Page 7 - May-June 2021 Vol 39 No 1

features in the project area.
The exposed rock along the existing
road was as old as the Ordovician
Chickamauga Group and Pennsylvanian
formation. The exposed rock cut gave a
good snapshot of the formation of the
sedimentary rocks over the years. The dips
and faults were evident in the exposed and
excavated rock cuts showing the varying
bedding planes, bedding angles and rock
structure.
The coal seams, selective sandstone
with coal seams, some of the weathered
shale, and sandstone were the probable
cause of the potential for APR that needed
to be protected during the excavation
of the material (and properly deposed
of in a selective deposal site and/or
properly encapsulated.) If this material
was exposed to water, the run-off would
be acidic and harmful to the creeks and
vegetation.
The different rock types encountered
were siltstone, sandy siltstone, sandstone,
shale, interbedded matrix of different
rocks, and coal seams. The bedding angles
varied from lying flat to 90 degrees. The
quality of the rock ranged from good
quality to moderately weathered, to very
severely weathered. The amount of sandy
clay over burden varied throughout the
limits of the walls.
Subsurface Investigations
S&ME conducted a subsurface
investigation, but it was limited by the
steep terrain and the ability to access
the actual wall locations to drill. From
the information taken in-field, design
parameters were determined for the
retaining walls, which also were necessary
to estimate the rock line along the wall.
Additional borings were done behind
the proposed wall to determine the
location and quality of the rock for the tie
back anchors. The location of the bedrock
profile information was limited, which
created a challenge for the contractor to
determine the rock quantity needed for
bidding the excavation and the retaining
walls.
The rock was estimated by interpolation
and extrapolation from the existing
borings, and a visual inspection noted
the visible exposed rock in the existing
roadway cuts. The amount of overburden
and condition of the rock was estimated to
determine estimated post spacing, and the
number of anchors and anchor lengths for
the cut-retaining walls.
Additionally, the contractor needed to
estimate which sections of the walls could
be constructed with or without shoring.
The preliminary information gave the
contractor some confidence on how the
walls could be constructed, but nothing
was certain. When the walls are bid by the
square foot of the wall face, there always
is risk in trying to estimate a cost for these draining no. 57 stone. Building the walls
type walls.
Two Phases of Construction
The roadway project was phase-
constructed in two sections. The first
project had nine retaining walls, eight
cut walls and one mechanically stabilized
earth (MSE) retaining wall. The total
exposed wall surface was 208,846 square
feet, and the total wall cost for the first
project was $22.7 million. The second
project had four cut retaining walls. The
total exposed wall surface was 156,792
square feet, and the total wall cost was
$14.2 million.
After the project was awarded, the
contractor did extensive subsurface
investigation in the location of the
retaining walls - horizontally and
vertically at the proposed post spacing -
to better define the existing rock line. It
was important to know the location of
competent rock, to determine the method
of construction, number of anchors per
post, and the approximate length of
anchors.
This information was used to determine
if the excavation for the wall would be
either a complete vertical rock cut, partial
rock cut and soil, or typical top-down
construction. Most of the retaining walls
did allow a vertical or near vertical cut,
with a few walls that required the typical
top-down construction with wood
lagging.
Retaining Walls Aided Construction
Efficiency
The contractor used galvanized HP
14 x 73 posts with pre-fabricated anchor
tubes, precast concrete panels, and 1-inch
150-ksi Williams anchors for the soldier
pile walls. The post spacing varied from
eight to 16 feet, depending on the applied
soil and rock loading and wall geometry.
These types of retaining walls expedited
the construction, speeding efficiency.
When the rock could be excavated to a
near vertical slope for the retaining wall,
the post could be installed freestanding
without the need of wood lagging. The
rock cuts placed minimum loading on
the walls, which required 1-inch 150-
ksi Williams anchors to be installed to
account for the fluid pressure from the
stone backfill and any loading from the
existing bedded rock.
The area between the back of the panels
and the rock face was backfilled with free
in this manner sped up construction but
did pose some construction issues that
needed to be overcome. Some of the issues
the contractor had to address were: 1) how
to install the anchors in the air, and 2)
how to keep the post from moving during
the anchor loading.
Methodology Using Free-Stand Posts
With the free-stand post, proper
support was needed to keep the post from
moving during the loading of the anchors
along with maintaining the required
hole opening for the anchors. The area
behind the precast panels was backfilled
with No. 57 stone. The contractor had
to develop a means for transferring the
load from the pile to the in-place material
while maintaining the hole for the anchor
installation.
The first method was to form a lean
concrete wedge from the back of the pile
to the in-place material. The contractor
would then drill the hole directly through
the lean concrete, allowing the concrete
wedge to adequately support the post
during the loading of the anchor. This
method provided the needed support but
was very expensive, requiring an extensive
amount of time to install the concrete
wedges.
Another method was to place 12-
inch PVC pipe at the anchor locations
following the 15-degree inclination of
the wall anchor. With the pipe properly
secured on both ends, panels could be
placed and backfilled with No. 57 stone.
The PVC pipe provided the hole for the
anchor in the stone backfill.
The anchor would be drilled through
the PVC pipe to the in-place material
to the required length. The No. 57 stone
would provide adequate support for the
pile, with the proper sequencing of the
anchor loading. This method proved to
be the most effective in both in cost and
construction time.
Drill Platforms Utilized
The contractor requested that Neel-
Schaffer design a drill platform with three
individual sections that could be adjusted
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