Part 24 (1/2)
CHAPTER XIII.
METHODS AND COST OF CONSTRUCTING RETAINING WALLS.
Concrete retaining walls may for construction purposes be divided into two cla.s.ses: Plain concrete walls of gravity section and reinforced concrete walls consisting of a thin slab taking the thrust of the earth as a cantilever anch.o.r.ed 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.
[Ill.u.s.tration: 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 a.s.sumed 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.
[Ill.u.s.tration: 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 3 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.
[Ill.u.s.tration: 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 38-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 2 ft. wide and 10 ft. long, by nailing 2-in. plank to 24-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 2 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 Chapter VIII. In cases where this object is not sought, panels of much larger size may be used. Working with panels 212 ft. of 2-in. plank it was found that each panel could be used 16 times before becoming unfit for further use, but as, owing to the nicety of molded surface demanded, panels were discarded when showing comparatively small blemishes, this record cannot be taken as a true indication of the life of such forms. These panel forms are used by the railway named for long abutments and piers as well as for retaining walls.
A different type of sectional form construction is ill.u.s.trated by Figs.
101 and 102. It has been extensively used for retaining wall work by the Chicago, Burlington & Quincy R. R. The studding and waling are framed in units as shown. The lagging is framed in panels for the rear of the wall, for the face of the coping, and for the inclined toe of the wall, and is ordinary sheathing boards for the main face of the wall. The make-up of the several panels is shown by the drawings. The reason for using ordinary sheathing instead of panels for the face of the wall is stated by Mr. L. J. Hotchkiss, a.s.sistant Bridge Engineer, to be that ”the sections become battered and warped with use, do not fit closely together, and leave the wall rough when they are removed.” The manner of bracing the form and of anchoring it down against the up-thrust of the wet concrete is shown by Fig. 102.
Two other examples of sectional form construction are given in the succeeding descriptions of work for the Grand Central Station terminal in New York City and for the Chicago Drainage Ca.n.a.l. In the former work it is notable that panels 5120 ft. were used, being handled by locomotive crane. The panels used on the drainage ca.n.a.l work and in the forms previously described are of sizes that can be taken down and erected by hand, and the means of handling them should always be given consideration in deciding on the sizes to be adopted for form panels not only in wall construction but in any other cla.s.s of work where sectional forms may be used. Wet spruce or yellow pine will weigh 4 lbs. per ft.
B. M., so that a panel 102 ft. made of 2-in. plank and three 24-in.
battens will weigh some 225 lbs. In form work where the panels are removed and re-erected in succession facility in handling is an important matter. When one figures that he may handle both the concrete and the form panels with it a cableway or a locomotive crane becomes a tool well worth considering in heavy wall work.
[Ill.u.s.tration: Fig. 101.--Forms for Retaining Wall Work, C., B. & Q. R.
R.]
Three details in retaining wall form work that are often sources of annoyance out of proportion to their magnitude are alignment, coping construction and wall ties. Small variations from line in the face of the wall are seldom noticeable, but a wavy coping shows at a glance.
For this reason it is often wise to build the coping after the main body of the wall has been stripped, or if both are built together to provide in the forms some independent means of lining up the coping molds. In the form shown by Fig. 101 the latter is done by bracing the coping panel so as to permit it to be set and lined up independently of the main form. A separate form for molding the coping after the main body of the wall is completed may be constructed as shown by Fig. 103. Bolts at B and C permit the yokes to be collapsed and the form to be s.h.i.+fted ahead as the work advances. This mold provides for beveling the top edges of the coping and also the edge of the overhang, and the beveling or rounding of these edges should never be omitted where a neat appearance is desired. It is not essential, however, that this finis.h.i.+ng be done in the molds. By stripping the concrete while it is still pliable the edges can be worked down by the ordinary cement sidewalk edger.
[Ill.u.s.tration: Fig. 102.--Sketch Showing Method of Bracing Form Shown by Fig. 101.]
[Ill.u.s.tration: Fig. 103.--Sectional Form for Constructing Coping.]