Drilled Shaft Clay-Shale TZ (Aurora and Reese) [Aurora and Reese, 1976]

[Aurora and Reese, 1976] loaded four instrumented drilled shafts to failure in clay-shale at two Texas sites (Montopolis and Dallas). The shafts were under 30 ft long and penetrated about 5 ft into the clay-shale; the peak load transfer was mobilized at movements of 1/4 in or less, and the construction method governed the capacity. The design conclusions of Chapter 9 give the side resistance as an $α$-method:

$$\tag{1} t_u = \alpha c_Q$$

where,

$t_u$ = ultimate unit side resistance $c_Q$ = shear strength of the clay-shale from a quick triaxial test with confining pressure about equal to the effective overburden pressure $α$ = shear strength reduction factor: 0.5 for the slurry displacement or casing method, 0.75 for the dry method

Where only dynamic penetration data are available, the report suggests, with caution, $c_Q$ (tsf) = $N_{THD}$/75 for the Texas cone penetrometer (Eq. 9.2).

t-z Curve

The report does not propose a load-transfer shape. OpenBrIM therefore applies the normalized curve of the Drilled Shaft Clay model of [O'Neil et al., 1999], with $t_u$ from Eq. (1) ($Z = z/D × 100$):

$$\tag{2} t = t_u(4.92Z) \ (Z \leq 0.12), \qquad t = t_u\frac{Z}{0.0951+0.893Z} \ (0.12 < Z \leq 0.74), \qquad t = t_u(0.9789-0.1158(Z-0.74)) \ (0.74 < Z \leq 2.0), \qquad t = 0.833t_u \ (Z > 2.0)$$

The recommendations apply to shafts with the geometry of the tests and to clay-shales with quick-triaxial strength below 10 tsf.

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[Aurora and Reese, 1976] Aurora, R. P. and Reese, L. C. (1976). Behavior of Axially Loaded Drilled Shafts in Clay-Shales. Research Report 176-4, Center for Highway Research, The University of Texas at Austin. (Summarized in Aurora and Reese, 1977, Proc. 9th ICSMFE, Tokyo, 371–376.)