Revisiting Okabe (1977), Cooke et al. (1981), Hansbo (1984), and Russo and Viggiani (1995) - The response of a wide pile group to load-Article

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The article revisits four monitored wide pile-group foundations to explain how load is distributed between perimeter and interior piles and what this means for design.<br /><br />Across the case histories, perimeter piles generally behaved much like single piles: shaft and toe resistance mobilized downward from the pile head. Interior piles behaved differently, engaging shaft resistance upward from the pile toe and showing little shaft resistance higher along the pile. Consequently, interior piles cannot be analyzed simply as individual piles loaded from the head.<br /><br />The observed load distribution depended on ground and foundation conditions. In the Okabe case, widespread soil subsidence caused downdrag on perimeter piles, shifting load toward the interior. Cooke and Russo and Viggiani found perimeter piles carrying greater loads under other conditions, although subsidence reduced that difference. Hansbo’s two adjacent buildings settled by about the same amount over 13 years despite having substantially different pile numbers and estimated loads per pile. This indicates that single-pile capacity is not a reliable basis for predicting the settlement response of a wide pile foundation.<br /><br />The article concludes that wide piled foundations should be designed primarily for serviceability and settlement, rather than assessed only by ultimate bearing capacity. Settlement reflects compression of the combined pile–soil “pier,” pile-toe penetration, and compression of soil below the pile toes. Analysis should account for factors such as subsidence, pile length and toe stiffness, raft rigidity, footprint ratio, and bearing walls. The author also argues that raft contact stress generally does not materially contribute to foundation bearing when piles occupy a substantial proportion of the footprint.

Keywords

wide pile-group foundations

load distribution

perimeter piles

interior piles

shaft resistance

toe resistance

downdrag

settlement analysis

serviceability design

pile-soil interaction