TUNNEL ENGINEERING

constructed to the correct grade. The sand can be
placed through movable pipes inserted and withdrawn
from the sides beneath the structure (sand
jetting), or through fixed pipes embedded with the
structure (sand-flow) with connections either external
on the roof or sides, or internal through valves.
One method of sand jetting, invented by the
Danish firm of Christiani & Nielson, is particularly
effective: A sand slurry is injected through a
movable nozzle, and the surplus water is pumped
off by another nozzle, the rolling motion depositing
the sand in a compact layer. With sand flow, the
rolling current deposits the sand around the
discharge point in a firm circular layer (pancakes),
often allowed to grow 20–26 ft radius. Discharge
points are built into the underside of the element
and are connected to pipes leading to convenient
points at which the pumped sand supply may be
connected.
As well as pile bents, elements have been
temporarily supported by jacks, penetrating
through the base of the section. The jacks bear on
previously placed concrete blocks. By adjusting the
jacks, the section is brought to exact grade. Then,
the sand foundation course is flushed in.
Immersion, Placing and Joining n Loosely
termed the “sinking” operation, these three
operations are performed with a high degree of
control and therefore of accuracy. Immersion and
lowering of each element is regulated by winches
on special barges or pontoons, or by cranes, from
which they are suspended. Alignment is controlled
by instruments set on fixed points and sighting on
targets mounted on temporary towers attached to
the ends of the sections or by sonar to pre-installed
targets to avoid using temporary towers. Steel-shell
elements have historically been connected with
short lengths of shell, which project beyond the end
bulkheads. The gap between the ends was covered
by hood plates extended from the lower and upper
half of the shell extensions. Form plates were
inserted into guides on the vertical edges of the
bulkheads. The space around the joint was filled
with tremie concrete as a preliminary seal. The
inside of the joint was drained, and closure plates
were welded to interior ribs of the shell extensions.
Finally, the concrete lining was completed.
Making rigid immersion joints today using
tremie concrete is unusual, with rubber-gasket
joints being almost universally used. Flexible
joints are generally sealed with a temporary
TUNNEL ENGINEERING
Tunnel Engineering n 20.51
immersion gasket or soft nosed gasket (Ginatype)
in compression, attached to the end of one
of the elements and mating with a flat steel face
on the other. The use of a secondary independent
flexible seal, capable of being replaced from
within the tunnel, is common practice (often an
omega-shaped seal). Each seal should be capable
of resisting the external hydrostatic pressure and
should allow for expected future movements.
Jacks pull the tubes into contact to provide an
initial seal. The joint is then drained, activating
the full hydrostatic pressure on the opposite
end of the tube. The pressure compresses the
gaskets completely, providing a secure seal. Then,
the bulkheads between the connected tubes can
be opened and the joint completed from the
inside.
Depending upon the construction sequence,
the last element may need to be inserted in the
remaining space, rather than appended to the end
of the previous element. In order to achieve this, a
small final gap will remain. This closure or final
joint corresponds to a short length of tunnel that
will need to be constructed in a special way.
Methods used have include tremie concrete to seal
a rigid joint, and for flexible joints:
† dewatering to complete the joint in the dry from
the inside;
† terminal block where a short closure section is
slid out from within one side until it meets the
other and any remaining gap is closed with a
rubber gasket in compression;
† wedge-shaped block dropped into the remaining
gap until it is sealed against both sides.
Backfill n Up to about half the height of the
element, the trench is backfilled with well-graded
self-compacting material to lock the elements
securely into place. Ordinary backfill is placed to
a depth of at least 5 ft over the top of the tunnel. If
any part of the tunnel projects above the natural
bottom, dikes should be built at least 50 ft away on
both sides to a height of 5 ft above the tunnel. The
space between the dikes should be filled with
backfill, covered with a stone blanket to prevent
scour where necessary.
Design n Tunnel elements are designed as rigid
structures to resist dead loads, live loads, exceptional
loads and extreme loads. Dead load includes
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