Applied loads may be separated into
vertical and horizontal components that can be evaluated by soil-structure
interaction analyses. Deep foundations must be designed
and constructed to resist both applied vertical and lateral loads. The
applied vertical load Q is supported by soil-shaft side friction Qs and base
resistance Qb . The applied lateral load T is carried by the adjacent lateral
soil and structural resistance of the pile or drilled shaft in bending.
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The applied loads should be sufficiently less than the ultimate bearing
capacity to avoid excessive vertical and lateral displacements of the pile or
drilled shaft. Displacements should be limited to 1 inch or less.
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Factors of safety applied to the ultimate bearing capacity to obtain
allowable loads are often 2 to 4. FS applied to estimations of the ultimate bearing
capacity from static load test results should be 2.0. Otherwise, FS should be at
least 3.0 for deep foundations in both clay and sand. FS should be 4 for deep
foundations in multi-layer clay soils and clay with undrained shear strength Cu >6
ksf.
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Development of soil-shaft side friction resisting
vertical loads leads to relative movements between the soil and shaft. The maximum
side friction is often developed after relative small displacements less than 0.5
inch. Side friction is limited by the adhesion between the shaft and the soil or
else the shear strength of the adjacent soil, whichever is smaller.
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Side friction often contributes the most bearing capacity in practical
situations unless the base is bearing on stiff shale or rock that is much stiffer
and stronger than the overlying soil.
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Side friction is hard to accurately estimate, especially for foundations
constructed in augered or partially jetted holes or foundations in stiff, fissured
clays.
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Failure in end bearing normally consists of a punching
shear at the tip. Applied vertical compressive loads may also lead to several
inches of compression prior to a complete plunging failure. The full soil shear
strength may not be mobilized beneath the pile tip and a well-defined failure load
may not be observed when compression is significant.
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