Bucket Foundation in Clay for OWT Subjected to Combined Cyclic Loads

Abstract

This thesis project investigates the bucket foundation in clay subjected to cyclic loads. The cyclic irregular loading history in this thesis was represented by the equivalent number of cycles Neq with a certain level of cyclic to average stress ratio tcy/ta which can be derived using cyclic shear strain accumulation procedure developed in NGI. The first part is the bearing capacity analysis of bucket foundation in clay. The second part is to produce the displacement contour diagrams.The bearing capacity analysis used in-house software BIFURC from NGI. The normalized failure envelopes were investigated for different degree of cyclic degradation expressed by Neq, soil profiles and bucket foundation geometry h/D. The two soil profiles used in this thesis project were constant cyclic shear strength with depth for OCR=40 and linearly increase cyclic shear strength with depth for OCR=1 & 40. Drammen clay was used as basis for the evaluation.The mobilized ultimate capacity were plotted for pure horizontal load, pure vertical load and pure overturning moment. The loads were applied in the decoupling point, meaning that the pure horizontal load fail under pure sliding. It was found that the three load component were equally influenced by cyclic degradation and that the increase and reduction in the total failure as function of Neq and Fcy/Fa could be defined by one curve for a given OCR.The cyclic effect during combined loading was studied through normalized failure envelopes. The normalized failure envelopes do not change for changing cyclic loading history for the same soil profile. The normalized failure envelopes changed in HM load plane due to different h/D and different soil profiles. The failure envelopes describes a full surface in the 3D load space HVM. Besides, the failure envelopes could be described by formulas suggested by Gourvenec and Barnett with some modifications for the failure envelopes in HM plane.Displacement contour diagrams were produced for a wide range of load combination in HVM load space. MATLAB was used to interpolate the results from bearing capacity analysis to produce the displacement contour diagrams. The displacement contours gives the stiffness for bucket foundation under cyclic loading. Finally, a procedure was made to account cyclic loading to determine the bearing capacity and stiffness for bucket foundation for OWT. The cyclic loads as input parameters are represented by Neq and Fcy/Fa. The other input parameters are bucket foundation geometry h/D and soil profile.