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p 46
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
2.12.1.2 Crowd
During the installation of a helical pile or tieback,
axial force or “crowd” is applied to the pile/
tieback shaft to advance the helix plates into the
soil. The density or stiffness of the soil dictates
the amount of crowd necessary to advance
the pile to a depth where the helix plates can
then provide the downward thrust. Multi-helix
pile configurations often install easier than
single-helix configurations due to the thrust
provided by the additional helix plates. At a
depth typically just a few feet below the surface,
little to no external force is necessary unless
deeper, dense soil layers or obstructions are
encountered. Additional crowd may be required
to either penetrate the dense layers or fully
embed the helix plates into dense bearing soil.
In soft soil conditions, it is important not to over-
crowd or restrain the advancement of the pile.
Applying the proper crowd is critical to maintain
the penetration rate and minimize disturbance or
mixing of the soils, especially within the final 3
to 5 feet of installation prior to pile termination.
Installation equipment not only needs to be sized
correctly to provide the proper hydraulic flow
and hydraulic fluid pressure for the drive head,
but also to provide the proper crowd for pile
advancement. The lack of appropriate machine
weight during installation into dense soils or
weathered bedrock may limit pile penetration,
resulting in less than anticipated tensile or
compressive capacities.
2.12.1.3 Penetration Rate
Helical piles or tiebacks should ideally be
advanced into the soil at a rate equal to the pitch
of a properly formed, conforming helix plate; i.e.,
3 inches per revolution. ICC-ES AC358 further
states that the pile advancement shall equal or
exceed 85 percent of the helix pitch per revolution
at the time of final torque measurement. Crowd
may be required to maintain adequate pile
penetration or advancement. Installation speeds
should be limited to less than 25 revolutions
per minute (rpm) within the last several feet of
pile installation to minimize soil disturbance. It
is good practice to further reduce installation
speeds to 10 rpm or less within the final 3 to
5 feet so the operator can concentrate on pile
alignment, crowd and rate of advancement.
Installation speeds may be further restricted by
soil conditions or operating equipment.
