After each caisson was built it was towed to its proper site, where it was held in place by temporary pile dock built completely around it. On these docks the concrete was placed; a 2 cu. yd. cubical mixer of the usual pattern being used for mixing. The concrete materials, consisting of sand, stone and cement was handled direct from barges alongside, into the mixer. The concrete was placed by a derrick located in the center of the caisson, which was a bad feature as the caisson was usually out of level and considerable difficulty was experienced in swinging the derrick. On the South caisson ¾ cu. yd. bottom dump buckets were used in placing the concrete, on the North caisson the size of these was increased to 1½ cu. yd. which reduced the cost of placing 15 cts. per cu. yd. There were placed in the South caisson 3,827 cu. yds. in 32 days of actual working time—120 cu. yds. per day of 10 hrs. The gross time was 2 months. On the North caisson 5,693 cu. yds. were placed in 46 days worked—124 cu. yds. per day. The gross time was 4 months.

The rates of labor were as follows per 10-hour day:

Foreman$5.00
Assistant foreman2.50
Hoisters2.50
Fireman1.60
Laborer1.50

Proportions concrete were 1: 2.5: 6.

The low price of sand in the North caisson was brought about by the finding of good building sand in the excavation for the anchorage, which work was done by the same contractor.

When the caissons had been sealed the iron material shafts were removed. This left holes 5 ft.×6 ft. extending from the roof of the caisson up to Mean H.W. which were filled with concrete. These shaft holes were 80 ft. deep on the South caisson and 100 ft. deep on the North caisson. They were partially filled with water and the concrete had to be placed with considerable care. Wooden chutes were used on the South caisson; they rested on the caisson roof, were filled with concrete and then raised allowing concrete to flow out at the bottom. The shaft holes were too deep on the North caisson for chutes and 20 cu. ft. bottom dump buckets were used. They had to be lowered to bottom of shaft each trip before dumping, a slow operation, which greatly added to the cost. Proportion for concrete 1-2.5-6.

The proportion for concrete in working chamber was the same as for all other concrete. The specifications called for 6 in. of mortar, of 1 part of cement to 2½ parts of sand, between the concrete and all bearing areas; that is, under the cutting edge and directly under the roof of the working chamber. The concrete was mixed in the cubical mixer and dumped on the bottom door of the material lock, the top door of the lock was then closed, the bottom door opened and the concrete fell through the shaft to the working chamber. It was then shoveled by the sand hogs into place. A 6-in. space was left below all bearing surfaces into which damp mortar was tightly rammed. Concreting the South caisson took 10¼ working days of 24 hours, the gangs working night and day in twelve 2-hour shifts; 1,566 cu. yds. of concrete and mortar were placed, or at the rate of 140 cu. yds. per 24 hours. The gross time including Sundays was 14½ days. The sand hogs worked in shifts of 2 hours each and received $3.50 for the two hours work. The twelve foremen received 1 dollar more: the average gang consisted of 12 sand hogs.

On the North caisson the organization was much better, owing to the experience gained on the first caisson; and in spite of the fact that the sand hogs, on account of the increased depth, received $4.00 for 1½ hours' work, or an increase of $22.00 per man per 24 hrs. over that on the South caisson, the work was done for less money. There were placed 1,566 cu. yds. of concrete in 7 working days of 24 hrs., or at the rate of 224 cu. yds. per day. The gross time was 11½ days including Sundays. The average number of men in the sand hog gangs was 18, with one foreman, who received $5 for 1½ hours work.

TABLE XVII.—ITEMIZED COST OF CONCRETING SOUTH CAISSON FOR BROOKLYN TOWER OF THE WILLIAMSBURG BRIDGE: COST OF CONCRETING CAISSONS ABOVE ROOF.

South Caisson (3,827 cu. yds.).
Materials.Quantity.Rate.Amount.
Cement4,480bbls.$1.57$7,034.00
Sand1,288cu. yds..60773.00
Broken stone3,421cu. yds.1.505,132.00
Water36.00
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Total3,827 cu. yds.$3.39$12,975.00
Labor.
Mixing and placing3,827 cu. yds.$0.90$3,432.00
Plant charges2,280.00
Plant labor742.00
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Total plant3,827 cu. yds.$0.79$3,022.00
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Total cost3,827 cu. yds.$5.08$19,429.00
General expenses, 10%3,827 cu. yds..511,943.00
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Grand total3,827 cu. yds.$5.59$21,372.00
COST OF CONCRETING SHAFTS.
South Caisson.
Materials.Quantity.Rate.Amount.
Concrete612½ bbls.$1.57$962.00
Sand193 cu. yds..4077.00
Stone493cu. yds.1.10542.00
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Total541cu. yds.$2.92$1,581.00
Labor.
Handling, mixing and placing541 cu. yds.$0.96$519.00
Plant charges, etc.541 cu. yds.1.06576.00
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Total541 cu. yds.$4.94$2,676.00
General expenses, 10%541cu. yds..49267.00
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Grand total541cu. yds.$5.43$2,943.00
COST OF CONCRETE IN WORKING CHAMBERS.
South Caisson. (1,435 cu. yds.)
Materials.Quantity.Rate.Amount.
Cement for concrete1,666bbls.$1.57$2,615.00
Cement for mortar459bbls.1.57720.00
Sand for both670cu. yds..40268.00
Broken stone1,181cu. yds.1.101,299.00
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Total materials1,435cu. yds.$3.42$4,902.00
Labor.
Top labor, mixing and placing1,435 cu. yds.$1.09$1,575.00
Pneumatic labor1,435 cu. yds.4.937,117.00
Compressor house labor1,435 cu. yds..19275.00
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Total labor1,435 cu. yds.$6.21$8,967.00
Plant.
Coal at $2.40 per ton1,435 cu. yds..10140.00
Concrete plant1,435 cu. yds..791,145.00
Pneumatic plant1.435 cu. yds.1.051,522.00
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Total plant1,435 cu. yds.$1.94$2,807.00
Totals1,435 cu. yds.$11.57$16,676.00
General expenses, 10%1,435 cu. yds.1.161,667.00
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Grand total1,435 cu. yds.$12.73$18,343.00