P.G. Watson

Failure envelopes for caisson foundations in calcareous sediments

Caisson or bucket foundations are being used increasingly for offshore developments, in soil conditions ranging from soft clay to dense sands. This paper addresses the performance of caisson foundations in fine-grained calcareous soil, under combined vertical (V) and horizontal (H) loading. Results of centrifuge model tests are presented, aimed at defining the shape of the yield envelope in V-H space, and the foundation response inside the yield envelope. It is shown that the tension and (compressive) bearing capacity for the caisson are similar, and that the maximum horizontal capacity is mobilised at close to zero vertical load. Work hardening of the yield envelope due to additional penetration of the caisson is demonstrated, as is the similarity of the yield function shape at different penetrations.

Life Cycle Changes in p-y Stiffness for a Conductor Pile Installed in Carbonate Silt

Lateral soil stiffness has a strong influence on the overall strength and fatigue life of well conductors and piles. This paper reviews data from centrifuge testing of a short model pile embedded in carbonate silt, which was subjected to packets of cyclic lateral displacement. Key conclusions are that (i) cyclic lateral stiffness is significantly affected by prior loading history due to the generation and dissipation of pore pressure, (ii) the generation process leads to the well-recognized softening of p-y curves, (iii) the dissipation process leads to a less-recognised stiffening of the response, and (iv) carbonate silts show a different shape of cyclic lateral response compared to non-carbonate clays, meaning that existing ‘fully degraded’ steady state p-y models are not appropriate. Future ideas to capture these improvements in lateral response modelling are set out, with the aim of allowing more accurate and reliable design of conductors and piles.