July/August 2021 - 102

AAR has occurred extensively and
includes both alkali carbonate reaction
(ACR) and alkali silica reaction (ASR).
Monolith 48 is situated directly
downstream of the left downstream
miter gate for the existing lock chamber
in Monolith M47. The replacement
project to anchor Monolith 48 will
buttress the concrete that contains the
miter gate pivot. While the lock
replacement project is underway, the
existing lock must remain in operation
until completion of a subsequent
project, which includes breeching of the
existing dam into the new lock chamber
and guide wall construction. The
successful demonstration of the system
on 61-strand anchors at Chickamauga
is believed to be the first time an anchor
of this size has been lifted off by this
method in the United States.
Methods and Analysis
Dam anchors seldom require load
adjustment; however, at Chickamauga,
USACE was concerned that concrete
growth may cause a gradual increase in
load due to the expansion of the concrete.
The concrete expansion leads to
an increase in the free stressing length
of an anchor, and thus adds load to an
anchor. To accommodate expansion, the
new anchors were designed to be locked
off at 56% ultimate tensile strength
(Fpu), which is lower than the typical
60% Fpu used. In addition, one anchor
was outfitted with a permanent
vibrating wire load cell to monitor the
load of the anchor over time.
While using a lower lock-off load
will allow for some future concrete
growth without overstressing the
installed anchors, provisions for
additional adjustment were included in
the design. A 61-strand anchor typically
has a design of 60% Fpu, or 2,144 kips
(9,537 kN). Based on the free stressing
length of 150 ft (45.7 m), the anchor load
increases by 50 kips (222 kN) for every
0.25 in (76 mm) of concrete expansion.
The anchors were designed with four,
½-in (152 mm) thick shim packs for a
total of 2 in (610 mm) of shim. This
102 * DEEP FOUNDATIONS * JULY/AUG 2021
would allow for a theoretical reduction
in anchor load of 200 kips (890 kN), or 2 in
(610 mm) of concrete expansion, over the
life of the anchors.
adjustment of the load on all of the 61
strands. First, the tension can be
increased to lift the anchor wedge plate.
Then, while the head is no longer
resting on the shim pack, some or all of
the shims can be removed. Finally, the
load on the jack is slowly released and
the wedge plate is lowered back onto
any remaining shims and the bearing
plate. This method, which essentially
locks off the anchor at a lower load, was
performed on the five new anchors in
March 2019 over serval days.
The stressing system also presented
Space-saving steel stair system at modified monolith
Anchor Installation and
Adjustment
The 61-strand anchors were installed
over a 5-month period in 2018 - 2019.
For this installation, multiple challenges
needed to be overcome, including
limited access to the work area. The five
anchors needed to be installed behind
the downstream gate of the lock
chamber on the river wall. Access to the
work area consisted of an 8 ft (2.4 m)
wide bridge for foot traffic and a 230 ton
(208,000 kg) crane to lift all materials
and equipment over the active lock. The
work area was roughly 28 ft by 35 ft (9 m
by 11 m) and was already crowded with
existing anchors, instrumentation and a
concrete staircase. The concrete
staircase was removed to create space
for the anchors and later replaced with a
ga l vani zed s t ee l s t a i r sys t em.
Additionally, to provide adequate space
for the labor and equipment necessary
to complete the work, a temporary
access platform was installed around
the outer edge of the work area.
To enable removal of the shims, a lift
off system was developed that includes
a threaded anchor wedge plate that
facilitates attachment of a hydraulic
jack using a threaded coupler and
pulling bars. This configuration allows
a s imu l t a n e o u s , c o o r d i n a t e d
many challenges. The lift off system
was designed and built specifically for
this project. The materials used to
fabricate the custom system needed to
be stressed to their maximum limits,
given the center hole size restriction of
the stressing jack. The 3 in (7.6 cm)
diameter ASTM A722 threaded bars
used were selected from a heat of Grade
150 (150 ksi Ultimate) that tested above
175 ksi (1,206 MPA). It was found that
during lift off, cracking was observed in
the welds of the jack chair. The welds
were repaired and additional strapping
was added to the chair to reduce
bending of the vertical chair legs. Each
Schematic of lift off system
July/August 2021

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