A Chapter on Foundations. 283 



4th. Next to sand, impervious clay may be in order of resist- 

 ance to pressure. This is a very common earth which yields 

 only to the pick, is not plastic and does not become so from effects 

 of moisture. A solid rock is scarcely more reliable against 

 pressure than this clay. 



5th. We now come to the less resisting soils among which the 

 first are the plastic clays. These earths give good resistance when 

 dry but undergo a soaking process from the effects of moisture 

 and becoming plastic they jield to a considerable degree and 

 many fine buildings have unaccountably failed a few years after 

 their erection, while the fact was, that their very presence con- 

 ducted the moisture to the bed of their foundations and became 

 the means of their own destruction. 



6th. And last brings us to the treacherous yielding alluvial 

 beds among which the engineer is obliged to flounder with uncer- 

 tainty in seeking a solution of the problem of stability. 



Having touched upon the various earths and soils which come 

 into practice, the problem is, how to construct with safety and 

 economy under varying circumstances. 



As before remarked, with a rock bed it is only necessary to 

 prepare the bed to receive the desired structure. 



With the second class, i. e., gravel bed, it is necessary to extend 

 the base beyond the thickness of the wall to guard against lateral 

 motion ; as a rule an increass of about one-half the thickness of 

 the wall will give a sufficient base. In some ordinary founda- 

 tions in gravel, trenches are dug to the desired depth and filled 

 with irregular fragments of stone and then grouted with thin 

 mortar of lime or cement, in many cases the dry stone foundation 

 alone is relied upon. The foundation of Fort Hamilton, New 

 York Harbor, a granite battery of two tier of guns, is built of dry 

 stone. The Fort has stood about fifty years with no sign of fail- 

 . ure. The ground is gravel and hard clay. In case of bridge 

 piers or abutments it is of course necessary to protect the bed 

 from erosion by the force of ice gorges or freshets. This portion 

 of the subject will be treated further on. 



In the case of sand for foundations, I have presumed to place 

 the same next in order to gravel. As before stated, sand is equal 



