... Theory and Application', Rankine Lecture, Geotechnique 34(2), 365-415 FRICTION COEFFICIENTS FOR PILES IN SANDS AND SILTS 20 21 22 23 24 677 Ridley, A M and Jardine, R J (1992), 'Ring-Shear Interface ... reported in this paper The controlling frictional parameter for piles in sands and silts is the critical state interface angle o~s o~s does not depend on relative density and, for a given interface ... show that FRICTION COEFFICIENTS FOR PILES IN SANDS AND SILTS 675 the ratio J( c = (7~c/ (7~o' which quantifies the radial effective stresses set up by installing displacement piles, increases...