Behavior of Axially Loaded Drilled Shaft Foundations With Symmetric Voids Outside and Inside the Caging-Article

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The study examined how symmetric voids affect the axial strength and stiffness of drilled-shaft foundations. Researchers tested 15 quarter-scale reinforced-concrete shafts under axial compression: 12 with voids and three intact controls. The voids occupied 10–40% of the shaft cross-section and were located in different positions relative to the reinforcement cage: Type A crossed the concrete cover and longitudinal bars, Type B extended into the concrete core, and Type C affected only the cover.<br /><br />All void types reduced capacity and stiffness, with damage depending on void size and location. Type B voids were most detrimental: at 40% of the cross-sectional area, they reduced axial capacity by about 47%. Type A voids produced reductions of up to about 37%, while Type C voids caused reductions of up to about 28%. Type B was especially damaging because it weakened concrete confinement around the reinforcement. Type C shafts performed better because concrete cover continued to confine the longitudinal bars.<br /><br />Cracking generally began near the voids, where stress concentrations developed. Failure involved concrete crushing and shearing, and, for Types A and B, outward buckling of the steel bars. Calculations using ACI and AASHTO provisions overestimated the measured capacity of intact specimens by nearly 20%. The authors recommend applying a design reduction factor and adjusting it based on void location and size identified through nondestructive testing.

Keywords

drilled-shaft foundations

axial strength

stiffness reduction

symmetric voids

reinforced-concrete shafts

void location

void size

concrete confinement

axial compression

nondestructive testing