Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/370

Rh SUBSTRUCTURE. applied to a mere frame with wooden walls which is floated to the site of the pier, and there sunk so as to form an enclosure, inside which concrete can be shot and can set undisturbed by the wash of the water. Concrete in a shell is a name which might be applied to all the methods of founding a pier which depend on the very valuable property which strong hydraulic concrete possesses of setting into a solid mass under water. The required space is enclosed by a wooden or iron shell; the soil inside the shell is removed by dredging, or some form of mechanical excavator, until the formation is reached which is to support the pier ; the concrete is then shot into the enclosed space from a height of about 10 feet, and rammed down in layers about 1 foot thick ; it soon con solidates into a permanent artificial stone. Tho shell, 94o. unless of small size or very strong form, requires to be braced to meet the outward pressure of the concrete. The Fia. 95. Caisson used for a Bridge over the Cher. concrete used for this purpose is often called be&quot;ton, to distinguish it from inferior mixtures used for foundations on land. It may consist of angular stones 1 to 2 inches diameter, mixed with strong hy draulic mortar in the proportion of from one to two volumes of stones with one of mortar; the final volume may be from 4 to I- of the un- Fig. mixed materials. Bdton used at Biarritz consisted of one part Portland cement, two parts sand, and three parts broken stones; at Genoa one part rich lime, two parts pozzuolana, three parts broken stones. Fig. 94 shows a section of foundations constructed by nuing a casing of piles with concrete in this manner; the is protected against scour by large stones heaped round e outside, part of the loose earth having been removed oy digging. Fig. 94a shows the manner in which the concrete foundation was finished after removal of the temporary external wall shown in fig. 94. Figs. 95 and 95&amp;lt;z show a longitudinal elevation and cross section of a wooden shell, or caisson without a bottom, intended to be sunk to receive concrete. Cast-iron guide piles and sheet piling are also used for this class of foundation. Cast-iron tubular shells are now frequently employed ; the tube forms a large hollow pile, which may descend into the ground by its own weight, or by added weight while the soil inside is removed by some kind of dredge or excavator, such as Milroy s, worked from the surface. When the lower edge of the shell has penetrated the formation to be used as a foundation, the water inside may be pumped out if the soil forms a water-tight joint, or the shell may simply be filled up with concrete shot into the enclosed water. The piers of Char ing Cross bridge (fig. 9G) were con structed in this way; the excavation _ inside the tube (14 feet diameter), was carried on by divers with helmets until the shell had entered a few feet into the London clay. The water was then pumped out and excavation continued ; the cylinders were loaded with about 150 tons to sink them to the final depth. Compressed air is now very gene rally employed inside a metal shell for those foundations in which the excavation requires the presence of workmen at the bottom of the shell, The metal shell is open at the bottom, but air-tight and water-tight at all other points ; there is a chamber called an air-lock at the upper part. This &quot; air-lock&quot; serves for the exit and entrance of the workmen and materials ; the air in this compara tively small space is lowered to the pressure of the atmosphere before the chamber is opened for the passage of men or materials to the open air ; the air is again compressed in the air-lock before it is opened for communication with the body of the shell in which the air is permanently kept at such a pressure as w,ill keep the water down to the required level. The shell thus acts as a diving bell acts. It is found that men cannot in general be safely employed under a greater pressure than two atmo spheres above the ordinary atmospheric pressure, cor responding to a depth in water of about 65 feet. The centre pier of Saltash Bridge was, however, in 1855 by this plan carried down to a depth of 87 feet 6 inches below high water. Recently the foundations of St Louis bridge over the Mississippi have by the same method been established at a depth of FIG. 96.- -Cylinder, Charing Bridge. Cross