The custom-house at New Orleans, La., is founded upon it plank floor laid '7 ft. lower than the street pavement. A timber platform lies next, consisting of logs 12 in. in diameter laid side by side, and crossed by other logs leaving spaces of 2 or 3 ft. between them; the spaces are filled with concrete, and the platform is covered with a foot of concrete; the walls rest upon inverted arches, the whole area beneath the build ing, about 300 ft. square, being thus utilized in supporting the structure above. During the first three years, the building sank an average of 19.1 in.; after sixteen years, the dif ference in the levels of the walls was 3 inches. Foundations are often laid below the surface of water by means of a coffer-dam. To form a coffer-dam a row of piles is driven inclosing thespace which the foundation is to occupy, and their tops are bound together by a continuous cap of heavy timber. A parallel row of planks, called sheet piles, is driven with their edges in contact if possible, and the tops are firmly bound to the frame first formed. A second row of large piles, capped, and faced with sheet piles like the first, is driven so ttS to leave a space of 5 to 20 ft. between the two linings of sheet piles, which face each other. This space is filled with layers of clay thoroughly packed and paddled together, forming a strong dam quite impervious to water. The water inclosed by the dam is pumped away, and the ground is open for the reception of a foundation. It sometimes happens that the bed of the stream is .rock, covered with a stratum of mud or earth too thin to support piles, and too porous for sustaining acoffer dam. A caisson has sometimes been used in such cases. This is a chest, or water tight box, large enough to receive the intended foundation. The lower courses may be laid in the box, the whole floated to its proper place, and there sunk. The bed of the stream should be nearly level for the reception of the chest; or the mud may be removed by dredging and the caisson be lowered into the cavity so formed; the water will then be likely to wash about the chest more mud or gravel, which will assist in keeping it in place. The water may then be removed from the interior of the caisson and the foundation be A method of laying foundations in deep water adopted in late years is called the " pneumatic ;" the manner of its application is either that of a " vacuum" or a " plenum," according as the pressure of the air within is below or above the usual pressure of the atmosphere. In either case, an iron cylinder, usually constructed in sections, is lowered into the water until its lower end rests on the bottom, while its upper end extends above the Surface. If the vacuum process be used, the cylinder is capped, and an air-pump reduces the pressure of the air within. The weight of the tube with the atmospheric pressure on its head, pushes it into the ground, while the water pressing in below the lower end stirs the earth and assists the descent. When descent stops, the air-pump may he reversed, and the water in the pipe will be slowly driven through the earth; a sudden release of the inner pressure will cause a second influx of water, a disturbance of the soil, and a farther descent of the tube. If the earth contains boulders or buried timbers, the movement of the tube may be stopped before reaching the depth desired by the engineer; or he may wish to remove the interior earth and replace it with masonry, even where the ground is too gravelly to keep out the water. In this case an air-lock is placed upon the top of the tube, air is forced into the interior, driving out the water, and workmen arc employed within to excavate the earth, and afterward to lay the masonry. The air-lock is a chamber which serves as a vestibule to the interior, and permits the maintenance of a nearly constant air-pressure within. A man enters the air-lock and closes the door behind him; he then opens communication with the interior of the tube, and when the pressure of air is equalized in the two spaces he passes within. In the Harlem bridge at New York the tubes of the pier were 6 ft. in diameter, in sections 10 ft. long; the air-lock was 6 ft. long; the man-boles were 20 in. in diameter. Compressed air was stored in a reservoir upon shore, and was communicated to the tube by a flexible pipe; the flow of air was regulated by a stop-cock. Stop-cocks permitted the discharge of the air into the water at times, thus assisting the tube to sink into the earth. It was found farther that the air-pressure would force sand from the bottom of the tube up through a pipe above the surface of the water, greatly facilitating the labor of excavation and lifting. The tube, while sinking, is liable to incline from. the vertical; this has been opposed by placing wedges under the too-rapidly sinking edge; by boring holes on the upper or retarded side of the tubes, that the issuing air may disturb the earth, and permit a more rapid movement of the tube; the most effective remedy has been the beating of the upper end of the tube with a heavy rain; the jar seemed to loosen the soil. The pneumatic caisson is a further development of the
