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 shifted 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 finishing 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.]
Wall ties are commonly used to hold the face and back forms to proper s.p.a.cing, but occasionally they are not permitted. In the latter case the bracing must be arranged to hold the forms from tipping inward as well as from being thrust outward. A good arrangement is that shown by Fig.
102. In fastening the forms with ties the choice is usually between long bolts which are removed when the molds are taken down and wire ties which are left embedded in the concrete. The selection to be made depends upon the character of the work. When sectional forms are used like the one shown by Fig. 101, for long stretches of wall of nearly uniform cross-section bolts are generally more economical and always more secure. If the bolts are sleeved with sc.r.a.p gas pipe having the ends corked with waste the bolts can be removed ordinarily without difficulty. To make the pipe sleeve serve also as a s.p.a.cer the end next the face may be capped with a wooden washer which is removed and the hole plastered when the forms are taken down. With bolt ties the forms can be filled to a depth of 15 to 20 ft with sloppy concrete. This is hardly safe with wire ties unless more wire and better tieing are employed than is usual. It takes four strands of No. 10 to give the same working stress as a -in. threaded rod and the tieing in of four strands of wire so that they will be without slack and give is a task requiring some skill. Bolts are much more easily placed and made tight. In the matter of cost of metal left in the wall, the question is between the cost of sc.r.a.p gas pipe and of wire; the pound price of the wire is greater but fewer pounds are used and the metal is in more convenient shape to cut to length and to handle. This convenience in shaping the tie to the work gives the advantage to wire ties for isolated jobs or jobs which involve a continual change in the length and s.p.a.cing of the ties. In general the contractor will find bolts preferable where sectional forms are used and wire ties preferable when using continuous forms.
One objection urged against the use of wire ties is that the metal is exposed at the face of the work when they are clipped off unless the concrete is chipped and the cavity plastered. To obviate this objection various forms of removable "heads" have been devised. Two such devices are shown by Figs. 104 and 105. In both the bolt is unscrewed, leaving the "heads" embedded. The head shown by Fig. 104 has the advantage that it can be made by any blacksmith, while the head shown by Fig. 105 is a special casting.
[Ill.u.s.tration: Fig. 104.--Tie for Wall Forms.]
[Ill.u.s.tration: Fig. 105.--Tie for Wall Forms.]
~MIXING AND PLACING CONCRETE.~--Where a long stretch of wall is to be built the system of mixing and handling the concrete must be capable of being shifted along the work. For isolated walls of short length this problem is a simpler one. Where the mixer can be installed on the bank above, wheeling to chutes reaching down to the work is the best solution. As shown in Chapter IV concrete can be successfully and economically chuted to place to a greater extent than most contractors realize. Where the mixer has to be installed at the foot of the wall wheelbarrow inclines, derricks, gallows frames, etc., suggest themselves as means of handling the concrete. It is not this cla.s.s of work, however, but the long stretches of heavy section walls such as occur in depressed or elevated railway work in cities that call for thought in the arrangement and selection of mixing and handling plant.
In building the many miles of retaining wall in the work of doing away with grade crossings in Chicago, Ill., trains made up of a mixer car and several material cars have been used. The mixer is mounted on a flat car set at the head of the train and is covered by a decking carrying two charging hoppers set above the mixer. The material cars are arranged behind, the sand and stone or gravel being in gondola cars. Portable brackets hooked to the sides of the gondola cars carry runways for wheelbarrows. Sand and stone or gravel are wheeled to the charging hoppers, the work being continuous since one hopper is being filled while the other is being discharged into the mixer. The mixer discharges either into a chute, wheelbarrows or buckets. The foregoing is the general arrangement; it is modified in special instances, as is mentioned further on. The chief objection to the method is the difficulty of loading the wheelbarrows standing on runways level with the tops of the gondola sides. The lift from the bottom of the car is excessive, and as pointed out previously, shoveling stone or gravel by digging into it from the top is a difficult task.
The delivery of the concrete into the forms was accomplished by chute where possible, otherwise by wheelbarrows or cranes, and in one case by belt conveyor. In the last instance the mixer car was equipped with a Drake continuous mixer and was set in front. Behind it came three or four gondola cars of sand and stone, and at the rear end a box car of cement. All material was wheeled on side runways to two charging hoppers over the mixer. The mixer discharged onto a belt conveyor carried by a 25-ft. boom guyed to an A-frame on the car and pivoted at the car end to swing 180 by means of a tag line. The outer end of the conveyor was swung over the forms. A -in. wire rope wrapped eight times around two drums on the mixer car and pa.s.sing through slots in the floor to anchors placed one 500 ft. in front and one 500 ft. to the rear enabled the train to be moved back and forth along the work. This scheme of self-propulsion saved the hire of a locomotive. In another case the mixer was discharged into buckets which were handled by a crane traveling back and forth along a track laid on two flat cars.
[Ill.u.s.tration: Fig. 106.--Side Elevation of Traveling Mixer Plant, Galveston Sea Wall.]
Another type of movable mixer plant used in constructing a sea-wall some 3 miles long at Galveston, Tex., is shown by Figs. 106 and 107. Two of these machines mixed and placed some 127,000 cu. yds. of concrete, in 1 cu. yd. batches. Two 12-HP. engines operated the derricks and one 16-HP.
engine operated the Smith mixer; all engines took steam from a 50-HP.
boiler. The rated capacity of each machine was 300 to 350 cu. yds. per day. The method of operation is clearly indicated by the drawings.
[Ill.u.s.tration: Fig. 107.--End Elevation of Traveling-Mixer, Galveston Sea Wall.]
