DRED3ING THROUGH WELLS.
A timber crib is frequently sunk by excavating the material through compartments left for that purpose, thus undermining the crib and causing it to sink. Holbw iron cylinders or wells of masonry with a strong curb, or ring, of timber or iron beneath them are sunk in the same way.
This method is applicable to foundations both on dry land and under water. It is also sometimes employed in sinking shafts in tunneling and mining.
The advantage of this method for foundations under water is that it is applicable to greater depths than any other method except the freezing process; and the disadvantage is that the descent of the crib or cylinder is liable to be stopped by logs, bowlders, etc.
The soil is removed from under the crib with a clam-shell or an orange-peel dredge, or with an endless chain and bucket dredge, or with the Eads pump (I 877).
The clam-shell dredge consists of the two halves of a hemispherical shell, which rotate about a horizontal diameter; the edges of the shell are forced into the soil by the weight of the machine itself, and the pull upon the chain to raise the excavator draws the two halves together, thus forming a hemispherical bucket which incloses the material to be excavated. A similar device consists of two quad rants of a short cylinder, hinged and operated similarly to the above. The orange-peel dredge (shown at A in Fig. 90, page 424) appears to have the preference for this kind of work. It consists of a frame from which are suspended a number of spherical triangular spades which are forced vertically into the ground by their own weight. The pull upon the excavator to lift it out of the mud draws these triangles together and incloses the earth to be excavated.
- - In one case in France, the soil was excavated by the aid of compressed air. An 8-inch iron tube rested on the bottom, with its top projecting horizontally above the water; and compressed air was discharged through a small pipe into the lower end of the 8-inch tube. The weight of the air and water in the tube was less than an equal height of the water outside; and hence the water in the tube was projected from the top, and carried with it a portion of the mud, sand, etc. Pebbles and stones of considerable size were thus thrown out. See f 876 for another use of a similar device.
There are four river piers. The crib for the largest is 100 feet long, 60 feet wide at the bottom and 40 feet at the top, and 104 feet high. It is divided, by one longitudinal and six transverse walls, into fourteen compartments through which the dredge worked. The side and division walls terminate at the bottom with a 12- by 12-inch oak stick, which served as a cutting edge. The exterior walls and the longitudinal division wall were built solid, of triangular cross section, for 20 feet above the cutting edge, and above that they were hollow. The gravel used to sink the crib was deposited in these hollow walls. The longitudinal walls were securely tied to each other by the end and cross division walls, and each course of timber was fastened to the one below by 450 1-inch drift-bolts 30 inches long. The timber was hemlock, 12 inches square. The four teen compartments in which the clam-shell dredges worked were 10 by 12 feet in the clear. The cribs were kept level while sinking by excavating from first one and then the other of the compartments. Gravel was added to the pockets as the crib sunk. When hard bottom was reached, the dredging pockets were filled with concrete deposited under water from boxes holding one cubic yard each and opened at the bottom by a latch and trip-line.
After the crib was in position, the masonry was started in a floating caisson which finally rested upon the top of the crib. Sink ing the crib and caisson separately was a departure from the ordinary method. Instead of using a floating caisson, it is generally con sidered better to construct a coffer-dam on top of the crib, in which to start the masonry. If the crib is sunk first, the stones which are thrown into the pockets to sink it are likely to be left projecting above the top of the crib and thus prevent the caisson from coming to a full and fair bearing.