MODELLING THE INSTALLATION OF STIFFENED CAISSONS IN OVERCONSOLIDATED CLAY

Design of suction caissons for installation in overconsolidated clay presents several geotechnical engineering challenges. These include (i) predicting the installation resistance and required 'suction' pressure, (ii) ensuring adequate skirt length to account for vertical plug heave, and (iii) accommodating the structural engineering stiffening requirements and their effects on the penetration resistance and plug heave. A suite of centrifuge tests in overconsolidated kaolin clay was carried out to investigate the effects of stiffener geometry on penetration resistance during direct jacking and suction installation. Three caisson geometries were compared: caissons with (i) no stiffeners, (ii) horizontal stiffeners only and (iii) both vertical and horizontal stiffeners. Results show negligible differences in penetration resistance between jacked and suction installation for each caisson type. The magnitude of soil heave within the caisson is seen to be highly dependent on the level of applied suction as well as on the volume of the stiffeners. Observations during and following testing indicated that minimal flow-round of the overconsolidated clay occurred for skirts with horizontal stiffeners. These included (i) linear penetration resistance profiles following penetration of the lowest horizontal stiffener, (ii) a wedge of clay observed only below the lowest horizontal stiffener following extraction, and (iii) unsupported plug heave heights following penetration. A comparison of measured data with back-calculated resistance factors suggests that current design methods adequately predict the measured penetration resistance assuming zero flow-round conditions, implying additional end bearing of the upper horizontal stiffener during penetration was negligible.