DFI Magazine September/October 2020 - 110
stabilized the shafts' bottoms against
hydrostatic pressure that was applied below
the bottom slab. Likewise, the barrettes
provided the necessary bearing capacity to
the final structure, when the weight of the
internal structure and of the backfill
became higher than the hydraulic thrust.
For the concrete casting, two tremie
pipes were used and positioned symmetrically within the excavation. This approach
limited the casting time, produced a better
quality and homogeneity of concrete, and
ensured a proper rising speed.
Monitoring Panel Deviations
The absolute necessity to maintain
deviations below 0.6% of the depth (±1 to
164 ft [±30 cm to 50 m] to guarantee an
adequate wall thickness in correspondence
with the joints) required utmost attention
while controlling the verticality of the
excavation. All the primary panels executed
with grab were subjected to verticality
controls carried out systematically in the
middle and at the bottom of the excavation
by means of the Koden ultrasonic system.
For secondary panels, a further control was
made with the Koden at the bottom of the
excavation, in addition to continuous
measurements with inclinometers mounted
on the hydromill frame. This allowed for
reconstructing the panels' real positions and
verifying joint contacts before starting the
shaft excavation.
Soil Block
To connect the TBM-bored tunnel and the
launching/reception shafts, rigid blocks of
consolidated soil were used (called breakin and break-out blocks, where the TBMs
started and finished). The rigid blocks
consisted of intersecting plastic concrete
diaphragm walls.
Soil blocks were executed from many
single and multiple panels, which were
overlapped 1.1 in (35 cm) so that their larger
dimension ran parallel to the tunnel axis. To
avoid settlements that would result from
consolidation of the soft clay underlying the
rigid blocks, 40% of the panels were
embedded into the deep sandy layer.
Shaft Approach Ramps
The approach ramps stretch from the
shafts for about 1,362 ft (415 m) on the
east side and 1,673 ft (510 m) on the west
side. Each of these two structures includes
an artificial tunnel in the deepest part that
becomes an open-air ramp in the most
superficial sections.
The structural diaphragm walls of the
longitudinal alignments, which acted as
temporary supports during the excavation
of the ramps, were developed with 256 in
(6,500 mm) long panels with a thickness of
47 in (1,200 mm) in the deepest part of the
ramps, and 40 in (1,000 mm) in their most
superficial sections.
Regarding excavation depth, the
project included 177 ft (54 m) deep panels
that were embedded into the underlying
sandy layer, alternated with panels whose
Excavation of shaft panel
110 * DEEP FOUNDATIONS * SEPT/OCT 2020
DFI Magazine September/October 2020
Table of Contents for the Digital Edition of DFI Magazine September/October 2020
CONTENTS
CONTENTS 2
DFI Magazine September/October 2020 - Intro
DFI Magazine September/October 2020 - 1
DFI Magazine September/October 2020 - 2
DFI Magazine September/October 2020 - CONTENTS
DFI Magazine September/October 2020 - 4
DFI Magazine September/October 2020 - CONTENTS 2
DFI Magazine September/October 2020 - 6
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