Placing the concrete in the forms is generally required to be done in layers; with wet mixtures this means little more than distributing the concrete somewhat evenly along the wall and slicing and puddling it to get rid of air and prevent segregation. Where mortar facing is required the face form described in Chapter VIII may be used. A reasonably good surface can be secured without mortar facing by spading the face. With dry concrete, placing and ramming in layers, calls for such care as is necessary in dry concrete work everywhere. Where new concrete has to be placed on concrete placed the day before, good bond may be secured and the chance of efflorescence be reduced by the methods described in Chapter VIII.
~WALLS IN TRENCH.~--In ca.n.a.l excavation, in subway work in cities, and the like, it is often necessary to dig trenches and build retaining walls in them before excavating the core of earth between the walls. The following examples of such work are taken from personal records:
_Example I._--A Smith mixer was used, the concrete being delivered where wanted by a Lambert cableway of 400 ft. span. The broken stone and sand were delivered near the work in hopper-bottom cars which were dumped through a trestle onto a plank floor. Men loaded the material into one-horse dump carts which hauled it 900 ft. to the mixer platform. This platform was 2424 ft. square, and 5 ft. high, with a planked approach 40 ft. long and contained 7,300 ft. B. M. The stone and sand were dumped at the mouth of the mixer and shoveled in by 4 men. Eight men, working in pairs, loaded the broken stone into the carts, and 2 men loaded the sand. Each cart was loaded with about 70 shovelfuls of stone on top of which 35 shovelfuls of sand were thrown. It took 3 to 5 minutes to load on the stone and 1 minute to load the sand. The carts traveled very slowly, about 150 ft. a minute--in fact, all the men on the job, including the cart drivers, were slow. After mixing, the concrete was dumped into iron buckets holding 14 cu. ft. water measure, making about cu. yd. in a batch. The buckets were hooked on to the cableway and conveyed where wanted in the wall. Steam for running the mixer was taken from the same boiler that supplied the cableway engine. The average output of this plant was 100 cu. yds. of concrete per 10-hour day, although on many days the output was 125 cu. yds., or 250 batches. The cost of mixing and placing was as follows, on a basis of 100 cu. yds.
per day:
Per day. Per cu. yd.
8 men loading stone into carts $12.00 $ .12 2 men loading sand into carts 3.00 .03 1 cart hauling cement 3.00 .03 8 carts hauling stone and sand 24.00 .24 4 men loading mixer 6.00 .06 1 man dumping mixer 1.50 .01 2 men handling buckets at mixer 3.00 .03 6 men dumping buckets and ramming 9.00 .09 12 men making forms at $2.50 30.00 .30 1 cable engineman 3.00 .03 1 fireman 2.00 .02 1 foreman 6.00 .06 1 waterboy 1.00 .01 1 ton coal for cableway and mixer 4.00 .04 ------- ----- Total $107.50 $1.07
In addition to this cost of $1.07 per cu. yd. there was the cost of moving the whole plant for every 350 ft. of wall. This required 2 days, at a cost of $100, and as there were about 1,000 cu. yds. of concrete in 350 ft. of wall 16 ft. high, the cost of moving the plant was 10 cts.
per cu. yd. of concrete, bringing the total cost of mixing and placing up to $1.17 per cu. yd. As above stated, the whole gang was slow.
The labor cost of making the forms was high, for such simple and heavy work, costing $10 per M. of lumber placed each day. The forms were 2-in.
sheeting plank held by 46-in. upright studs 2 ft. apart, which were braced against the sides of the trench. The face of the forms was dressed lumber and all cracks were carefully puttied and sandpapered.
The above costs relate only to the ma.s.sive part of the wall and not the cost of putting in the facing mortar, which was excessively high. The face mortar was 2 ins. thick, and about 3 cu. yds. of it were placed each day with a force of 8 men! Two of these men mixed the mortar, 2 men wheeled it in barrows to the wall, 2 men lowered it in buckets, and 2 men put it in place on the face of the wall. If we distribute this labor cost on the face mortar over the 100 cu. yds. of concrete laid each day, we have another 12 cts. per cu. yd.; but a better way is to regard this work as a separate item, and estimate it as square feet of facing work.
In that case these 8 men did 500 sq. ft. of facing work per day at a cost of nearly 2 cts. per sq. ft. for labor.
_Example II._--The building of a wall similar to the one just described was done by another gang as follows: The stone and sand were delivered in flat cars provided with side boards. In a stone car 5 men were kept busy shoveling stone into iron dump buckets having a capacity of 20 cu.
ft. water measure. Each bucket was filled about two-thirds full of stone, then it was picked up by a derrick and swung over to the next car which contained sand, where two men filled the remaining third of the bucket with sand. The bucket was then lifted and swung by the derrick over to the platform of the mixer where it was dumped and its contents shoveled by four men into the mixer, cement being added by these men.
The mixer was dumped by two men, loading iron buckets holding about cu. yd. of concrete each, which was the size of each batch. A second derrick picked up the concrete bucket and swung it over to a platform where it was dumped by one man; then ten men loaded the concrete into wheelbarrows and wheeled it along a runway to the wall. One man a.s.sisted each barrow in dumping into a hopper on the top of a sheet-iron pipe which delivered the concrete. The two derricks were stiff-leg derricks with 40-ft. booms, provided with bull-wheels, and operated by double cylinder (710-in.) engines of 18-HP. each. About 1 ton of coal was burned daily under the boiler supplying steam to these two hoisting engines. The output of this plant was 200 batches or 100 cu. yds. of concrete per 10-hr. day, when materials were promptly supplied by the railroad; but delays in delivering cars ran the average output down to 80 cu. yds. per day.