FOUNDATIONS.
In considering the elements of such buildings, I wish to devote a few words to the question of foundations, because in the excessive loads imposed by this class of buildings, and in the frequent necessity of constructing them upon sites where alluvial drift or quicksands form compressible foundations, there is afforded an opportunity for the widest range of engineering skill in dealing with the problem. In such instances, a settling of the building must be foreseen and provided for, in order that it may be uniform under the whole structure. This is generally accomplished by means of independent foundations under the various points of pressure, arranged so as to give a uniform intensity of pressure upon all parts of the foundation. It is considered important to limit the load upon such foundations to two tons a square foot, although loads frequently exceed this amount.
There is a large building in New York City which has recently been reconstructed, and the foundations rearranged, where the load reached to the enormous amount of six to ten tons per square foot. It was a frequent occurrence in the class of high mills spoken of to impose loads of so much greater intensity upon the wall foundation than upon the piers under the columns of the mill, that the floors became much lower at the walls than at the middle.
The stone for such foundations should be laid in cement rather than in mortar, not merely because cement offers so much greater resistance to crushing, but because its setting is due to chemical changes occurring simultaneously throughout the mass. The hardening of mortar, on the other hand, is due to the drying out of the water mechanically contained with it, and its final setting is caused by the action of the carbonic acid gas in the air.
Although quicksands are never to be desired, yet they will sustain heavy loads if suitably confined. When inclined rock strata are met with, all horizontal components of stress should be removed by cutting steps so that the foundation stones shall lie upon horizontal beds.
Foundations are frequently impaired by the slow, insidious action of springs or of water percolating from the canal which supplies the water power for the mill; and the proper diversion of such streams should be carefully provided for.
In the question of foundations, there is much of a general nature which is applicable to all structures; but, at the same time, each case requires independent consideration of the circumstances involved.