July/August 2023 - 84

and the rock was open hole drilled to the
required depth.
Facing rebar and shotcrete were
assembled and installed on the same
day excavations were performed to
reduce soil loss before the soil nails
were installed for each lift and
sequence section. After soil nails were
installed, each soil nail was locked off at
50% of the design load to create an
active condition. The lock off sequence
also was considered a form of modified
proof testing of each soil nail.
During construction, a daily on-site
inspection was performed to confirm
the soil nails and shotcrete facing were
installed in accordance with the
design. This allowed the use of the
observational method for making field
modifications to the design on an asneeded
basis in real-time through
communication with a representative
of the design engineer present on site.
The layout of the foundation and
excavation heights were modified
during construction due to varying
rock elevations and existing site
conditions not indicated on the plans.
Observing these changes, the wall
design was analyzed on a case-by-case
basis resulting in a modified layout
consisting of installing additional
nails or eliminating nails where they
were unnecessary.
Observation and documentation of
all soil nail testing, including four
verification tests and seven proof tests,
to confirm the soil and rock bond
stresses were consistent with the design
methodology. To confirm there was
limited movement of the wall and the
existing structure, an automated
deformation monitoring program was
implemented using two automated
robotic total stations accurate to 1
second vertical and horizontal, with an
accuracy of 0.05 in (1.3 mm) at 1,000 ft
(305 m) using optical survey prisms and
total station. Deformation monitoring
points installed on the existing structure
showed minimal northing, easting and
elevation fluctuations throughout
construction and after the completion of
the soil nail wall, confirming the soil
nail wall was performing as intended.
Conclusion
An efficient alternative underpinning
solution that was executed through
careful analysis and planning, the
design was coordinated using effective
communication between the architect,
engineer, contractor and earth support
application and capability to tolerate
large surcharges with little deformation,
an active soil nail wall is considered a
viable option when underpinning
existing foundations.
Acknowledgments
Eastern Excavation, the underpinning
contractor on the project, gave a
practical view during the design
process of what was achievable from a
construction standpoint. Turner
Construction served as the project's
general contractor with Eskew Dumez
Ripple as the project architect and Guy
Nordenson and Associates serving as
the project's structural engineer.
Through automated, near real-time deformation
monitoring, this active soil nail wall design proved to
be an effective and safe solution given the existing
and proposed foundation geometry and complex
ground conditions.
designer. Through automated, near realtime
deformation monitoring, this
active soil nail wall design proved to be
an effective and safe solution given the
existing and proposed foundation
geome t ry and compl ex ground
conditions. With onsite inspection, the
use of the observational method
illustrated the importance of exercising
engineering judgment throughout the
construction phase. The observational
approach allowed for a more organized
and efficient design and acknowledged
potential for modifying the design
during the construction process based
on in-situ conditions as well as
promoting a safer alternative than
traditional underpinning. Given its safe
Jonathan Ernst, P.E., is a project manager at
GTR. He has completed a bachelor's degree
in civil engineering and a master's degree in
geotechnical engineering from the University
of Massachusetts Lowell and has 10
years of experience in the deep foundation
design and foundation testing industry.
Les Chernauskas, P.E., is co-founder and
president of GTR. He is a registered Professional
Engineer in Massachusetts, New York,
Connecticut, Rhode Island and Michigan
and holds bachelor's and master's degrees in
civil engineering. His background includes
more than 30 years of geotechnical consulting
experience including foundation design,
earth support and temporary structures.
84 * DEEP FOUNDATIONS * JULY/AUG 2023
July/August 2023

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