The Setting
The tall, nearly vertical, 80-degree retaining wall
overlooking Lock and Dam #1 on the Mississippi River in Minneapolis needed
repair.
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Using the hydraulic drill specially designed and constructed
in-house by The Judy Company of Kansas City, workers install
15-foot percussion drilled rock anchors 85 feet above ground level
on the retaining wall.
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The
1,370-foot long retaining wall, constructed in the 1950s, was
designed
to protect the west bluff adjacent to the lock and a sprawling historic
Veterans Home on top of the bluff overlooking the Lock. The bluff, an
exposed rock face almost 100 feet high, was stabilized by the retaining
wall built of concrete cribs on the lower 65 feet. The cribs created a
vertical wall face in front of the natural rock face of the bluff.
Different length cribs were used because the distance
created between the wall and rock face of the bluff varied from 5 feet to
12 feet. The distance, called a “bin,” was backfilled with rock. Above
the existing crib wall is 30 feet of exposed limestone with a 7-foot
cast-in-place concrete wall near the top.
The
Problem
Over
time, the Army Corps of Engineers had recorded small movements
in some areas in the crib wall sections, indicating instability
of the bluff. The exposed limestone was disintegrating, dropping
broken rock and undermining the cast-in-place wall.
The
Corps issued a contract to Lametti & Sons of Hugo, Minn., to
reinforce and rehabilitate the wall, and to extend its height.
First, the top 3 feet of crib wall was to be removed. Then high
capacity, post-tensioned rock anchors with concrete walers were
to be installed along the crib wall in limited sections.
To
rehabilitate the exposed limestone above the crib wall, 15-foot
rockbolts were to be installed and the surface shotcreted with
reinforced concrete. Finally, the cribwall was to be capped with
a concrete walk, and a poured-in-place wall constructed along
the base.
Access
was the project’s greatest challenge. Adding to the difficulty
of height and steepness, there was only limited access along the
base of the wall. For the first 700 feet, there was a narrow
paved area at the base of the wall. The remainder of the wall
was over water or along unimproved riverbank. An equally great
problem was that no standard equipment exists for drilling and
installing rock bolts and anchors in instances of such limited
access and steep, high walls.
The
Solution
The innovative climbing drill conceived and built by The Judy
Company can climb, telescope, and reach up to 85 feet from the
ramp with a footprint of only 8 feet by 16 feet. A crawler base
levels itself on grades of up to 7 percent and the drill works on
a vertical, 90-degree slope.
Known for innovative solutions to difficult access construction,
The Judy Company used the new drill to install 88 high capacity
rock anchors through the crib walls and 1,350 rock bolts in the
upper limestone face at Mississippi River Lock and Dam Number 1.
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The
Judy Company, Inc., a Kansas City, Kan., geotechnical specialty
contractor, was awarded the subcontract to install 88 high
capacity rock anchors through the crib wall and 1350 rock bolts
in the upper limestone face. In service since 1922, The Judy
Company is known for innovative solutions to difficult access
construction and construction problems. Engineers from The Judy
Company and Lametti teamed up to consider approaches to
anchoring the wall, including the conventional barge-mounted
crane and swinging drill platform approach.
However,
this approach would have been difficult because the crane would
have to be moved every time river traffic locked and barged
through. Swinging platforms would also have been troublesome. It
is difficult to drill into a wall without a landed base to
anchor the rig because drilling pressure forces the platform
away from the wall.
Judy’s
engineers decided that access would be best achieved from an
anchored position at the base of the wall. A 12-foot-wide bridge
would be constructed using railcars from the paved area to the
riverbank and a berm for work access would be built along the
bank to the end of the wall.
To
achieve the needed access, The Judy Company designed and built
an innovative elevating platform drill that could climb,
telescope, and reach up to 85 feet from the ramp with a
footprint of only 8 feet x 16 feet. The crawler base could level
itself on grades of up to 7 percent and work on a vertical,
90-degree slope. The drill would be required to penetrate the
crib wall and coredrill up to 60 feet deep for the high capacity
anchors. For the upper, relatively quick setting rock bolts, the
platform would be reconfigured with a percussion drill to drill
through rock and concrete for the 15 feet deep, smaller diameter
holes.
The
Judy Company’s engineers, along with a structural consultant,
designed the drill and platform using ANSI, OSHA and Army Corps
of Engineers safety standards and requirements.
Truss-style
leads from a pile driver were used as vertical members that were
attached to a self-propelled crawler drill fitted with
outriggers. The angle of the leads was hydraulically controlled
allowing the leads to be lifted away from the wall. Hydraulics
also permitted the rig to move under its own power.
For
installation of the high capacity rock anchors, the mast and
pedestal from a rotary drill used for coredrilling rock anchors
were attached to the climbing work platform along with a
hydraulic drill and power pack for the hoist controls.
For
the 1,350 rock bolts to be installed through the concrete wall
at the top of the bluff, a swinging drill platform was designed
and built. The drill mast was designed to be controllable in 3
axes to accommodate the various orientations. For installation
of the majority of the anchors, it was necessary to reconfigure
the climbing tower drill for more height. The leads were
extended an additional 30 feet and a special drill platform
rigged with a percussion drill was built. To minimize the number
of setups necessary, Judy’s engineers fitted the platform with
an indexing slide mechanism that allowed 5 foot of indexing in
either direction, permitting the installation of 8 bolts per
setup, significantly enhancing production. To facilitate
handling of the longer bolts, the platform and handrails were
designed to telescope.
The
unit was pre-assembled and tested at The Judy Company’s yard
in Kansas City and ready for installation of the rock anchors
within three months.
Using
the newly built rig, casing was seated in competent rock and the
holes were advanced up to 60 ft. in sandstone. The 252 kip
capacity anchors were pre-grouted inside corrugated sheathing
for additional corrosion protection and the assembly was
installed and grouted in place.
After
the anchors were installed, Lametti, the general contractor,
cast a concrete waler to allow the anchors to be pre-loaded
against the cribwall. An innovative stair-step construction
sequence allowed the forms and scaffolds to be supported from
the waler below it, simplifying falsework requirements.
Next,
the 1,350 15-foot rockbolts were installed into the upper
limestone face. Installation of the shorter anchors required
different drilling procedures and equipment because of the
nature of percussion drilling and due to shorter bolt spacing.
The limestone face was 5-8 feet behind the face of the crib
wall. To allow the drill to pass the concrete walers, it was
necessary for the workers to use the telescoping feature of the
work platform. While drilling on the upper rows, the work
platform was extended to the face of the wall for the crew to
access the work.
The
rock bolts were placed on 5-foot centers along the wall. To
minimize the number of setups of the climbing drill, the
hydraulic slide mechanism indexed the mast 5 feet either left or
right to allow two columns of rock bolts to be installed from
one setup. On one setup, two rows of four bolts each could be
installed, saving a great deal of time. Installation of eight
bolts on one setup was complete before the crew moved to the
next hole.
The
ingenuity of The Judy Company’s equipment design, combined
with close contractor-sub-contractor cooperation, assured the
completion of the majority of the work within a single season,
reducing schedule time and remobilization costs.
