The Effect of Construction Technique on Axial Capacity of Drilled Foundations

A program of field loading tests was conducted to measure the axial response of drilled foundations constructed using a variety of different drilling techniques. The research was performed at the Auburn University National Geotechnical Experimentation Site at Spring Villa, Alabama in Piedmont geology composed of silty soils formed by weathering of parent metamorphic rocks. A total of 10 drilled shafts (0.9 m diameter by 11 m deep) were constructed using techniques including dry construction with casing advanced ahead of the hole and with drilling slurry composed of polymer fluids and mineral (bentonite) fluids. The results demonstrate the· great potential influence that differing construction techniques may have on the load transfer in side shear of drilled foundations. The mineral slurry resulted in significantly lower side shear relative to the other techniques. INTRODUCTION The axial capacity of drilled foundations is affected by the conditions at the soil/concrete interface immediately adjacent to the shaft. O'Neill (2001) has shown that the effect of details related to construction can be significant and includes such factors as stress relief, moisture migration from the concrete to the geomaterial, borehole roughness, and borehole smear. The relative importance of various details are dependent upon geological conditions; for instance, soft cohesive rock appears to be particularly sensitive to construction techniques. The influence of construction drilling fluids on the resulting axial capacity of drilled shaft foundations is reviewed by O'Neill and Reese (1999). Some case histories