Proceedings-DFI/EFFC 11th International Conference on Piling and Deep Foundations, 2014, Stockholm, Sweden, (DFI)
Ultimate Shaft Friction, Displacement Response and Set-Up for Axially Loaded Piles in Clay
Kjell Karlsrud, Norwegian Geotechnical Institute, Norway
New semi-empirical design procedures for the determination of ultimate shaft friction and load-displacement response of axially loaded piles in clay are proposed. Hereunder also prediction of how the capacity will build up with time following the pile installation, primarily as result of reconsolidation. A main basis for developing the new procedures was detailed review and re-assessment of results from a series 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; α-approach using the normalised undrained strength based on direct simple shear tests as a main correlation parameter, and β-approach using 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. It is recommended to use both methods in design practice, and select design values 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 post-peak reduction. The proposed procedure for predicting the time for set-up is based on linear radial consolidation theory. The main input parameters are the insitu permeability and virgin modulus number (or virgin compression index) of the undisturbed clay. The same basic formula and correlations are applicable to both open and closed-ended or partially plugging piles.
|article #1893; publication #100 (IC-2014)|