A Comprehensive Comparison and Analysis of In-Situ Dynamic and Static Tests of a Cased Drilled Shaft-Article

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The study evaluates the axial capacity and soil–shaft interaction of temporary cased drilled shafts in layered Zagreb clays. The foundation system supports a 19-story tower and comprises 34 shafts, 1.5 m in diameter and 22–24.5 m long. Researchers compared a full-scale static test on a 0.6 m diameter shaft with dynamic testing on a 1.5 m shaft, semi-empirical capacity methods, and FLAC3D simulations.<br /><br />Static testing indicated an ultimate capacity of about 3,000 kN by Davisson’s criterion, with most resistance provided by shaft friction. Strain-gauge data became unreliable at higher loads, possibly because of bending, creep, or measurement corrections. The O’Neill–Reese and Chen–Kulhawy adhesion-factor methods matched results in the softer upper clays but underestimated shaft resistance in deeper, stiffer clays by roughly 1.5–2.5 times. The study suggests that uncertain shear-strength estimates and local clay variability may explain this difference.<br /><br />Dynamic testing estimated total capacity at approximately 11,390 kN. FLAC3D simulations reproduced key test responses and indicated that interface adhesion had less influence on dynamic behavior than on static capacity. Overall, static and dynamic tests gave broadly comparable capacity estimates when differences in test scale and interpretation are considered. The authors caution that Eurocode 7 strength factors should not be combined uncritically with adhesion-factor methods; adhesion should be determined using unfactored undrained shear strength. The findings offer guidance for drilled-shaft design in Zagreb and similar clay deposits.

Keywords

drilled shafts

axial capacity

soil-shaft interaction

Zagreb clays

temporary casing

static load test

dynamic testing

shaft friction

FLAC3D simulation

Eurocode 7