LOAD CAPACITY OF DRILLED SHAFTS
The load capacities of drilled shafts are calculated similarly to driven piles except that the empirical adhesion, friction, and end bearing factors are different.
TSA (fi ne-grained soils):
Adhesion (O’Neill and Reese, 1999)
where pa is atmospheric pressure (101 kPa).
- End bearing:
where z is the embedded depth of the drilled shaft in the end bearing layer and (su)b is the average undrained shear strength over two diameters below the base. The (su)b value must be obtained from field or laboratory tests within 2 pile diameters below the pile tip. If (su)b , 25 kPa, the value of Nc should be reduced by one-third.
Coarse-grained soils (O’Neill and Reese, 1988):
z 5 depth (m) measured from the ground surface to the middle of the soil layer.
Gravels and sandy gravels (GW and GP)
- End bearing (Quiros and Reese, 1977, correlation for cemented sand)
The soil near the top of the drilled shaft is subjected to environmental and construction effects, while the soil just above the base may develop tensile cracking. Consequently, the upper 1.5 m of the shaft and one pile diameter above the base are ignored in calculating skin friction for drilled shafts . The b for drilled shafts in coarse-grained soils have been obtained from back calculations from load tests on 1-m-diameter drilled shafts in cemented sand at particular locations (Texas Gulf Coast region and Los Angeles, California). They are not related to any soil parameters. They have to be used with careful judgment based on experience.