Compressed Air Work And Diving 1909

BUOYANCY AND $KIN FRICTION 41 the buoyancy of the caisson and the skin friction to over- come. In open water-bearing strata the buoyancy of the caisson will be obtained from the amount of water which woulcl be displaced by the caisson measured from high water level, the bottom of the chamber being taken as closed in, which it actually is by the pressure of the air. If the caisson has vertical sides, the buoyancy, or upward reaction, can be obtained by multiplying the area it stands upon, or cross-sectional area, by the head of water at the level of the cutting edge. In actual practice there are, however, many modifica- tions of these rules. In salt water, as at Barmouth Bridge, described in the next chapter, a fresh water spring may necessitate the air pressure being raised to a point much higher than that requirecl by the head of salt water in order to keep the water from rising in the working chamber. It very often happens, on the other hånd, that the working chamber can be kept quite clry by a pressure of air lower than that due to the head of water measured from water level to cutting edge. It is rather difficult to determine beforehand what the skin friction will amount to, but it is usually from Libout 2^ to 6 cwt. per sq. ft. To overcome skin friction and upward reaction of air, or buoyancy, it will be necessary in almost every case to add to the weight of the caisson itself. With large caissons such weight is usually addecl in the form of concrete which is put in Libove the roof of the working chamber, which will have consequently to be made strong enough to bear it. When the caisson has reached the bottom intended for it, the working chamber and shafts will ^llso be fillecl up with concrete, so that the whole will become a huge monolith which will form the Foundation