224 TOWERS AND TANKS FOR WATER-WORKS. 



upon the tops of the stringers and over the concrete de- 

 posited between them. 



Quicksand. When sand is so completely saturated as to 

 become fluid, it is termed "quicksand"; it has no peculiar 

 qualities or inherent properties, but is generally given an 

 individual classification. 



Any saturated sand is "quick" when the upward pressures 

 of the underground waters are sufficient to overcome the 

 tendency of gravity to keep its particles at rest. Sand of 

 coarse grains resists this upward tendency to a greater extent 

 than the finer varieties; hence quicksand is usually a very 

 fine grained sand, and from the fact that it must be found 

 immersed in water, the constant friction of its particles 

 moving upon each other grinds the sharp points and angles, 

 until the grain becomes rounded or "water-worn," the usual 

 condition of the grains of the so-called quicksand. 



Increasing Bearing Values. In very soft material, where 

 the necessity of reen forcing the bearing-value of the soil is 

 apparent, and where there exists an underlying soil of better 

 material, the piles, when driven through the top soil, pene- 

 trating into the strata below, act as so many columns whose 

 ultimate bearing is the crushing strength of the material of 

 which the pile consists, but where there is no such lower soil 

 the piles are supported in the soft material only by the friction 

 of that material against their sides, and the determination of 

 their safe bearing- value is more problematical. Rankine gives 

 as a rule for the safe bearing of piles under this last condition 

 the area of the head of the pile in inches by 200; thus a 12- 

 in. pile, having an area of head of 78 sq. in., would give a 

 safe bearing of 7.8 tons. 



A simple rule frequently used for the safe bearing value 

 of piles is one formulated by Major Sanders, of the U. S. 

 Engineer Corps, from experiments made with common 

 wooden piles at Ft. Delaware, and is as follows : 



