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p 25
Chapter 2
Helical Foundation Systems
CHAPTER 2
HELICAL FOUNDATION SYSTEMS
2.7.1.1 Critical Depth
In granular soils the helical pile capacity is largely
a function of the vertical effective overburden
stress at the helix plate depth. Therefore,
one may expect that the pile capacity would
increase without bound as the effective stress
increases with increasing pile depth. According
to the Individual Bearing and Cylindrical Shear
Method equations, the helical pile capacity
should increase by simply extending the pile
deeper into granular soils. In reality, there is a
critical depth within uniform granular soils where
a further increase in vertical effective stress
results in little to no increase in the end bearing
capacity of the pile. Certainly, if the strength of
the granular soil increases with depth, you would
expect an increase in pile capacity, but not due
to an increase in the overburden stress. This
concept is well-documented in many foundation
design textbooks and design manuals.
Critical depth may range from 10D to 40D (where
D is the largest helix plate diameter), depending
upon the relative density and position of the
water table. FSI recommends critical depths of
20D to 30D be considered for design purposes.
For example, if the helix plate depth is greater
than an assumed critical depth of 20D, limit the
vertical effective stress at the helix plate to that
value corresponding to the critical depth of 20D.
Figure 2.7.1.b
Recommended Bearing Capacity
Factors N
c
and N
q
versus Soil Friction Angle
