March/April 2023 - 16

Instrument redundancy shown at Maryland Abutment during preconstruction phase (left), and during superstructure construction phase (right)
redundancy and understand the bridge behavior was to
overlap the quantity and type of instrument in a single critical
location to provide multiple methods of structure evaluation.
At one of the deepest bridge locations, navigation span
Pier 14 was equipped with:
* Two tilt sensors - one at the bridge deck elevation and one
at the pier water level elevation.
* Three structural monitoring prisms - one at the bridge
deck elevation and two at the pier water level elevation,
each facing a different total station.
* One strain gauge on an existing steel truss member.
* One vibration sensor measuring vibrations on the existing
pier cap.
Continuous monitoring of the data trends was performed,
sometimes through evenings and weekends during critical
construction stages, to evaluate triggered alerts for validity
and consideration of the types of causes. With supply chain
shortages starting to develop in the summer of 2020 and
contractor schedule ramping up, the baseline monitoring
phase quickly transitioned into the construction monitoring
phase with little opportunity to increase the instrument
coverage. Having a strong geotechnical analysis of the
instruments in place during the beginning stages of
construction was integral to project success.
Throughout the project the instrument data was routinely
analyzed for performance and adjustments were made to
tighten or loosen design alert threshold levels accordingly.
Temperature-induced effects were observed on both a daily
and seasonal cycle, with greatest impacts during the winter
period as the freezing point was routinely crossed. Another
constant factor in data analysis that required early design
changes was the effect of dynamic traffic loads on the existing
16 * DEEP FOUNDATIONS * MAR/APR 2023
bridge deck designed for anticipated flexural movement. Tilt
sensors installed on the bridge deck parapet wall typically
experienced tilt fluctuation magnitudes of +/-0.3 degrees
greater than accompanying tilt sensors installed at the water
level elevation on existing pier caps.
Results of Instrumentation and Monitoring
Beginning in May 2020, vibration sensors were installed on
the existing bridge from the Maryland abutment to the
Virginia abutment for structure vibration monitoring.
Vibration sensors were regularly relocated to various bents
and piers to coincide with adjacent pile driving operations as
they advanced. An evaluation of each vibration sensor dataset
over at least a 20-month-long monitoring period, in
conjunction with data observed from tilt sensors and prisms
at same locations, showed no long-term structural effects on
the existing bridge due to construction-induced vibrations.
Over the course of the project, pile driving activity at several
locations produced elevated vibration levels in existing
structures above baseline levels. However, vibration readings
were never observed above the design alert threshold level of
0.5 in/s (13 mm/s) established by the owner's specifications.
The precast concrete piles were designed to develop their
side resistance in stiff Nanjemoy and Marlboro soils as well
as dense to very dense Aquia sands. New pile driving impacts
that were expected to occur at the adjacent bridge
substructure units, which consisted of a combination of steel,
concrete and timber piling support, had calculated ranges up
to 0.7 in (18 mm) of lateral ground displacement and 1.1 in/s
(28 mm/s) of anticipated peak particle velocity.
On July 6, 2021, increased lateral ground movement of up
to 2 in (50mm) was observed at VA-INC-4, at a depth of 55 ft
(18 m) below grade, during and after pile driving activity at the
adjacent Virginia Abutment A.

March/April 2023

Table of Contents for the Digital Edition of March/April 2023