Dred3ing Through Wells

The largest crib was sunk through about 53 feet of water, 20 feet of mud, 45 feet of clay and sand, and 17 feet of sand and gravel. It rests, at 134 feet below high water, upon a bed of gravel 16 feet thick, overlying bed-rock. The timber work is 110 feet high, includ ing the floor of the caisson, and extends to 14 feet below high water (7 feet below low water), at which point the masonry commences and rises 39 feet. On top of the masonry a steel tower 100 feet high is erected. The masonry in plan is 25 by 87 feet, and has nearly vertical faces. The lower chord of the channel span is 130 feet and the rail is 212 feet above high water.

The other piers are nearly as large as the one here described. The cribs each contain an average of 2,500,000 feet, board measure, of timber and 350 tons of wrought iron.

Atchafalaya Bridge.

This bridge is over the Atchafalaya bayou or river, at Morgan City, La., about 80 miles west of New Orleans. The soil is alluvial to an unknown depth, and is subject to rapid and extensive scour; and no stone suitable for piers could be found within reasonable distance. Hence iron cylinders were adopted. They are foundation and pier combined. The cylinders were sunk 120 feet below high water—from 70 to 115 feet below the mud line—by dredging the material from the inside with an orange peel excavator. Fig. 90 shows the excavator, A, and the appliances for handling the cylinders.

The cylinders are 8 feet in outside diameter. Below the level of the river bed, they are made of cast iron 1} inches thick, in lengths of 101 feet, the sections being bolted together through inside flanges with 1-inch bolts spaced 5 inches apart. Above the river bottom, the cylinders are made of wrought-iron plates inches thick, riveted together to form short cylindrical sections with angle-iron flanges. The bolts and spacing to unite the sections are the same as in the cast-iron portions.

The cylinders were filled with concrete and capped with a heavy plate. Two such cylinders, braced together, form the pier between two 250-foot spans of a railroad bridge.

The only objection to such piers relates to their stability. These have stood satisfactorily since 1883.

Hawkesbury Bridge.

The bridge over the Hawkesbury River in south-eastern Australia is remarkable for the depth of the foundation. It is founded upon elliptical iron caissons 48 by 20 feet at the cutting edge, which rest upon a bed of hard gravel 126 feet below the river bed, 185 feet below high water, and 227 feet below the track on the bridge. The soil penetrated was mud and sand.

The caissons were sunk by dredging through three tubes, 8 feet in diameter, terminating in bell-mouthed extensions, which met the cutting edge. The spaces between the dredging tubes and the outer shell were filled with gravel as the sinking progressed. The caissons were filled to low water with concrete, and above with cut-stone masonry.

As these caissons were to be sunk to an unprecedented depth, it was considered wise to construct them with a flare at the bottom, that is, to make the bottom larger than the upper portion, so as to decrease the resistance due to friction. Experience showed that making the bottom larger was a mistake, since it seriously increased the difficulty of guiding the caisson in its descent.

Brick Cylinders.

In Germany a brick cylinder was sunk 256 feet for a coal shaft. A cylinder 251 feet in diameter was sunk 76 feet through sand and gravel, when the frictional resistance became so great that it could be sunk no farther. An interior cylinder, 15 feet in diameter, was then started in the bottom of the larger one, and sunk 180 feet further through running quicksand. The soil was removed without exhausting the water.

A brick cylinder—outer diameter 46 feet, thickness of wall 3 feet—was sunk 40 feet in dry sand and gravel without any difficulty. It was built 18 feet high (on a wooden curb 21 inches thick), and weighed 300 tons before the sinking was begun. The interior earth was excavated slowly, so that the sinking was about 1 foot per day, —the walls being built up as it sank.

In Europe and India masonry bridge piers are sometimes sunk by this process, a sufficient number of vertical openings being left through which the material is brought up. It is generally a tedious and slow operation. To lessen the friction a ring of masonry is sometimes built inside of a thin iron shell. The last was the method employed in putting down the foundations for the present Tay bridge.* Valves from Experiments. The friction between cylinders and the soil depends upon the nature of the soil, the depth sunk, and the method used in sinking. If the cylinder is sunk by either of the pneumatic processes (§ 859 and 860), the flow of the water or the air along the sides of the tube greatly minishes the friction. It is impossible to give any very definite data. Table 66 gives the values of the coefficient of friction for materials and surfaces which are likely to occur in sinking foundations for bridge piers. Each result is the average of at least ten experiments.