Multi-Source Geotechnical Characterization and Axial Bearing Capacity of Cast-in-Place Bored Piles in Alluvial Soils

Abstract Axial design of bored piles requires combining measurements that sample different soil volumes, strain regimes, and failure processes. This paper compares SPT resistance, CPTU soundings, cross-hole seismic profiles, laboratory strength data, and static compression tests of eight 800 mm diameter bored piles (18.2–22.2 m long) at the Dhi-Qar Combined Cycle Power Plant, southern Iraq. SPT, Vs, and borehole descriptions all identify a common stiffness transition at 13–16 m, with median SPT N rising from 12 to 56 and mean Vs from 217 to 517 m/s. However, the apparent SPT-Vs correlation (pooled r = 0.835) is largely a depth artifact: both variables are strongly depth-dependent, and depth-adjusted descriptive correlations fall to r = 0.45 after linear depth adjustment and r = 0.39 after within-profile demeaning. Because the observations are nested within four profiles, these correlations are descriptive rather than independent inferential tests. Penetration-implied undrained strength is 2.40–4.21 times laboratory strength; known biases explain only about half of this gap. Small-strain stiffness ratios (6,100-6,800) confirm that seismic stiffness cannot substitute for strength parameters. Proof tests at twice service load produced head movements of 2.50–7.53 mm (0.31–0.94% of D), of which 13–70% is elastic pile shortening rather than ground deformation. The eight-pile dataset is too small for reliable resistance-factor calibration. The study's contribution is methodological: multiple sources can agree on stratigraphy while disagreeing on design parameters, and reconciliation requires explicit treatment of spatial support, strain regime, and transformation uncertainty.