January/February 2022 - 17

Drilling row 1 ground anchors at Peregrine Slide
Planned anchor locations vs. PTI drilling guidance
Making holes for the long ground anchors at the Peregrine
site presented construction challenges for our drilling
contractor. With a pile spacing of 7 ft (2.1 m), there were
problems with ground anchors interfering with adjacent
anchors during the drilling operation. Adjacent anchors that
were damaged during drilling required costly repairs and
interjected delays into the construction schedule. These
construction challenges were overcome by the contractor's
proactive quest for solutions.
In its Recommendations for Prestressed Rock and Soil Anchors,
the Post-Tensioning Institute provides guidance on drilling
ground anchors (PTI DC35.1-14, 2014). The guidance states
that, " A tolerance of 0.035 rad (2 degrees) is routinely
achievable using normal drilling methods. Tighter tolerances
down to 0.01 rad (0.5 degrees) will require special drilling
methods. " With this guidance in mind, the long anchors at
Peregrine Slide had the potential to interfere with each other
during the drilling operation if standard drilling practices
were used.
Overcoming Anchor Challenges
To avoid conflict between adjacent anchors, our drilling
contractor and our Caltrans inspection and surveying teams
partnered to implement tighter controls when positioning the
drill rig to begin drilling holes. Rather than setting the ground
anchor hole alignment perpendicular to adjacent piles, our
team started setting the drill angle perpendicular to the
surveyed retaining wall layout line (RWLOL). Vertical piles
can be slightly out of alignment and do not provide consistent
data for horizontal drilling. By providing survey marks on the
timber lagging between piles indicating an offset to the
RWLOL, our team was able to pull consistent measurements
from the RWLOL to set the drill steel on a consistent
perpendicular line along the length of the wall. This practice
drastically reduced ground anchor interference problems on
the project.
With future projects, based on feedback with our
geotechnical and structure designers, there are other design
considerations that would help ease construction difficulties
on these large walls. For similar projects, it is recommended
to alternate the drill angle between adjacent piles to ease
constructability of the ground anchors. The inclination angle
should be alternated at adjacent piles along the length of the
wall (e.g., pile #1 = -15 degrees, pile #2 = -20 degrees, pile
#3 = -15 degrees, etc.), repeating the pattern at each level of
anchors, with the inclination angle remaining the same along
the height of the pile. Horizontal ground anchor spacing is
typically less than vertical anchor spacing, so that alternating
inclination angles along the length of the wall should allow
for greater flexibility with drilling tolerances and reduce
problems with ground anchor interference.
Another potential solution to the ground anchor
interference problem is to simply increase the span between
ground anchors. Wall designers may run into lagging span
limitations and problems with higher load ground anchors,
though, so this may not be feasible in all situations.
A separate problem at the Peregrine Slide Project
concerned ground anchor elongation variance. A standard
acceptance criterion for ground anchors is that the measured
elongation values during the stressing operation need to fall
within 80% of theoretical values. The 80% criterion verifies
future anchor performance by measuring the behavior of the
DEEP FOUNDATIONS * JAN/FEB 2022 * 17
January/February 2022

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