January/February 2025 - 15

Generalized soil profile
west side of the bridge to 180 ft (55 m) on the east. Below the
alluvial soils lies a decomposed Columbia River basalt that
ranged greatly in thickness. This was
underlain by weathered Columbia River
basalt, giving way to intact Columbia
River basalt. The deeper rock is
primarily located on the eastern end of
the bridge. Basalt bedrock found across
the site had unconfined compressive
strengths ranging from 105-24,407 psi
(724-168,280 kPa). Permanent drilled
shaft steel casing was specified to be
taken to the top of the intact basalt.
Groundwater is a difficult rival. Near
surface, it was controlled by the river
level. But just to make it interesting,
mother nature threw in artesian
groundwater from within the basalt.
Piezometric head ranged from elevation
+20 to +30 ft (6-9 m), which meant the
contractor had to maintain a top of
casing elevation as much as +40 ft (12 m).
Construction and Challenges
There were numerous unique challenges in the design,
construction and testing of these large diameter drilled
shafts: artesian groundwater, proximity to adjacent battered
H-piles and existing bridge supports, overwater work
platforms, and scaling up every component of the drilled shaft
installation process. Since permanent casing was required to
top of rock, some of the largest and most powerful drilled shaft
construction equipment in the world
was needed, which was custom built,
costly and with long lead times.
Although the equipment had to be
bigger, overwater access limited the
work area to several small temporary
steel platforms. The size of the
construction equipment meant the
machines would impart tremendous
operating loads to each of the work
platforms, which required massive
steel structures to react against. There
was also great preconstruction debate
about whether available oscillators or
rotators could overcome the enormous
frictional resistance to install such
large and deep casings. Normal things that changed because
of the scale included management of high volumes of
polymer support fluid, complex rebar lift plans, doing
multiple cage splices of the largest rebar cages in the industry,
designing a concrete mix with extended workability due to
the large volumes, and planning and placement of more than
1,000 yd (915 m) of concrete per shaft, which all played a
factor.
To drill the 12.25 ft (3.7 m) diameter
Transition of decomposed basalt to competent basalt
Pier 2 Shaft B (test shaft)
shaft casing installation at an excavated
depth of 250 ft (76 m), the contractor
used a custom built oscillator, thought to
be one of the world's largest, with
increased lifting force and torque to
overcome the enormous frictional forces
during casing installation. The machine
was matched to a duty cycle crawler
crane that excavated the alluvium and
weathered rock from within the casings
with a massive spherical grab and some
chiseling. Rock sockets were excavated
with one of the largest Bauer rotary drill
rigs in the industry, anywhere from
6-11 ft (1.8-3.3 m) below the casing with
a minimum rock socket diameter of
11.5 ft (3.5 m). In some locations there
was artesian groundwater necessitating
the extension of casing tops and working
platforms high above the piezometric
groundwater head.
Rebar cages, designed for seismic loading, were
exceptionally big and heavy, with weights approaching
300,000 lb (136,000 kg). Given the tight working areas, long
cage lengths and heavy lifts, cages were installed in up to four
sections mechanically spliced over the hole. This required an
extensive amount of planning and engineering into the rebar
cage construction and installation.
The project's oscillator
DEEP FOUNDATIONS * JAN/FEB 2025 * 15
January/February 2025

Table of Contents for the Digital Edition of January/February 2025
