FO UNO AT10NS. 221 



Frequently the stone is not found in horizontal, continu- 

 ous layers, but in seamy strata, offering a bearing-surface of 

 more or less irregularity and composition. For the suitable 

 preparation of such a subfoundation the overlying earthy 

 matter and any decomposed or decayed stone must be re- 

 moved to " bed-rock" or the solid layer, which is then blasted 

 or sledged to a surface as nearly perpendicular to the pressure 

 to be imposed as possible. Interstices or fissures of the rock 

 should be filled with broken stone or concrete, and where the 

 bearing will not be entirely upon stone, but upon contiguous 

 earth, at such junction especial care should be taken to thor- 

 oughly compact the softer material or to remove it altogether, 

 substituting broken stone, or preferably concrete, bedded as 

 well as possible to the more unyielding natural stone by cut- 

 ting the bed-stone in steps, or making some other effective 

 union ; otherwise unequal settlement, the result of unequal 

 resistance, will result. 



Clay. Clay, when dry and likely to remain so, is an ordi- 

 nary and excellent foundation, being easily excavated and 

 having a safe bearing- value for ordinary structures; but clay 

 is a treacherous material in that it so readily absorbs moist- 

 ure, its seamy veins acting often as conduits for underground 

 streams of varying magnitude. When clay absorbs water, its 

 tendency is to swell and soften, and under such conditions, 

 when confined, it exerts a material pressure upon the sides or 

 bottoms of foundations, tending to bulge and crack them. 

 When unconfined it spreads in every direction, oozing and 

 squeezing from under the weight imposed and becoming un- 

 stable and uncertain in action. Exposed to the moisture of 

 the air it becomes more or less saturated, and at low tm- 

 peratures the mass freezes, expands, and disintegrates after a 

 thaw, proving a most intractable material. From this fact, 

 in preparing the subfoundations in such material, the exca- 

 vations should extend well below the frost-line, and the ex- 



