Large Diameter Double Underreamed Drilled Shafts

Abstract

A double underreamed drilled shaft with 5 ft (1.5 m) diameter bells and 2.5 ft (0.8 m) diameter shaft was instrumented and load tested. Results of the load test indicate that the underreams act independently at very small displacements and that settlement of only about 0.2% of the base diameter causes the underreams to act as a unit. The ultimate resistance of the unit consists of bearing on the lower base and shear resistance on the cylinder circumscribing the bells. A double underreamed production drilled shaft with 12 ft (3.6 m) diameter bells was instrumented to determine long term settlement and to verify the support mechanism. Both load test and production drilled shafts were designed for a bearing pressure slightly below the preconsolidation pressure. Settlement at the working load of the production drilled shaft was only 1.25% of the base diameter or 1.8 in. (46 mm) indicating that long term settlement at the working load is within tolerable limits.