Optimization technique to determine the p-y curves of laterally loaded stiff piles in dense sand

Lateral loading is often the governing design criteria for piles supporting offshorewind turbines and with the recent growth of this sector, the reliability of traditional design approaches is receiving renewed interest. To accurately abeb the behavior of a laterally loaded pile requires a detailed understanding of the soil reaction that ismobilized as the lateral deflection of the pile occurs. Currently, the p-y curvemethod iswidely adopted tomodel the response of laterally loaded piles. The limitations of existing p-y formulations arewidely known, and there is acceptance that load tests on large-diameter stiff monopiles are urgently required to formulate appropriate designmethods. However, interpretation of the data frominstrumentation placed on stiffmonopiles is not straightforward. This paper proposes an optimization technique to derive the soil reaction profile along the shaft of instrumented piles, fromwhich the correlated p-y curves can then be obtained. Themethod considers force equilibrium, pile deflection, and additional boundary conditions. A set of fourth-order polynomial equations are abumed to model the soil reaction profile under each load step during a monotonic load test. By minimizing the difference between themeasured and calculated bendingmoment and considering equilibriumof the shear forces acting on the pile, the soil reaction profile and the concentrated tip resistance can be obtained simultaneously. A stiff instrumented test pile installed in over-consolidated sand was load tested and the resultswere used to test the performance of the proposed method. The results are compared with othermethods used in literature and practice. Themethod provides a consistent framework to derive p-y curves from measured strain data. The results of the field test and derived p-y curves confirmed that existing designmethods do not accurately capture the lateral loading response of piles in dense sand.