DFI Magazine September/October 2020 - 109

which generally ensures better performance in terms of deviations and
verticality. The presence of soft clay for
about 80% of the excavation depth would
have hampered correction of deviation by
the milling unit. This would have resulted
from the driving flaps penetrating the soft
ground instead of rectifying the direction of
excavation. Furthermore, given the fine
matrix material, the use of a hydromill's
drilling wheels and centrifugal pump
(which tend to shred and mix soil) would
have increased the mud weight. That
would have made it difficult to separate the
clay from the bentonite.

Panel's primary cage

Therefore, the primary panels of the
diaphragm walls and barrettes for the
shafts were executed using hydraulic grabs.
A hydromill was used, however, for
secondary panels in order to get proper
overlap, ensuring a hydraulic seal and
structural continuity for the whole length
of the milled joints.

The shafts were multicellular and
included five compartments in the launching zone, where TBMs for each tunnel were
assembled. The shafts had only two
compartments in the reception zone,
where TBMs were disassembled. In the
launching zone, more space was needed
both during the TBM's assembling phase
(for back-up and thrust frame installation)
and during the excavation phase (for spoil
removal, TBM's segments supply, mud
supply, etc.).
Instead of adopting the classic rectangular form for all shafts, a polycentric
shape was developed by intersecting
circular shafts, with an
inner diameter of
80.4 ft (24.5 m). This
geometry chosen by
AC-OCI JV took
advantage of the arch
effect provided by the
circular shape of the
various sectors of the
shafts to allow space
within which the TBM
could be lowered and
assembled, and it permitted excavation up
to the maximum design depth without
using props or tie rods.
The continuity of the arch effect was
obtained by means of diaphragm wall partitions executed in correspondence to the
intersections of the circular shafts. Selecting

a circular shape to support the structure
during excavation produced circumferential stresses on the diaphragm wall. So,
special attention was required for excavation
verticality controls to ensure that an
adequate contact surface existed between
the joints of adjacent panels and to limit
compressive stresses to a maximum value of
7.5 MPa (1087.8 psi), or 25% of the concrete's characteristic strength.
For the 47 in (1,200 mm) thick shafts'
diaphragm walls, Trevi - AC JV opted for
110 in (2,800 mm) long single primary
panels, executed by means of hydraulic
grabs. Single closing secondary panels with
the same dimension were executed with a
hydromill able to reach 167.3 ft (51 m)
below ground level so panels were
embedded at least 9.8 ft (3 m) within the
sand formation.
Special attention was paid to the panels
placed at the cells' intersecting points,
where the perimeter arches converged and
where highly concentrated stresses would
develop, especially after the demolition of
the transversal walls' central part. After
analyzing various solutions, single panels
reinforced with shaped cages were executed
(see below) to not interfere with joint milling.
Isolated diaphragm wall elements
(barrettes), 100 in (2,800 mm) long, were
added inside the compartments. During the
transition phase and throughout the
excavation of the tunnel, these barrettes

Cell intersection details, with example cage

DEEP FOUNDATIONS * SEPT/OCT 2020 * 109
DFI Magazine September/October 2020

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