Ultimate Shaft Friction and Load-Displacement Response of Axially Loaded Piles in Clay Based on Instrumented Pile Tests

New semiempirical design procedures for the determination of ultimate shaft friction and load-displacement response of axially loaded piles in clay are proposed. A main basis for developing the new procedures was detailed review and reassessment of results from a series of pile load-test programs published over the past 30 years on piles well instrumented to monitor soil and pore pressures as well as load distribution along the pile shafts. Two new alternative procedures are proposed for predicting the ultimate shaft friction: an approach, which uses the normalized undrained strength based on direct simple shear tests as a main correlation parameter, and a approach, which uses the overconsolidation ratio (OCR). Both approaches include the effects of the plasticity index of the clay, Ip, which is shown to have an especially large impact on the correlations when Ip<25%. In practical design, it is recommended that both methods be used and design values be selected for the ultimate shaft friction that represents an average for the two methods. Pile diameter, length, or stiffness or whether the pile is open- or closed-ended seem to have little impact on the shaft friction. However, a moderate effect of pile length or flexibility on the total ultimate pile capacity comes out of the proposed t-z curves, which include some postpeak reduction.