Driven Pile Clay TZ (Coyle and Reese) [Coyle and Reese, 1966]

From laboratory tests and full-scale field tests on instrumented steel friction piles in clay, [Coyle and Reese, 1966] correlated the load transferred to the soil with the pile movement and presented the load-transfer ratio as a function of displacement:

$z$ (in)	0.00	0.01	0.02	0.04	0.06	0.08	∞
$t/t_u$	0.00	0.18	0.38	0.79	0.97	1.00	1.00

where,

$t$ = unit side resistance (skin friction) $t_u$ = ultimate (nominal) unit side resistance, entered as the ultimate shear stress of the layer $z$ = vertical displacement of the pile

A very small displacement, under 0.1 in, mobilizes the full side resistance.

The journal article is not yet on file, so the table is implemented as it is reported in the secondary literature; it will be checked against [Coyle and Reese, 1966] once the original is obtained. API RP 2A-WSD (2007), Section 6.7.2, also refers to this work for t-z curves in clay.

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[Coyle and Reese, 1966] Coyle, H. M. and Reese, L. C. (1966). Load transfer for axially loaded piles in clay. Journal of the Soil Mechanics and Foundations Division, ASCE, 92(SM2):1–26. (Article not on file.)