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Helical anchor design for excavations.
Helical anchor design is governed by simple geotechnical engineering
principles. In excavations, helical anchor design is controlled
primarily by the bearing resistance on each helical plate. In
some cases, the shaft resistance can also be included. Since the
helix shaft is in tension the helical anchor does not have to
be examined in buckling. In principle the ultimate capacity of
a helical anchor is determined as the most critical of the two
basic modes of failure:
a) Individual bearing method: This is the sum of the bearing
capacity at each helical plate. In this mode the total ultimate
pullout resistance of a helical anchor is determined as:
Qtotal = sum {Ahelix (9 c + gamma' x Depth x Nq)}
The geotechnical helical anchor capacity should be smaller than
the individual structural capacity of each plate.
b) Cylinder strength method: In this mode the capacity
is calculated as the shear resistance of a cylinder of soil contained
by the helical plates, plus the bearing resistance of the helical
plate closer to the excavation. The cylinder method can be applied
only in cases where the helical anchor has atleast two plates.
Qcylinder = AhelixPlate1 (9 c + gamma' x D x Nq) + side resistance
In practice the individual bearing method and the cylinder strength
method have generally produced smaller geotechnical capacities
than actually experienced by pullout tests. For this reason, a
number of researchers have proposed the torque installation method
that relates the pullout resistance to the installation torque
which can be measured during each helical anchor installation.
c) Torque correlation method: The Torque Correlation Method
is an empirical method that distinguishes the relationship between
helical pile capacity and installation torque and has been widely
used since the 1960's. The process of a helical plate shearing
through the soil or weathered bedrock in a circular motion is
equivalent to a plate penetrometer test. The method gained notoriety
based on the study performed by Hoyt and Clemence (1989). Their
study analyzed 91 helical pile load tests at 24 different sites
within various soil types ranging from sand, silt and clay soils.
They demonstrated the direct correlation of the installation torque
of a helical pile to its ultimate capacity in compression or tension.
The common denominator discovered from the study was a parameter
referred to as the torque correlation factor, Kt.
The equation is:
Pu = Kt T
Where:
Pu is the ultimate capacity of the helical pile or anchor [lb
(kN)].
Kt is the empirical torque factor of the central shaft of the
pile [ft-1 (m-1)].
T is the final installation torque [ft-lb (m-kN)].
It's important to point out that the tests analyzed by Hoyt and
Clemence (1989) were in tension. It was shown in sub-sequential
studies that the tension capacity of helical piles was 16 to 33
percent less than the measured compression capacity. The difference
is attributed to the fact that the lead helical plate is bearing
on relatively undisturbed soil in compression applications. In
tension applications, the leading and trailing helical plates
are bearing on soil affected by the installation of the helical
plates. It has become common practice to use the same torque correlation
factor for a helical pile of the same size for tension and compression
and ignore the slight increase in compression capacity. This creates
a more conservative compression capacity for helical piles when
compared to the Individual Bearing Method. Also unlike the Individual
Bearing Method, the number of helical plates on a pile is completely
independent of the piles capacity based on the Torque Correlation
Method.
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