. The street railway review . BACK OF THE DRIVING SHIELD. are walled up with a double ring of iron plates, filled inwith concrete. In sinking these shafts the first operationis the building of a section of the lining at the surface of theearth. The ground is then dug away from under untilthe section of lining sinks down, when another section isbuilt upon the first, and the digging and building up ofthe lining continues until the required depth is depth of the shaft at Blackwall is over seventy-fivefeet and the diameter fifty-eight feet. At the bottom ofthe shaft is a circular openi
. The street railway review . BACK OF THE DRIVING SHIELD. are walled up with a double ring of iron plates, filled inwith concrete. In sinking these shafts the first operationis the building of a section of the lining at the surface of theearth. The ground is then dug away from under untilthe section of lining sinks down, when another section isbuilt upon the first, and the digging and building up ofthe lining continues until the required depth is depth of the shaft at Blackwall is over seventy-fivefeet and the diameter fifty-eight feet. At the bottom ofthe shaft is a circular opening on the side towards theriver, through which the cutting shield was pushed. Economy of time required for construction was securedby having the cutting shield under construction at thesame time that the shaft was being sunk, instead of wait-. / THE, IN COMPRESSED AIR. ing to put it together until after the bottom of the shaftwas reached. The shield was built on a timber frame-work and weighed 250 tons. When all was ready thedock and shaft were filled with water and the shieldfloated over the shaft down which it was lowered bysimply pumping out the water. After the plates covering the circular opening in theside of the shaft were removed, the cutting shield waspressed through it and into the earth by the twenty-eight hydraulic jacks provided for this purpose, each ofwhich could exert a pressure of 100 tons. At the for-ward end of the shield, men are at work on platforms atdifferent levels, so as to attack the whole face of theexcavation at the same time. The earth is thrown intotram cars, hauled to the shaft and elevated to the surface,where it is dumped. At one time a mass of cement-like
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Keywords: ., book, bookcentury1800, bookdecade1890, booksubjectstreetrailroads
