Axial Capacity of Drilled Shaft in Overconsolidated Clay: Surprising Installation Techniques Impact-Paper

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The study examines how construction methods affect the axial capacity of drilled shafts in stiff to hard, highly plastic, fissured Potomac Formation clay in northern Virginia. The shafts were 0.75 m in diameter and 21 m long, and were designed primarily to rely on side resistance.<br /><br />Two bidirectional static load tests showed substantially lower side resistance than expected from local experience and comparable projects. In the first test, shafts were installed using segmental casing advanced ahead of excavation, with a water–polymer mixture sprayed on the casing to ease installation. Estimated upper-shaft side resistance was about 25 kN/m², with localized values up to approximately 48 kN/m². For the second test, polymer was eliminated and casing oscillation during removal was increased to roughen the shaft surface. Side resistance improved, reaching about 53 kN/m² in some zones, but remained more than 50% below the anticipated 100–160 kN/m² range. The second test mobilized a combined side and base resistance of 3,900 kN; an adjusted capacity of 3,000 kN was proposed at 25 mm displacement.<br /><br />The authors attribute the reduced side resistance mainly to soil remolding at the concrete–clay interface during casing installation and removal. Reworked clay found on the removed casing supported this explanation. Polymer use may have caused further remolding and contributed to the poorer first-test results.<br /><br />For shafts in very stiff to hard, high-plasticity clay where segmental casing is required, the authors recommend reducing assumed side-resistance values and avoiding polymer on the casing. They also advise completing load tests before production construction begins or using a higher factor of safety. Whether side resistance increases with time remains unknown because a planned retest was not conducted.

Keywords

drilled shafts

axial capacity

side resistance

Potomac Formation clay

segmental casing

load testing

soil remolding

polymer mixture

construction methods

northern Virginia