TM 5-018-7
duce both uplift and downdrag forces. Vermiculite,
foundation by positive skin friction and can cause
pea gravel, or other pervious materials that will allow
large tensile stresses to develop in the shaft. Negative
access of water to the expansive material should be
skin friction, which adds to the structural loads and in-
avoided.
creases the end bearing force, develops when the sur-
rounding soil moves down relative to the shaft, Nega-
d. Design. The heave or settlement of the founda-
tive skin friction is associated with the settling of the
tion usually controls the design and should not exceed
adjacent fill, loading of surrounding compressible
specified limits set by usage requirements and toler-
soils, or lowering of the groundwater level.
ances of the structure. The design of drilled shafts, in
addition to bearing capacity, should consider the meth-
ated by the equation
od of construction, skin resistance, and uplift forces.
The computer program HEAVE (WES Miscellaneous
(6-2)
Paper GL-82-7) may be used to help determine the
movement of drilled shafts for different lengths and
adhesion, tons per square foot
diameters of the shaft, and the diameter of the under-
ratio of horizontal to vertical effective
ream for different loading forces.
stress
vertical effective stress, tons per square
small relative displacements between the shaft and the
foot
adjacent soil. Positive (upward directed) skin friction,
angle of friction between the soil and
which helps support structural loads, develops when
shaft, degrees
the shaft moves down relative to the soil. Uplift of ad-
jacent swelling soils also transfers load to the shaft
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