usually move parallel to the grade line, though this was modified
considerably by the way the mucking was done and by the stiffness of the
ground at the bottom of the shield.

On the average, the shields were shoved by from ten to twelve of the
bottom jacks, with a pressure of about 4,000 lb. per sq. in. The jacks
had 9-in. plungers, which made the average total force required to shove
the shield 2,800,000 lb. In the soft ground, where shutters were used,
all of the twenty-seven jacks were frequently used, and on several
occasions the pressure exceeded 6,000 lb. per sq. in. With a unit
pressure of 6,000 lb. per sq. in., the total pressure on the shield with
all twenty-seven jacks in operation was 5,154 tons.


INJURIES TO SHIELDS.

There were only two instances of damage to the essential structural
features of the shields. The most serious was in Tunnel _D_ where the
cutting edge at the bottom of the shield was forced up a slightly
sloping ledge of rock. A bow was formed in the steel casting which was
markedly increased with the next few shoves. Work was suspended, and a
heavy cast-steel patch, filling out the bow, was attached to the bent
segments, as shown in Fig. 2, Plate LXXIII. No further trouble was
experienced with the deformed portion. The other instance was in Tunnel
_B_, from Long Island, where a somewhat similar but less serious
accident occurred and was treated in a like manner.

_Bulkheads._--At Manhattan, bulkheads had to be built near the shafts
before the tunnels could be put under pressure. After 500 ft. of tunnel
had been built on each line, the second bulkheads were constructed. The