COST OF COFFERDAM FOUNDATIONS. It is universally admitted that estimates for the cost of foundations under water are very unreliable, and none are more so than those contemplating the use of a coffer-dam. The estimates of the most experienced engineers frequently differ greatly from the actual cost. The difficulties of the case have already been discussed Examples. The following example is interesting as showing the cost under the most favorable conditions. The data are for a railroad bridge across the Ohio River at Point Pleasant, W. Va.* There were three 250-foot spans, one 400-foot, and one 200-foot. There were two piers on land and four in the water; and all extended about 90 feet above low water. The shore piers were founded on piles—driven in the bottom of a pit—and a grillage, concrete being rammed in around the timber. The foundations under water were laid by the use of a double coffer-dam (§ 816). The water was 10 feet deep; and the soil was 3 to 6 feet of sand and gravel resting on dry, compact clay. The foundations consisted of a layer of concrete 1 foot thick on the clay, and two courses of timbers. The quantities of materials in the six foundations, and the total cost, are as follows: The total cost of foundations, including labor of all kinds, derricks, barges, engines, pumps, iron, tools, ropes, and everything necessary for the rapid completion of the work, was $64,652.62.
In the construction of the bridge over the Missouri River, near Plattsmouth, Neb., a concrete foundation 49 feet long, 21 feet wide, and 32 feet deep, laid on shore, the excavation being through clay, bowlders, shale, and soapstone, to bed-rock (32 feet below surface of the water), cost $39,607.23, or $42.81 per yard for the concrete laid.t The following example gives the details of the actual cost, exclusive of contractor's profits, of a coffer-dam and concrete pier on a pile foundation in water averaging 5 feet deep.$ The coffer-dam consisted of triple-lap sheet piling of the Wakefield pattern, the planks being 2 inches thick and giving a coffer-dam wall 6 inches thick. The coffer-dam inclosed an area 14 by 20 feet, giving a clearance of 1 foot all around the base of the concrete pier, and a clearance of 2 feet between the coffer-dam and the outer edge of the nearest pile. The sheet piles were 18 feet long, were driven 11 feet deep into sand, and projected 2 feet above the surface of the water.
There were twenty-four foundation piles, which were 40 feet long and which were driven 33 feet. Upon the heads of the piles rested a concrete base, 12 by 18 feet at the bottom, 7 feet thick, and 9 by 15 feet on top. The concrete pier was 7 by 13 feet at the bottom and 5 by 11 feet at the top. There were .100 cu. yd. of concrete in the pier and the base. The detailed cost of the work, which is typical of similar work, is as follows: If the cost of the plant be distributed among the other items in proportion to the time employed, the additional cost will be as follows: Coffer-dam—$74.00 or $9.00 per M. ft. B. M., making a total cost of $45.00 per M. ft. B. M. Excavation—$21.00 or 36 cents per cu. yd., making a total cost of 93 cents per cu. yd. Foun dation piles—$42.00 or $1.75 per pile, making a total cost of $5.08 per pile for driving, or a total cost of 12.7 cents per lin. ft. Con crete—$53.00 or 53 cents per cu. yd., making a total cost of $5.19 per cu. yd., or $6.25 including forms.
For data on the relative cost of different methods of con structing foundations, see Art. 6, page 456.
Uncertainty as to what trouble and expense a coffer-dam will develop usually causes engineers to choose some other method of laying the foundations for bridge piers. Coffer dams are applicable in shallow depths only; hence one objection tc founding bridge piers by this process, particularly in rivers subject to scour or liable to ice gorges, is the danger of their being either un dermined or pushed off the foundation. When founded in mud oz sand, the first mode of failure is most to be feared. This danger is diminished by the use of piles or large quantities of riprap; but such a foundation needs constant attention. When founded on rock, there is a possibility of the piers being pushed off the foundation; for, since it is not probable that the coffer-dam can be pumped per fectly dry and the bottom be thoroughly cleaned before laying the masonry or depositing the concrete, there is no certainty that there is good union between the base of the pier and the bed-rock.
Coffer-dams are frequently and advantageously employed in laying foundations in soft soils not under water, as described in