March/April 2023 - 17

The 36 in (915 mm) square prestressed concrete pile had a
required driving design resistance of 954 kip (4,250 kN),
minimum plan depth of 93 ft (28 m), with blows per foot
ranging from four to seven at the depths where soil
displacement was greatest. Re-evaluation of the original
geotechnical report analysis determined that the soil
displacements were caused by a combination of shoving and
heaving between various soil strata with varying
consistencies as the nearby pile was driven.
Due to the magnitude of the lateral soil movement profile,
data from the inclinometer and other nearby instrumentation
on the bridge were analyzed to determine the pile driving
impact on existing bridge foundations. Lateral ground
movement at the edge of the existing abutment due to pile
driving was predicted to be approximately 0.4 in (10 mm) from
original geotechnical analysis. Utilizing the measured
inclinometer movements and proximity factors, additional
finite analysis determined that soil movement at the closest
existing pile was approximately 0.6 in (15 mm), on average.
With expected soil movements above original design
tolerance, the instrumentation data at the abutment was
analyzed in relation to structure tolerance of +/-0.5 in (12 mm)
total settlement or heave, as set forth in the project
specifications. After pile driving, the data from the
inclinometer indicated no additional movement, the
abutment structural monitoring prism indicated no
settlement nor heave, and nearby tilt sensor data showed no
rotational movement, allowing construction and bridge
service to safely continue.
As the project evolved into phases of simultaneous
superstructure and substructure installations on the critical
path, an unanticipated construction risk factor developed.
Adjacent to existing bridge pier 14, steel fender pile driving
operations and new pier column concrete pour were
scheduled to occur within 50 ft (15 m) of each other on the same
day. The concern was that pile driving at that proximity would
induce excessive vibration of freshly placed concrete. In a realtime
collaboration with the project team, FTG coordinated a
vibration monitoring plan for the new column to measure the
impact of the pile driving. A vibration sensor was installed on
the formwork of the new column to be poured to record data
continuously throughout the concrete pour and during the
initial 24-hour curing period. Peak particle velocity design
tolerance of 0.25 in/s (6 mm/s) was established for fresh
concrete with considerations to drive energies and wave
velocity paths through new and existing piles within the
fender-to-pier cap layout. Vibration readings were monitored
during and after the pour for verification of vibration impact
within tolerance. Collaboration of all project team members
determined that pile driving could continue safely adjacent to
the newly placed column without the threat of damage and
that the column was not subject to adverse amplitudes.
Inclinometer readings of VA-INC-4 before and after nearby pile driving activities
DEEP FOUNDATIONS * MAR/APR 2023 * 17
March/April 2023

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