Page 8 - May-June 2021 Vol 39 No 1

LARGE Walls
from page 7
vertically and connected horizontally. That Williams 150-ksi No. 8 bar. The hole
would allow the contractor to disconnect
the end section once the anchor had been
installed, and move it forward to the
opposite end to reconnect the unit to the
adjacent platform.
This process was repeated as the
anchors were installed at this level. This
allowed the drill rig to drive continuously allowed to cure for 48 hours. The anchor
down the wall as the anchors were
installed. The subcontractor for the
drilling and installation of the anchors
for the walls for both projects was
Colorado-based GeoStabilization
International (GSI). GSI was very pleased
with the drill platforms because they
allowed an uninterrupted method of
installation of the upper anchors in the
retaining walls.
Ninety percent of the walls in the rock
cuts were installed with the installation
of the freestanding post, cast in place
panels, and backfilling without loading
the anchors. PVC pipes were used at the
anchor locations to maintain the anchor
hole during backfilling. The 1-inch, 150-
ksi Williams threaded rod anchors were
loaded to 76 kips per anchor from the
bottom to the top to minimize movement
of the wall.
In the areas with competent rock, the
installed anchors were loaded to 40 kips
per anchor and tested with a calibrated
torque wrench. The calibration of the
torque wrench was done by on-site
testing of in-place anchors. In some of
the areas of Wall 3 on the second project
and Wall 4 on the first project, colluvium
soil was encountered and caused some
issues in the anchor installation, with the galvanized, additional grout cover was
anchors failing proof testing.
Challenges in Anchoring
In an area on Wall 4 in project one, GSI
had trouble installing and loading the
anchors between piles 14 and 20 due to
the colluvium material in the area. The
wall height in this area was around 26.5
feet, with a three-anchor section.
The anchors got excessive in length due
to the colluvium zone, which caused the
grout to migrate to the stream below. The
anchor hole advanced through multiple
seams of weathered rock and a mixture
of gravel and soil, which made it difficult the anchors to more than 100 feet in
to keep the hole open. GSI installed two
sacrificial test anchors in the colluvium
8 TPW May/June 2021
zone to determine which anchor type was
more feasible.
The first anchor installed was a
was pre-treated with a low mobility
grout to stabilize the hole, which utilized
temporary casing. The anchor had a total
length of 50 feet (20-foot free zone and
30-foot bond zone).
The low mobility grout was installed
while the casing was removed and
was drilled again and installed. A 20-foot
PVC bond breaker was installed for the
free zone.
The second sacrificial anchor installed
used a Z51-mm (two-inch) hollow-bar
injection anchor (self-drilling). A six-
inch sacrificial hole was required for the
self-drilling anchor. A 50-foot anchor
with a 40-foot bond length and 10-foot
free length was used, creating a free zone
challenge for GSI.
The anchor used a grout injection
method where the grout was injected
through the hollow stem bar and port
holes in the sacrificial drill bit. An anti- material was worked out with the TDOT
washout additive was added to the grout
that congealed when it came in contact
with ground water.
No turbidity or discoloration in the
water in the Emory River was noticed
after the anchors were installed. The
first anchor experienced pull-out failure,
but the self-drilling anchor achieved its
maximum test load without experiencing
failure.
There were concerns about corrosion,
so a Z51-mm (two-inch) anchor was
used on a six-inch hole. This anchor was
provided, and additional sacrificial steel
in the anchor was incorporated into the
design. The self-drilling anchor was used
in this area and no further issues were
encountered.
Working Around Conventional Anchors
A similar issue occurred in a limited
area on Wall 3 on the second project,
where colluvium soils were encountered,
and the conventional anchors were
failing due to collapsing holes and the
inability to get a good bond zone in the
colluvium. In order to achieve the design
load, the anchors were advanced until
competent rock was located, extending
length.
Two sacrificial anchors were used to
establish bond capacity without reaching
bedrock. That gave the contractor/
subcontractor the option to use new/
longer bonding zones rather than
chasing bedrock. The sacrificial anchors
established new bonding length in the
non-rock material.
A new bonding zone of 30 feet was
adequate to achieve the design load.
It was GSI’s choice to use permanent
or temporary casing to keep the hole
open. The contractor chose to use the
original 1-inch, 150-ksi rod with the
new 30-foot bonding zone length for
the colluvium zone. This section of the
wall was originally designed for a partial
rock cut loading but ended up with a full
colluvium soil loading.
The area of the colluvium soil was
between pile 15 through pile 25 in Wall
3. The wall height was around 28.3 feet
in this area, with a three-anchor section.
The slope behind the wall did not exceed
a 2:1 slope.
The colluvium soil comprised of a soil
and rock matrix. An estimated loading
and new bond strength for this type of
geotechnical section. The revised loading
required a higher anchor load of 95 kips.
Wall Issues: Under Control
Another issue that occurred was
maintaining the 15-foot bench between
the walls. This was a huge concern since
the base of the upper wall was anchored
into the rock bench by 30-inch caissons.
The blasting had to be strictly controlled
in order to maintain the integrity of the
benches. In some areas where the rock
was severely weathered, rock anchors and
a temporary gunite facing was used to
maintain the stability of the bench until
the lower wall was constructed. In some
areas, wood lagging was used to keep the
weathered rock from slumping off.
The lagging secured the weathered rock
and loose material between the posts. The
HP 14 x 73 post at the 16-foot spacing
had enough capacity without anchors to
restrain the No. 57 stone loading before
the anchors were installed.
Economical, Efficient Construction
The pre- and post-cast panel
construction provided an economical and
efficient way of constructing a wall in this
terrain. The contractor was very pleased
with the construction of the walls, and
there were very few issues that had to be
addressed during the construction.