This paper examines factors controlling the ultimate shaft friction developed on buried piles in normally consolidated clay-sand mixtures and explores the relationship between this friction and the cone penetration test (CPT) end resistance. Results from tension load tests on buried piles in a laboratory pressure chamber and geotechnical centrifuge are presented. Variable rate CPTs combined with Rowe cell, undrained simple shear and interface direct shear tests are used to characterise the mixtures. It is shown that the measured drained CPT end resistances could be predicted using parameters assessed from the laboratory tests and a simple expression derived from cavity expansion theory in an elastic Mohr-Coulomb soil. The tension pile tests indicate how changes in radial effective stresses during loading can be inferred from interface shear tests and these combined with the cavity expansion analysis allow derivation of a relationship between shaft friction (τf) and CPT end resistance (qt). This expression reveals the significant effects on the qt/τf (= βc) ratio of soil stiffness, stress level, dilation and drainage during cone penetration and is shown to be consistent with a recently proposed empirical formula for bored piles.