Drilled shafts are often designed by representing soil layers asideal geomaterials, such as clay, sand or rock and using simple correlation factors to convert measured strength values into values of unit shaft and base resistance. The effects of apparently minor inclusions in layers of otherwise uniform soil and soft rock on load transfer, particularly in shaft resistance, are addressed. Also considered are the effects of the use of polymer drilling slurry and artificial roughening of the borehole on load transfer. Data from the loadtesting of six full-sized drilled shafts at three sites indicated that thin sandstone layers could increase load transfer by one-third in dense sand anad that thin bentonite layers could decrease load transfer by two-thirds in clay-shale. No adverse effects could be detected in shaft load transfer by the use of polymer drilling slurry, and the rifling of a borehole wall in clay-shale increased the shaft load transfer by about 40 percent over that in an unrifled shaft.
Abstract