The brick arch, water-proofing, and rock
packing were laid up in lifts, in the same manner as in the Twin Tunnel,
with grout pipes built in at intervals of about 8 ft. The concrete arch
was placed in sections, from 25 to 50 ft. in length, with a rather wet
mixture and a back form on the steep slope of the extrados.
The concrete for the sand-walls and lower part of side-walls was handled
on tracks and platforms laid on cantilever beams at mid-height of the
trestle, as shown by Fig. 3, Plate LXII. For the walls above the
springing line, the tracks were laid on top of the I-beam ties, and some
of the arch concrete, also, was delivered from the mixer at that level
and hauled up an incline to the level of the top of the arch. By far the
greater part, however, was turned out from mixers set on the completed
arch, and was transported on tracks hung in part from the street
timbering.
_Completion._--Except in the heavily-timbered portions, such as at Fifth
Avenue, where the load had to be transferred from posts to the completed
masonry section by section, the lining of the tunnels presented no
special difficulty. The large number of small forms to be set, and the
mutual interference of the concreting and duct-laying operations proved
to be the most troublesome features of the work.
Pages:
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64