The tube of cast-iron rings is adapted to be built in the tail of the
shield. Where no shield was used, after the excavation was completed and
all loose rock was removed, timbers were fixed across the tunnel from
which semicircular ribs were hung, below which lagging was placed. The
space between this and the rough rock surface was filled with concrete.
This formed a cradle in which the iron tube could be erected, and, at
the same time, occupied space which would have been filled by grout, at
greater cost, had a shield been used.

As soon as each ring of iron was erected, the space between it and the
roof of the excavation was filled with hand-packed stone. At about every
sixth ring a wall of stone laid in mortar was built between the lining
and the rock to serve as a dam to retain grout. The interstices between
the hand-packed stones were then filled with 1 to 1 grout of cement and
sand, ejected through the iron lining. The concrete cradles averaged
1.05 cu. yd. per ft. of tunnel, and cost, exclusive of materials, $6.70
per cu. yd., of which $2.25 was for labor and $4.45 was for top charges.
The hand-packed stone averaged 1-1/2 cu. yd. per ft. of tunnel, and cost
$2.42 per cu. yd., of which $0.98 was for labor and $1.44 was for top
charges.


ERECTION OF IRON LINING.

The contractors planned to erect the iron lining with erectors of the
same pattern as that used on the shield under the river, mounted on a
traveling stage. These will be described in detail in Mr. Japp's paper.
Two of these stages and erectors worked in each tunnel at different
points. The tunnel was attacked from so many points that these erectors
could not be moved from working to working. The result was that about