The test specimen was comprised of a 1.8 m (6 ft) diameter column extending 12.2 m (40 ft) and a 2 m (6.5 ft) diameter shaft extending 14.6 m (48 ft) below the ground line, as shown in Figure 3. Steel reinforcement was comprised of 36 #14 longitudinal bars with an average yield and an ultimate tensile strength of 490 MPa (71 ksi) and 703 MPa (102 ksi) respectively and shop-welded #8 hoops at 152 mm (6 inches) spacing with average yield and ultimate tensile strength of 496 MPa (72 ksi) and 738 MPa (107 ksi), respectively. The longitudinal and transverse steel ratios are roughly 2 percent and 0.7 percent, respectively. Normal-weight ready-mix concrete was used in the specimen, with an average tested cylinder compressive strength of 42 MPa (6.1 ksi).
Aim of monitoring:
The test shaft was extensively instrumented to enable high-precision section curvature measurements and to measure contact pressure at the soil-shaft interface around the shaft perimeter. The testing was done to characterize the soil-shaft interaction across a wide displacement range to review the adequacy of existing design guidelines. The instrumentation monitored the component stresses of resistance and the effects of nonlinear soil resistance to relative displacements between the soil and shaft.
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The observed behavior suggests that large-diameter shafts could have added capacity from side friction, which the current design models do not incorporate.