Model Tests to Determine Properties of Concrete and Aggregates in Verification Core Holes at the Bottom of Drilled Shafts

Verification core holes are important tools in ascertaining the properties of the bed rock at the bottom of drilled shafts. However, it is unknown whether verification core holes fill during concrete placement; if they fill, the mechanical properties of the filling are unknown. The diameters of the core holes are in the 15-30 cm range, and core holes are at least 1.5 m long; empty core holes or core holes filled with weak and deformable materials may significantly decrease the capacity of drill shafts. The paper describes the experiments performed to answer these two questions by using 15- and 30-cm verification core hole models, and drilled shaft depths between 9- and 30-m depth. The smallest slump allowed by the Texas Department of Transportation (TxDOT) was used: 15 and 18 cm in dry and wet conditions, respectively. In dry conditions, verification core holes completely filled with concrete, whose compressive strength exceeded the TxDOT specifications. The compressive strength of the concrete obtained in the 30-cm core holes was higher than the concrete strength in the 15-cm core holes by 60% to 150%. In wet conditions, unless the tremie or pump pipe was inserted into the bottom of the core hole, the bottom half of the verification core filled with uncemented gravel-sand mixture (angle of internal friction, φ=52°), while the upper half of the verification core filled with weakly cemented material whose P-wave velocity was found to be 610 m/s.