A Geometry-Aware Two-Stage Semi-Analytical Method for Installation Resistance and Uplift Capacity of Premixed Helical Rigid-Core Composite Piles in Clay

Installation through fresh cement–clay and uplift after curing require distinct resistance descriptions for premixed helical rigid-core composite piles (PHRCP). We develop a semi-analytical framework combining geometry-strength kernels, identifiable effective parameters, and transparent reference-to-target elimination. Installation and service components are calibrated separately. Model tests included one PHRCP and one ordinary helical pile (HP) at each helix diameter, 66 and 110 mm. PHRCP reduced depth-averaged torque by 34.0% and 32.2% and thrust by 41.7% and 61.8%; measured uplift capacity increased by 136% and 51%. Thrust comparisons are conditional on adopted channel corrections. Coefficients and enlargement δ = 46.6 mm identified at 66 mm were frozen at 110 mm. The transferred diameter was 186.6 mm (−1.4% against the post-test mean); the conditional external-envelope-based capacity estimate was 1896.5 N versus 1792.4 N measured (+5.8%). HP capacity was underestimated by 19.3%, while installation peak agreement did not ensure curve reproduction. Target observations informed model-form selection, so the assessment is a retrospective parameter-transfer check. The formulation makes geometric dependencies and calibration limitations explicit. Single specimens, uncertain strength calibration, post-test geometry, and unverified failure assumptions preclude general predictive or field reliability claims.