Gravity caissons are proposed as the wharf structure for the Port of Singapore Authority's (PSA) Third Container Terminal. Conveyance of cargo at this terminal will be undertaken by new pilotless automated guided vehicle (AGVs) operation which may be adversely affected by ground movement exceeding 1/30 between 2 points over a distance of 3m. In addition, efficiency and utilisation rate of a quay crane used to service quay-side container handling operations may be severely affected by a differential level between the front and rear crane rails exceeding l/500 over a distance of 35.6m. Thus, there is a need to study the movement of the gravity caissons as well as that of the reclaimed land behind it wider the anticipated construction and working loads. This is the task of an on-going joint PSA-NUS (National University of Singapore) research programme.The effects of sandbed relative density (RD), caisson base width, the presence of rock-sill overlying sandbed and sandbed thickness were studied by means of centrifuge model tests on the NUS geotechnical centrifuge. The simulated construction sequences included infilling of caisson, backfilling of caisson and live load imposition on the caisson. The centrifuge studies showed that the caisson would generally settle uniformly during the infilling stage. During thebackfilling and live loading stages, it would settle further with some tilting and sliding in the seaward direction whereby the maximum movement was observed around the caisson toe. The results indicated that the caisson settlement decreased as the sandbed RD increased. Then, asthe caisson base width was reduced from 18m to 14m, the settlement was relatively unaffected while the horizontal movements were significantly large for the 14m caisson. The smaller caisson width of 14m may not have sufficient selfweight to prevent sliding and tilting failure during the backfilling stage. Furthermore, the presence of rock-sill overlying the sandbed was found to be effective in reducing the magnitude of caisson horizontal displacements andincreasing the sliding stability of the caisson. Finally, the different sandbed thickness along the length of the wharf could result in differential displacements between adjacent caissons. Finite Element Analyses (FEM) were also carried out to interpret the centrifuge results in terms of soil mechanical behaviour. Both models, Mohr-Coulomb and Modified Cam-clay, predicted the infill-induced settlement and backfill induced sliding satisfactorily. Neither model was able to predict the backfill-induced tilt and liveload induced sliding accurately. The Mohr-Coulomb model was found to give a better representation of the foundation soil over the modified Cam-clay model in terms of predicting the experimental observations. The FEM analyses also revealed that the compression was mainly one dimensional and that the sandbed compressed uniformly during the infilling stage. During the backfilling stage, localised plastic yielding occurred around the vicinity of the caisson toe. During the live loading stage, the growth of plastic failure zone in front of the toe and the absence of it at the heel caused the caisson to tilt apart, from the expected settlement response to the vertical live loads.
Caisson, Relative Density, Infilling, Backfilling, Liveloading
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