The Field Behaviour of Drilled and Grouted Piles in Rock for Offshore Foundations-Paper

Paper

Pdf Summary

The study reports field testing of 14 steel piles drilled and grouted into sandstone rock sockets in Northern Brittany, France. Socket diameters ranged from 0.33 to 1.30 m. The tests included monotonic tension and compression loading, plus one-way axial cyclic loading, to inform offshore wind-turbine foundation design.<br /><br />Results showed that unit shaft resistance at failure decreased as socket diameter increased. This trend is consistent with resistance developing through constrained dilation at the grout–rock interface: larger sockets generate less dilation-induced normal stress for a given interface roughness and rock-mass stiffness. The Seidel and Collingwood design method, which accounts for roughness, stiffness and diameter, agreed with the measured capacities. In contrast, conventional methods that relate shaft resistance mainly to unconfined compressive strength may overestimate capacity when applied to large piles based on small-diameter test data.<br /><br />Socket roughness was strongly affected by drilling technique but was similar for the 0.70 and 1.30 m sockets drilled using comparable methods. The smaller sockets, drilled differently, were smoother.<br /><br />Cyclic tests produced both stable and metastable responses. Stable behaviour involved limited displacement accumulation and no failure within 1,000 cycles; metastable behaviour involved continuing accumulation that could lead to failure over hundreds of cycles. The proposed stability boundaries were somewhat lower than comparable boundaries for piles in sand, suggesting greater cyclic sensitivity in the tested rock.<br /><br />The findings support incorporating socket diameter, roughness and rock-mass stiffness into design, while highlighting the need for further testing across other rock types and strengths.

Keywords

steel piles

rock sockets

sandstone

shaft resistance

socket diameter

monotonic loading

cyclic loading

offshore wind turbine foundations

interface roughness

design method