For subsea structures placed on soft seabed comprising fine-grained sediments, geotechnical design may be equally governed by the foundation settlement and stability requirements. For this reason, the soil stress history parameter, i.e. yield stress (σy), becomes as important as the strength parameter, i.e. undrained shear strength (su). Common practice in characterisation of fine-grained sediments focuses more on deriving the su profile than on evaluating the σy profile. More often, the underlying connections between these two important parameters are overlooked in the interpretation process. As a result, the selected design line of σy may be incompatible to that of su. This paper presents a balanced characterisation of fine-grained soils, focusing on interpretation of in-situ su and σy using a coherent and simple framework underpinned by the Stress History and Normalised Soil Engineering Properties (SHANSEP) relationship. Interpretation procedure, performance and advantages of the proposed framework are demonstrated by a real application to an offshore site dominated by fine-grained carbonate sediments.
In this framework, the two SHANSEP parameters, including normally consolidated normalised strength ratio (S) and empirical constant (m), need to be quantified for the site of interest. This is achieved by performing a series of laboratory strength and consolidation tests on undisturbed samples, with the strength tests being performed respectively under in-situ and elevated stress conditions for determining in-situ su and the values of S and m. Once the SHANSEP parameters and the su profile are derived, the σy profile can be calculated. These two profiles can then be refined further by comparing against the measured su and σy at discrete depths respectively from the strength and consolidation tests.
Since this proposed framework allows the user to consider the results from the in-situ and laboratory tests (including both strength and consolidation tests) holistically, more reliable and coherent interpretation of the su and σy profiles can be achieved. In addition, the framework allows for an easy estimation of consolidation-induced strength gain due to the permanent load applied by the subsea structures by simply using the derived σy profile and updating the vertical stress in the parameterised SHANSEP relationship. This may reduce the conservatism in the foundation design and provide optimisation opportunities.