Effects of Installation Parameters of Drilled Displacement Piles on In Situ Stress Conditions in Stiff Clays-Paper

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The study examines how installation parameters affect soil stresses and capacity of drilled displacement piles (DDPs) in stiff to hard clay. Unlike bored piles, DDPs displace rather than remove soil, so auger geometry, rig capability, rotation, torque, and penetration rate influence the surrounding ground.<br /><br />Two 4 m test piles with identical 450 mm augers were installed at a Queensland clay site using rigs of different capacities. The weaker rig could not maintain the calculated minimum penetration rate of 1.8 m/min; the stronger rig, using automated control, sustained approximately 2.0 m/min. Cone penetration tests conducted before and seven days after installation measured changes in soil strength.<br /><br />Adequate penetration increased effective stresses in the clay by about 43%. Slow, inconsistent penetration remoulded and sheared the clay, reducing stresses by approximately 16%. Capacity estimates reflected these effects: the properly installed pile was predicted to reach about 950 kN, compared with 680 kN for the inadequately installed pile and 720 kN under undisturbed ground conditions.<br /><br />The authors conclude that installation rate is a major determinant of DDP performance in stiff clays. They recommend specifying minimum rates, accounting for installation conditions in design, selecting rigs capable of maintaining those rates, and conducting pre-construction load tests to verify site-specific performance.

Keywords

drilled displacement piles

stiff clay

installation parameters

penetration rate

soil stress

pile capacity

auger geometry

rig capability

cone penetration test

load test