Polytechnic Association Proceedings. 859 



idea has been to obtain a foundation which will afford an equal 

 support to all parts of the structure. This is almost as important 

 as to obtain an unyielding support; indeed, a small degree of 

 settlement occurs in all foundations, and even in the superstructure 

 itself, when the walls are very high, or carry very heavy loads. 

 The finest cut-stone facing yields slightly, and the coarser masonry 

 of the interior, and that forming the foundation, considerably. 



When the natural material below the foundation is not firm 

 enough to sustain the structure, or is liable to be scoured away, or 

 to become softened irregularly, and it is not deemed advisable to 

 sink it below these influences, resort is had to bearing piles. 



Before commencing the discussion of this branch of the subject, 

 it may be worth while to allude to an erroneous popular idea, that 

 the support derived from piles is obtained by driving them until 

 they reach a hard substratum. From what has been already stated 

 in regard to the weight which earths will support, it will be seen 

 that a pile, sharpened at the point, will not have more than one- 

 fifth of a square foot of area resting upon such hard substratum, 

 and as these piles are rarel}'^ driven nearer than three feet from 

 center to center, the earth below must be firm enough to sustain 

 nearly fifty times as much weight as we have assumed upon the 

 ordinary foundations. No kinds of earth are able to sustain such 

 pressures without yielding. 



If the material between the foundation and the hard stratum 

 below is very soft and allows the piles to yield laterally, they 

 should be considered simply as columns of support, and if very 

 long will be bent and broken, or will sway off laterally and allow 

 the structure to fall. These consequences do not often follow, and 

 hence we may assume that all except the softest materials does 

 give considerable support to the piles from the cohesion of the 

 sides, and as will be presently shown, gives in most cases nearly 

 the whole sustaining power which is derived from them. 



There are cases, however, where piles passing first through very 

 soft material and driven some distance into a harder substratum 

 derive support from the latter; but in this case, it is again emphati- 

 cally repeated, that the support is mainly derived from the adhesion 

 of the adjacent material to the exterior surface, and not from its 

 its sectional area at the bottom. 



Piles are sometimes used to compress a soil, which is not quite 

 compact enough, in its natural condition, to sustain the structure, 

 which they do to the extent of their o^vn bulk, compared with the 



