Part 20 (2/2)

~DAM, CHAUDIERE FALLS, QUEBEC.~--The dam was 800 ft. long and from 16 to 20 ft. high, constructed of 1-2-4 concrete with rubble stone embedded.

The rubble stones were separated at least 9 ins. horizontally and 12 ins. vertically and were kept 20 ins. from faces. At one point the rubble amounted to 40 per cent. of the volume, but the average for the dam was 25 to 30 per cent. The stone was broken at the work, some by hand, but most by machine, all to pa.s.s a 2-in. ring. Hand-broken stone ran very uniform in size and high in voids, often up to 50 per cent.

Stone broken by crusher with jaws 2 ins. apart would run 20 to 30 per cent. over 2 ins. in size and give about 45 per cent. voids; with crusher jaws 1 ins. apart from 98 to 100 per cent. was under 2 ins. in size and contained about 42 per cent. of voids. It was found that if the crushers were kept full all the time the product was much smaller, particularly with Gates gyratory crusher, though a little more than rated power was required when the crusher was thus kept full. This practice secured increased economy in both quant.i.ty and quality of product. The concrete was made and placed by means of a movable traveler shown by Fig. 92. Concrete materials were supplied to the charging platform of the traveler by means of a traveling derrick moving on a parallel track. In placing the concrete on the rock bottom it was found necessary in order to secure good bond to scrub the rock with water and brooms and cover it with a bed of 2 ins. of 1-2 mortar. The method of concreting in freezing weather is described in Chapter VII.

CHAPTER XII.

METHODS AND COST OF CONSTRUCTING BRIDGE PIERS AND ABUTMENTS.

The construction of piers and abutments for bridges is best explained by describing individual examples of such work. So far, in America, bridge piers have been nearly always of plain concrete and of form and section differing little from masonry piers; where reinforcement has been used at all it has consisted of a surface network of bars introduced chiefly to ensure monolithic action of the pier under lateral stresses. In Europe cellular piers of reinforced concrete have been much used. Plain concrete abutments differ little in form and volume from masonry abutments. Reinforced concrete abutments are usually of L-section with counterforts bracing the upright slab and bridge seat to the base slab.

Form work for reinforced abutments is somewhat complex; that for plain abutments and piers is of simple character, the only variations from plain stud and sheathing construction being in the forms for moldings and coping and for cut-waters. For piers of moderate height the form is commonly framed complete for the whole pier, but for high piers it is built up as the work progresses by removing the bottom boards and placing them at the top. Opposite forms are held together by wire ties through the concrete. Movable panel forms have been successfully employed, but they rarely cheapen the cost much. Sectional forms, which can be s.h.i.+fted from pier to pier where a number of piers of identical size are to be built, may frequently be used to advantage. An example of such use is given in this chapter.

Derricks are the recognized appliances for hoisting and placing the concrete in pier work; they are the only practicable appliance where the pier is high and particularly where it stands in water and mixing barges are employed. For abutment work and land piers of moderate height derricks and wheelbarrow or cart inclines are both available and where much s.h.i.+fting of the derricks is involved the apparently more crude method compares favorably in cost.

The methods of placing concrete under water for pier foundations are described in Chapter V, and the use of rubble concrete for pier construction is ill.u.s.trated by several examples in Chapter VI. The following examples of pier and abutment construction cover both large and small work and give a clear idea of current practice.

[Ill.u.s.tration: Fig. 93.--Pier and Cofferdam for a Railway Bridge.]

~COST OF CONSTRUCTING RECTANGULAR PIER FOR A RAILWAY BRIDGE.~--This pier, Fig. 93, was built in water averaging 5 ft. deep. The cofferdam consisted of triple-lap sheet piling, of the Wakefield pattern, the planks being 2 ins. thick, and spiked together so as to give a cofferdam wall 6 ins thick. The cofferdam enclosed an area 1420 ft., giving a clearance of 1 ft. all around the base of the concrete pier, and a clearance of 2 ft. between the cofferdam and the outer edge of the nearest pile. The cofferdam sheet piles were 18 ft. long, driven 11 ft.

deep into sand, and projecting 2 ft. above the surface of the water.

