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Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
2.9.2.1 Temporary and Permanent
Wall Facing
Helical soil nail walls can be categorized as
temporary or permanent, and the design of the
wall facing and nail head connection details will
vary based upon this determination.
Whether
the soil nail wall is temporary or permanent,
the wall facing and helical soil nail connection
detail must be completed and/or approved by
the engineer of record.
Helical soil nail walls are used most often in
temporary shoring applications, with reinforced
shotcrete the most common temporary wall
facing material. Shotcrete is concrete conveyed
through a hose and projected through a nozzle
at high velocity onto a working surface. The
shotcrete is applied/sprayed in thin lifts until the
design thickness requirement is met for the wall.
For temporary wall applications, the shotcrete
is typically applied to a thickness of 3 to 4
inches. Internal reinforcement of the shotcrete
may consist of welded wire fabric (WWF), steel
reinforcing bars (rebar), or fiber reinforcement.
WWF with rebar walers at the nail heads is
typically favored due to ease of installation.
Permanent helical soil nail walls may either have
an additional thickness of shotcrete applied
or another facing attached to the temporary
shotcrete layer. For permanent soil nail walls
with shotcrete facing, the typical wall thickness
varies from 6 to 12 inches, not including the
thickness of the temporary facing. Cast in place
and precast concrete facings can also be used
in conjunction with the temporary shotcrete wall
facing. Facings can be attached to the shotcrete
wall to form decorative facades.
2.9.2.2 Limit States
The design of the helical soil nail wall must
consider two distinct limiting conditions;
Strength Limit States and Service Limit States.
The Strength Limit States refer to failure of
the system due to loading forces greater than
the strength of the system or its individual
components. Specifically, the following potential
failure modes must be evaluated:
• External failure modes
• Internal failure modes
• Facing failure modes
External failure modes include global stability,
sliding and bearing failure. Internal failure modes
include soil nail pullout failure, soil nail tensile
failure and soil nail shear failure along the failure
plane. Facing failure modes include flexure failure,
punching shear failure and head stud failure.
The service limit states do not include failure of
the structure, but rather consider serviceability
issues such as wall deformation, wall settlements
or cracking of the facing.
For further information related to designing for
these potential failure modes, please refer to
FHWA Geotechnical Engineering Circular No. 7
(Lazarte, Elias et al. 2003).