DFI Magazine September/October 2020 - 111

depth ranged from 121 ft (37 m) to 69 ft
(21 m). The two transversal panels
adjacent to the deeper panels were also
deepened to the sandy layer since they
would serve, as the shafts' barrettes do, as
foundations for the final structure.
Because soft clay was present at the
excavation bottom, the designer developed
nonreinforced transverse diaphragm walls
that were connected to each longitudinal
panel to ensure wall stability. The transverse walls started from the excavation
bottom level and extended to a depth of
16.4 to 32.8 ft (5 to 10 m).

three tremie pipes for each multiple
longitudinal panel, and two tremie pipes
for transverse panels.

Cross Passages, Safe Havens
Trevi - AC JV also helped build the cross
passages and safe havens at the east side of
the two tunnels, and the North Safe Haven
at the west side. The plan dimensions of the
east cross passages were approximately
203.4 ft (62 m) long and 32.8 ft (10 m)
wide, expanding to a width of 46 ft (14 m)
at the Safe Havens of the tunnels, where
maintenance of the cutting head of the
TBMs occurred.

with either a grab or the hydromill. The
work carried out by Trevi - AC JV for the
construction of the North Road Tunnel
involved 194,680 yd2 (163,000 m2) of
structural diaphragm walls for the
launching/reception shafts, as well as the
east and west cut and covers. All told, 103,
550 yd2 (86,600 m2) of plastic diaphragm
walls were built to develop the east and
west soil blocks, cross passage CP6 and safe
havens, plus the north haven at crosspassage CP2.

Approach ramp plan and 3-D view

Construction Machinery
The sequencing of primary panels, follower
panels and secondary panels allowed all to
be executed with hydraulic grabs (except
transverse closing elements). The connection joint was developed using steel
sheet piles. The hydromill was employed
for the transverse closing panels, since
good contact was required between them
and the longitudinal diaphragm wall. This
approach required designing special
reinforcement cages for the diaphragm
walls. This resulted in a recess within the
central portion, so as to avoid the milling of
bars, in case of possible deviations of the
transversal panels; in this way, the structural integrity of the longitudinal elements
was preserved. In this case, the concrete
casting of the panels was carried out using

The dimensions of the West Cross
Passage were approximately 164 ft (50 m)
long and 15 ft (4.6 m) wide. Only the North
Safe Haven, 57.4 ft (17.5 m) long and 15 ft
(4.6 m) wide, was part of Trevi's scope of work.
Like the break-in and break-out blocks,
the soil for the cross passages was
consolidated by using intersecting plastic
concrete diaphragm walls; the single and
multiple panels overlapped by 1.1 in
(35 cm) to ensure a proper hydraulic seal
during tunnel excavation. The soil improvement depth varied from approximately 167 ft (51 m) to 169 ft (51.5 m),
according to the tunnel's depths.

Conclusion
The project primarily involved constructing diaphragm walls, consisting
mainly of multiple-bite panels excavated

A unique aspect of the work was the use
of plastic concrete blocks instead of jetgrouting blocks for soil consolidation at the
break-in/break-out and cross passages.
This was due to equipment availability for
the plastic blocks from diaphragm wall
development.
The work was performed in about 35
months, ending in December 2017. After
the diaphragm development work, the
remaining structures were excavated as
planned, and on schedule.
Giuseppe Lui is a senior civil engineer of the
overseas department of Trevi. He has 18 years of
experience in underground engineering, and
specializes in metros and tunnel projects.
Federico Fanti is a civil engineer in the Design
Research and Development Department of Trevi. He
has over 9 years of experience in the design of deep
foundations, soil improvement and structural design.

DEEP FOUNDATIONS * SEPT/OCT 2020 * 111

DFI Magazine September/October 2020

Table of Contents for the Digital Edition of DFI Magazine September/October 2020