Effect of Interface Conditions on the Response of Laterally-Loaded Monopiles in Sand-Paper

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The paper investigates how pile–soil interface friction affects the behaviour of laterally loaded offshore wind-turbine monopiles in sand. Three-dimensional finite-element analyses were conducted for two full-scale pile geometries with different embedment lengths, using a calibrated model of Dunkirk sand and interface friction values representing critical-state, residual and frictionless conditions.<br /><br />For realistic interface-friction values, the effect on overall load–displacement response and pile deflection was small. For the shorter pile, differences in lateral force were generally limited to about 1–2% at working and ultimate loads, though they reached around 5% at intermediate displacements in denser sand. For the longer pile, differences were below 2%. Thus, the typical reduction from critical-state to residual interface friction has little practical influence on global pile response.<br /><br />The analyses also examined the four soil-reaction components used in the PISA design approach. Lower interface friction slightly reduces the distributed moment generated by shaft shear, while other reactions—particularly those at the pile base—redistribute to partly compensate. In the extreme frictionless case, shaft shear and its distributed-moment contribution disappear, reducing pile capacity by approximately 25–40%, depending on pile geometry and deformation mode. Other soil reactions increase but do not fully offset this loss.<br /><br />The findings support the assumption that realistic interface-friction variations have a marginal effect on laterally loaded monopiles in sand, unlike their potentially substantial influence on axially loaded piles.

Keywords

pile-soil interface friction

laterally loaded monopiles

offshore wind turbines

sand

finite-element analysis

embedment length

load-displacement response

pile deflection

PISA design approach

shaft shear