The concrete base resting on the foundation piles was 1218 ft. The concrete pier resting on this base was 713 ft. at the bottom, and 511 ft. at the top. The pier supported deck plate girders. There were 100 cu. yds. of concrete in the pier and base.

The cost of this pier, which is typical of a large cla.s.s of concrete pier work, has been obtained in such detail that we a.n.a.lyze it in detail, giving the costs of cofferdam construction and excavation as well as of mixing and placing the concrete.

Setting up and taking down derrick and platform:

4 days foreman at $5.00 $ 20.00 days engineman at $3.00 2.25 days blacksmith at $3.00 2.25 days blacksmith helper at $2.00 1.50 22 days laborers at $2.00 44.00 ------- Total $ 70.00

Cofferdam-- 7 days foreman at $5.00 $ 35.00 4 days engineman at $3.00 12.00 38 days laborers at $2.00 76.00 1 ton coal at $3.00 3.00 ------- Total labor on 7,900 ft. B. M. at $16.00 $126.00 7,900 ft. B. M. at $20.00 158.00 ------- Total for 58 cu. yds. excavation $284.00

Wet Excavation-- 1.8 days foreman at $5.00 $ 9.00 1.5 days engineman at $3.00 4.50 9 days laborers at $2.00 18.00 ton coal at $3.00 1.50 ------- Total labor on 58 cu. yds. at 57c. $ 33.00

Foundation Piles-- 960 lin. ft. at 10c $ 96.00 4 days setting up driver and driving 24 piles at $20 per day for labor and fuel 80.00 ------- Total $176.00

Concrete-- 100 cu yds. stone at $1.00 $100.00 40 cu. yds. sand at $0.50 20.00 100 bbls. cement at $2.00 200.00 5 days foreman at $5.00 25.00 50 days laborers at $2.00 100.00 5 days engineman at $3.00 15.00 2 tons coal at $3.00 6.00 ------- Total, 100 cu. yds. at $4.66 $466.00

8 days carpenters at $3.00 24.00 2,400 ft. B. M. 2-in. plank at $25.00 60.00 1,000 ft. B. M. 46-in. studs at $20.00 20.00 Nails, wire, etc 2.00 ------- Total forms for 100 cu. yds. at $1.06 $106.00

Summary-- Setting up derrick, etc. $ 70.00 Cofferdam (7,900 ft. B. M.) 284.00 Wet excavation (58 cu. yds.) 33.00 Foundation piles (24) 176.00 Concrete (100 cu. yds.) 466.00 Forms (3,400 ft. B. M.) 106.00 --------- Total $1,135.00 Transporting plant 20.00 20 days rental of plant at $5.00 100.00 --------- Total cost of pier $1,252.00

Regarding the item of plant rental, it should be said that the plant consisted of a pile driver, a derrick, a hoisting engine, and sundry timbers for platforms. There was no concrete mixer. Hence an allowance $5 per day for use of plant is sufficient.

It will be noted that no salvage has been allowed on the lumber for forms. As a matter of fact, all this lumber was recovered, and was used again in similar work.

Referring to the cost of cofferdam work, we see that, in order to excavate the 58 cu. yds. inside the cofferdam, it was necessary to spend $284, or nearly $5 per cu. yd. before the actual excavation was begun.

The work of excavating cost only 57 cts. per cu. yd., but this does not include the cost of erecting the derrick which was used in raising the loaded buckets of earth, as well as in subsequently placing the concrete. The sheet piles were not pulled, in this instance, but a contractor who understands the art of pile pulling would certainly not leave the piles in the ground. A hand pump served to keep the cofferdam dry enough for excavating; but in more open material a power pump is usually required.

The above costs are the actual costs, and do not include the contractor's profits. His bid on the work was as follows:

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