pneumatic process. In the method described, the tube is first placed, and the interior masonry afterwards inserted; but when the caisson ig used the masonry is built upward while the pier is stink downward, and the weight serves to force the whole into the soil. The lower part is a structure of iron having walls and a roof; the walls are strong enough to sustain the lateral pressure of the water and earth, and the roof is able to carry the load of masonry which may be piled upon it. The lOwer edge of the wall is ao thin that it may be easily forced into the soil. From the lower portion tubes rise to form a communication between the caisson and the air-chambers above the water, and to afford a passage for the workmen and for raising material. The air-locks may be at the tops of the tubes, but better practice places them at the bottom, just above the caisson. The foundations of the Tay bridge, celebrated first for the splendid engineer ing achievements of its construction, and later for the utter destruction of its fall, were laid by this method. Each pier consisted of two columns of masonry, so joined at the bottom as to form one large compartment under the pier. At first single columns were sunk separately, but their bases were too narrow, and they were overturned before they were finished. The lowest chamber of the caisson was 22 ft. 7 in. long, 10 ft. 6 in. wide, and 3 ft. high. It was surmounted by a conical frame 5 ft. high, partially closed at the top by a flange 2f ft. wide, upon which the masonry rested. The bodies of the cylinders were of cast-iron, / of an inch thick, 9f ft. in diameter, and in sections about 4 ft. high. The lowest section was so formed that the two columns were joined by masonry arched over the central part of the caisson. A space of about 2 in. wide, left between the masonry and the iron, was afterward filled with concrete. After the pier was lowered to a permanent position the lower chamber was filled with concrete, and finally the cylindrical interior passage was filled. The piers were partly built near the shore, being supported by pontoons, and at a proper stage of the tide were floated into place, and carefully lowered by the aid of hydraulic power.
Each of the piers and abutments of the bridge over the Mississippi at St. Louis was built in a large caisson, having one large air-space in the base, where the workmen excavated the sand. The base compartment was 9 ft. high, the sides being, for the large pier, / of an inch thick, f in. in the smaller. Massive cross partitions of timber were built to sustain the roof of the chamber upon which the masonry was placed. There were three shafts, each connected with the air chamber by an air-lock placed below the masonry, where it would not have to be moved as the sinking proceeded. The support given by the timbers which rested on the bottom, the friction on the sides of the caisson, and the buoyancy of the air were the means relied on to sustain the pier in its descent to the bed-rock. When the rock was reached it was 110 ft. below the surface of the water. Doubts had been raised whether the workmen could endure the increase of ordinary atmospheric pressure to 41 atmospheres, but danger was avoided by frequent changes; so that men were not kept in the compressed air for more than an hour at one time. Exposure to intense pressure for several hours produces paralysis, and in some instances death. All combustible articles burn vigorously in the compressed air, even woolen cloth being extinguished with difficulty. The lamps were inclosed in very strong glass cases, which communicated with the open air, and allowed the combustion to act under ordinary pressure. When the rock was uncovered its depressions were filled with concrete, continued up the sides of the caisson to prevent the water from washing it; the central mass was filled nearly full of wet sand, and concrete was rammed between the sand and the roof. The sand was used to avoid cost, and was supposed to be as good in its place as cement. , The foundations of the piers of the East river bridge, to connect New York and Brooklyn, were built upon caissons and sunk by the pneumatic process. The Brooklyn caisson is 168 ft. by 102 ft. ; the New York caisson, 172 by 102. The Brooklyn pier rests on a firm subsoil at the depth of 50 ft, below the surface of the water; the New York pier rests on a compact layer 2 or 3 ft. above the bed-rock at the depth of 78 feet.