Evaluation of the strength of cement-treated clay with a broad range of mix ratios and curing periods was conducted using unconfined compression tests (UCTs). The influence of cement content, total water content, and curing period on the unconfined compressive strength of cemented clay are investigated. It is found that, at constant total water content, higher cement content results in higher unconfined compressive strength, while the total water content has an opposite effect. A power function can be used to correlate the unconfined compressive strength with the cement content or the total water content. For a fixed mix ratio, the unconfined compressive strength of cement-stabilized clay increases with the curing period, the effect of which can be characterized by a semi-log formula. Also, a strength-prediction model that considers both mix ratios and curing periods for cement-admixed marine clay is developed and validated; the model can capture the effect of clay type by considering the plastic index of untreated soils. It is also proved that the proposed framework for strength development is also applicable for other cement types.

In the design of sand compaction piles, the increase in undrained shear strength of the soft clay around the sand piles is often ignored. This paper examines the long-term gain in strength in the soft clay using centrifuge modelling and finite-element analysis. Stress, pore pressure and strength measurements in the centrifuge models are used to benchmark the finite-element results. Parametric studies are then conducted using finite-element analyses to explore the effects of the friction angle and modulus of the soft clay on the trend and quantum of strength increase. The findings show that the ratio of improved strength to original shear strength decays as a function of the logarithm of radial distance. A method of estimating strength increase is proposed, based on two parameters that are related to the friction angle and modulus of the soil. For multiple piles driven with almost no time lag for pore dissipation, strength gain does not accumulate with successive piles. On the other hand, if there is sufficient time for pore pressure dissipation between the driving of successive piles, the strength gains from the successive piles can be accumulated.

The rapid jack-in process of spudcan footing tends to generate substantial build-up of excess pore pressure below the spudcan underside. An intimate knowledge of the excess pore pressure concentration is thus of significance for understanding both the short-term and long-term mechanical behavior of spudcan. As yet there is a dearth of information in this area. This paper presents a comprehensive research to explore the pattern and magnitude of the excess pore pressure field surrounding an advancing spudcan. Both centrifuge experiments and effective-stress large deformation finite element analyses (LDFEAs) were involved, while the former served mainly to benchmark the latter and the latter provided a complete information database of the excess pore pressure. The bulb-shaped excess pore pressure concentration zone was identified below the spudcan underside. Sizes of pressure bulbs as well as the magnitudes of excess pore pressures inside them bore close relationship to the penetration depths and soil's properties. The results of parametric study led to the development of a simple yet useful approach to estimate the spudcan-penetration-induced excess pore pressure in soil without carrying out the complicated computation. Necessary charts and tables were provided to facilitate its usage. It is expected that the development of such an approach can aid the practicing engineer to obtain a quick and rough estimate of excess pore pressure generated in the midst of the jack-up spudcan penetration and prepare for its short-term and long-term implications.

This paper examines possible causes of additional spudcan settlement after preloading using both centrifuge model tests and small strain finite element analysis, in which spudcan settlement due to cavity collapse, consolidation settlement and settlement due to cyclic loading are incorporated. Back-analyses of seven jack-up rigs in the Gulf of Mexico show that even complete cavity collapse could only explain part of the measured additional settlements in the majority of the cases. Small strain finite element analyses also show that spudcan consolidation settlement is likely to account for even less of the additional settlement than cavity collapse in the sites considered. On the other hand, centrifuge model tests show that large amplitude cyclic rocking has a very significant effect on spudcan settlement, even if half of the preload has been removed. However, this effect cannot be explained by the exceedance of the yield envelope since the loading combination had not exceeded the yield envelope. One possible explanation is the stiffness and strength degradation of the soil under cyclic loading. In view of this, a conservative approach is recommended in instances where large amplitude cyclic rocking, such as that arising from storm loading, is expected shortly after preloading. The presence of lattice legs is found to reduce the spudcan settlement during large amplitude cyclic rocking.