88. Formerly, caissons were used exclusively for the foundations of bridge piers, but the advent of steel skeleton construction, and consequent erection of very lofty buildings, have caused caissons to be used for building foundations.

89. A caisson is a chamber of iron or wood that is used in the construction of deep foundations.

There are two different methods of reaching the required foundation.

First, when the desired depth is reached by excavating the material from the interior of a large timber or iron box, or cylinder, strongly constructed for the purpose, and then forcing the structure to sink against the exterior friction on its sides, by sufficient weights or loads placed in the caisson, until the required depth is reached. Structures of this kind are called open caissons or open cribs.

The second method is used when the foundations are to be carried to a great depth in very soft material or in water. The lower part of the caisson is then formed into an air chamber resting on the soil at the bottom. Air is pumped into this at a pressure corresponding to the depth below the surface, and the excavation is carried on by men working at the bottom, as in a large diving bell. Such a structure is called a pneumatic caisson.

After the crib or caisson is sunk to the proper level, either to rock or compact gravel, the inside space is filled with concrete; on this the piers or foundations are built, if the open caisson is used; and on top of the caisson, if the pneumatic method is used.

90. The work of sinking the cribs or caissons, is outside of the architect's province, and is usually entrusted to some firm of engineering contractors who make a specialty of work of this description, and who leave everything in readiness to construct the foundation walls or piers.

inverted arches.

91. When a front or other wall is composed of isolated piers, it is well to combine all their footings into one, and to step the piers down, as shown in Fig. 29, in which a shows the concrete footing course, b, the stepped-up foundations of the piers, and c, the piers resting on the footings.

92. When there is not sufficient depth for this purpose, use is sometimes made of inverted arches; these are to be avoided unless the foundation wall is from necessity very shallow, as great care is required to lay them properly, the slightest settlement in them having a disastrous effect upon the piers.

Cribs For Caissons 31

Fig. 29.

The end arch of the building must have sufficient pier or other support; otherwise it may throw the pier out, as shown by the dotted lines at a, a in Fig. 30.

Cribs For Caissons 32

Fig. 30.

This difficulty is overcome by the use of an iron rod, with iron plates and nuts, as shown in Figs. 31 and 32, thus securing the skewbacks in place.

93. The New York building law, in referring to inverted arches, provides as follows:

"If in place of a continuous wall, isolated piers are to be built to support the superstructure (where the nature of the ground and the character of the building make it necessary), inverted arches shall be turned between the piers, at least 12 inches thick, of the full width of the piers and resting upon a continuous bed of concrete of proper area, and at least 18 inches in thickness; or two footing courses of large stone may be used, the bottom course to be laid edge to edge and the top course laid end to end; or one course of concrete and one course of stone.

"The stones shall not be less than 10 inches thick in each course; the concrete shall not be less than 18 inches thick; the area of the lower course shall be equal to the area of the base course that would be required under a continuous wall, and the outside piers shall be secured to the second piers with suitable iron rods and plates."

94. Fig. 31 (a) represents two piers connected by a brick and concrete inverted arch, each pier being 3 feet square.

Cribs For Caissons 33

Fig. 31.

At a is shown the 18 inches of concrete under the 12 inches of brickwork b. At c, c' are shown the stone skewbacks from which the brick arches spring, and d is the 2-inch iron rod tying the end pier e' to the second pier e, and thus preventing the thrusting out of the end pier. At (b) is shown the inverted arch in section.

Fig. 32 shows an inverted arch built of stone 24 inches thick. At (a) is shown the stone arch a, maintained in position by the iron tie-rod b, and at c, the brick foundation piers are shown upon the skewbacks d. At (b) is shown a section of the arch a, an end view of the tie-rod b, and elevation of the pier c.

Cribs For Caissons 34

Fig. 32.

95. The best form of inverted arch is the three-centered or elliptical, (as shown in Fig. 33); next, the pointed; third, the circular; and lastly, the segmental arch.

Fig. 33 represents the method of getting the lines for the centering in an elliptical arch. Divide the space shown on the line from a to b into three equal parts, then draw the three circles c, c, c so that the circumferences of these circles will be tangent at d, d. Then carry the perpendicular line e-f through the center of the middle circle; the point / where it intersects the circumference of the circle, gives the center of the arch from g to h. From f draw lines through d, d to g and h. The intersection of these lines at d, d gives the centers of the arch from g to a and from h to b. At k is shown the 12 inches of brick in the arch, and at / the concrete under it. This form of arch is used frequently in the construction of sewers.

Cribs For Caissons 35

Fig. 33.