks shown. Five bottom panels making a form 2½ ft. high and 50 ft. long are placed first. When the concrete has been brought up nearly to the top of these panels, a second row of panels is placed on top of the first. When it is judged that the concrete is hard enough the lowermost panels are loosened and made free by removing the wedges, blocks and straps and the panels are drawn out endwise from behind the studding and used over again for one of the upper courses. The small size of the panels makes it practicable to lay bare the concrete while it is yet soft enough to work with a float or to finish by scrubbing as described in