SKIN FRICTION OF CYLINDERS. 



387 



information for constructive purposes, the data so obtained 

 applies only to a limited area immediately surrounding the hole, 

 therefore the positions should be systematically arranged at 

 moderate distances apart, and the holes carried down to such a 

 depth as to enable as accurate an opinion as possible being 

 formed as to the nature of the strata, the probability of water 

 being met with, and as to the sufficiency of the bed on which it 

 is proposed to place the foundations. As regards the latter 

 condition particularly, it is important, in order to obtain reliable 

 information on which to form an opinion as to the supporting 

 power of the bed, that proper boring appliances be used, so 

 as to recover specimens of the strata in as natural a state as 

 possible. 



As a rule, the penetration of an impervious foundation bed, 

 and the possible tapping of a water-bearing strata beneath, 

 should be avoided, provided, of course, that there proves to be a 

 sufficient thickness of impervious strata to ensure a good 

 foundation. 



When it is necessary to ascertain the thickness of an under- 

 lying bed, the bore-holes should be put down clear of the 

 foundations, and, if possible, in positions, where, in the event of a 

 water-bearing strata being tapped, the flow of water can best be 

 kept under control. 



Friction. The support afforded to cylinder foundations by 

 surface friction will depend upon the nature of the strata and the 

 depth to which the cylinder is sunk. 



For cast-iron cylinders sunk through gravel, the skin or 

 surface friction has been estimated at between 3 and 4 tons on 

 the square yard for small depths, and at 4 to 5 tons per square 

 yard when the depth is from 20 to 30 feet. 



In sinking the brick- and-concrete cylinders in the silt of the 

 Clyde, it was found to be about 3i tons per square yard. 1 



In constructing the cylinder foundations for the Prague- 

 Smichow Bridge, it was observed that by increasing the air 

 pressure the cylinders could be raised, and it was also noted that 

 when the internal pressure in the chamber exceeded by 7J Ibs. 

 per square inch the atmospheric pressure, the cylinder could be 

 maintained in suspension. These observations gave a ready 

 means of determining approximately the amount of surface 

 friction developed, for the total weight of the caisson was 

 1 H.P.1.C.K, voL 1. p. 131. 



