Keynote Lecture: US vs EU codes-Paper

Paper
Pdf Summary
The paper argues that piled-foundation design should prioritize predicting settlement rather than relying on nominal pile capacity and safety factors. It presents the Unified Method, which models pile response through shaft and toe force–movement relationships (t-z and q-z functions), pile compression, soil settlement, and the depth where pile and soil movements are compatible. The method considers applied loads and long-term ground subsidence; the forthcoming AASHTO update is expected to move closer to Canadian and US Corps of Engineers approaches.<br /><br />The author challenges the Eurocode practice of adding downdrag to structural load and subtracting it from geotechnical capacity. Based on full-scale observations, downdrag and drag force are environmental effects relevant chiefly to the pile’s structural strength, not reductions in its geotechnical bearing capacity.<br /><br />Pile groups require particular attention to the different behavior of interior and perimeter piles. Perimeter piles engage shaft resistance from the ground downward and generally respond more stiffly; interior piles engage soil from the pile toe upward, with strain compatibility governing load transfer. Raft rigidity and subsidence influence how loads shift between these piles, as well as settlement and bending response. Raft contact stress is part of the system’s response, not a reduction in the foundation’s applied load. Conventional group-efficiency factors are therefore considered unreliable substitutes for settlement analysis.<br /><br />The paper also emphasizes settlement of soil below pile toes and the combined stiffness of piles and intervening soil. It recommends using effective-stress parameters and accounting for site-specific geology, groundwater, fills, excavations, neighboring loads, and other changes over the structure’s life. Close collaboration between geotechnical and structural engineers is essential to ensure predicted total and differential settlements remain acceptable.
Keywords
piled-foundation design
settlement prediction
Unified Method
t-z and q-z functions
pile groups
downdrag
drag force
Eurocode
AASHTO update
effective-stress parameters