n setting fixes the tubbing firmly in position.
With increase in depth, however, the thickness and weight of the cast
iron tubbing in a large shaft become almost unmanageable; in one
instance, at a depth of 1215 ft., the bottom rings in a shaft 14-1/2 ft.
in diameter are about 4 in. thick, which is about the limit for sound
castings. It has therefore been proposed, for greater depths, to put
four columns of tubbings of smaller diameters, 8-1/2 and 5-1/2 ft., in
the shaft, and fill up the remainder of the boring with concrete, so
that with thinner and lighter castings a greater depth may be reached.
This, however, has not as yet been tried. Another extremely useful
method of sinking through water-bearing ground, introduced by Messrs A.
& H. T. Poetsch in 1883, and originally applied to shafts passing
through quicksands above brown coal seams, has been applied with
advantage in opening new pits through the secondary and tertiary strata
above the coal measures in the north of France and Belgium, some of the
most successful examples being those at Lens, Anzin and Vicq, in the
north of France basin. In this system the soft ground or fissured
water-bearing rock is rendered temporarily solid by freezing the
contained water within a surface a few feet larger in diameter than the
size of the finished shaft, so that the ground may be broken either by
hand tools or blasting in the same manner as hard rock. The miners are
protected by the frozen wall, which may be 4 or 5 ft. thick. The
freezing is effected by circulating brine (calcium chloride solution)
cooled to 5 deg. F. through a series of vertical pipes closed at the
bottom, contained in boreholes arranged at equal distances apart around
the space to be frozen, and carried down to a short distance below the
bottom of the ground to be secured. The chilled brine enters through a
central tube of small diameter, passes to the bottom of the outer one
and rises through the latter to the surface, each system of tubes being
connected above by a ring main with the circulating pumps. The brine is
cooled in a tank filled with spiral pipes, in which anhydrous ammonia,
previously liquefied by compression, is vaporized _in vacuo_ at the
atmospheric temperature by the sensible heat of the return-current of
brine, whose temperature has been slightly raised in its passage through
the circulating tubes. When hard ground is reached, a seat is formed for
the cast iron tubbing, which is built up in