Polytechnic Association Proceedings. 857 



lesistance ■which it meets with is less than that due to the pressure 

 of the load, or it will, iu shallow pits, cause an uplift of the adja- 

 cent material. This operation is familiarly exhibited, when an 

 embankment of hard earth is placed upon a marsh where the sub- 

 stratum is in a semi-fluid state. The embankment at first sinks by 

 the compression of the soil beneath, and as the weight is increased, 

 the material below is forced out laterally, and forces up the super- 

 incumbent earth. I have noticed an elevation of several feet in 

 height, extending a hundred feet or more beyond the banks, pro- 

 duced in this manner, even when the marsh was covered with trees 

 of large size. 



The weight of loaded bridges may be assumed as producing 

 from four to eight tons per square foot of the area of piers, and 

 of large buildings it is frequently twice as much, and hence the 

 foundations are generally expanded to at least twice this area, when 

 the structure rests upon moderately firm soil, not liable to displace- 

 ment; but the degree of expansion of the base depends upon the 

 foregoing conditions, and must sometimes be greatly increased 

 beyond that named. 



The present State Engineer carried a railroad bridge across the 

 Montezuma marshes upon this principle of the expansion of the base, 

 carried to an extent that I have never before met with. The load 

 on each pier wa^ about one hundred and thirty tons, and was dis- 

 tributed over eight hundred and seventy-four square feet of the 

 marsh by a grillege of timber, giving about three hundred pounds 

 per square foot. The work has been standing eighteen years with- 

 out yielding. 



In these expansions of the foundations, care must be taken that 

 the slope or projections have sufficient vertical strength to safely 

 carry the weight which will be brought upon them. A large pro- 

 portion of the whole weight is thrown upon these projections, by 

 the common method of making the front stone and joints closer 

 than those of the interior. In bridge piers the walls are ijattered 

 one-twelfth to one twenty-fourth with some of the bottom courses 

 projecting in oflT-sets of six to twelve inches, depending upon the 

 thickness and strength of the stone. 



In cases where the foundations are always wet, and not subject 

 to deca}--, timber forms a convenient and inexpensive method of 

 extending the support a few feet beyond the exterior lines of the 

 masonry, and where there is an ultimate decay anticipated, the tim- 



