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p 26
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
2.7.2 Cylindrical Shear Method
The design equation for determining helical pile
capacity by the cylindrical shear method was
originally developed by Mitsch and Clemence
(1985) and later modified for simplicity.
The cylindrical shear method assumes the
development of a soil friction column (cylinder)
between the upper and lower helix plates along
with individual bearing of either the upper or
lower helix, depending upon loading direction.
The ultimate bearing capacity is then determined
by the summation of shear strength of the soil
cylinder, shaft adhesion/friction and end bearing
of either the upper or lower helix. For deep
cylindrical shear failure to occur, spacing of the
helix plates along the shaft is generally less than
or equal to 3 times the diameter of the leading
plate and the uppermost helix plate is embedded
to a depth of at least 5 diameters. Skin friction
along the shaft is generally ignored for shaft
sizes less than 6 inches in outside diameter.
Figure 2.7.2.a
illustrates the load transfer
mechanism for the cylindrical shear method in
compression loading.
The helical pile capacity by the cylindrical shear
method can be calculated as:
Q
u
= 2πRL(c+K
o
q’tanΦ)+A
h
(cN
c
+q’N
q
)
Where,
Q
u
= Ultimate Pile Capacity (lb)
R
= Average Helix Radius (ft)
L
= Total Spacing Between All Helix
Plates (ft)
c
= Cohesion at the Helix Depth (lb/ft
2
)
K
o
= Dimensionless At-Rest Earth
Pressure Coefficient
Φ
= Soil Friction Angle (degrees)
A
h
= Area of the Top or Bottom Helix
Plate (ft
2
)
q’
= Effective Vertical Overburden Stress
at A
h
(lb/ft
2
)
N
c
, N
q
= Dimensionless Bearing Capacity
Factors
Refer to Section 2.7.1 for discussions
regarding Bearing Capacity Factors and
Critical Depth.
Based upon previous research, the individual
bearing method and cylindrical shear method
should provide similar results at helix spacings
of 2.5D to 3.5D. FSI promotes the use of the
individual bearing method for determination of
pile capacity due to its relative simplicity and
since the original form from which this method
is derived is already widely accepted by the
geotechnical engineering community.
Figure 2.7.2.a
Cylindrical Shear Method