This paper introduces an advanced geotechnical numerical modelling approach, which can be used to simulate the gradual deposition and large-strain consolidation of tailings in a multi-dimensional space. The findings from the modelling can be used not only to inform planning and design at mine closure but also help the management of tailings impoundments. For example, the results can be used to determine the settlement of tailings with time and thus inform backfilling planning; or inform on the tailings settlement and thereby assist with the design of an effective drainage network to divert surface water. Following a state-of-practice review of tailings consolidation modelling, the recently developed multi-dimensional modelling technique (the Norwegian Geotechnical Institute [NGI] model) is described in the paper, with validations against available analytical solutions and comparisons with commonly used predictions presented by Townsend & McVay (1990). An example of the application of the NGI model is presented to demonstrate its capability and performance in modelling a full-scale scenario. The NGI modelling approach is built on commercially available specialised geotechnical modelling software, FLAC (and FLAC3D), through its embedded programming language, FISH. The NGI model extends FLAC's existing capability of large-strain consolidation calculation to simulate the gradual deposition process of the tailings. The deposition of the tailings slurry is divided into many discontinuous layers, and these layers are activated one after another from the bottom up. Activation of each new layer (on top of the existing tailings surface) is followed by a large-strain consolidation stage, with the consolidation time being determined as a function according to the volume of the layer and discharge history. Rock backfilling can be modelled in a similar fashion or can be customised. A user-defined constitutive model (as part of the NGI model) has been developed to reproduce the key characteristics of the tailings during consolidation, including the variation of compressibility and permeability with reducing voids ratio. The consolidation of the tailings is modelled in a large-strain mode (i.e. the coordinates of the grid are updated frequently) in order to capture its effect on the consolidation behaviour and the deformation occurred prior to addition of a new layer. The NGI model is also capable of performing complex three-dimensional problems accounting for varying consolidation boundary conditions, non-uniformity of the tailings material, and irregular pit geometries. As illustrated by the example application, this approach can be used to predict the development of tailing consolidation settlement with time, the amount of water expressed during consolidation, the capacity of the pit for tailings storage and the required amount of rock for backfilling. Further development is ongoing in order to expand its modelling capability, such as prediction of increase in tailings strength with consolidation, modelling drying and consoli...

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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.

Shallow foundations are used offshore to support a fixed and floating platforms and a range of subsea facilities. This article outlines the types and applications of shallow foundations used offshore; the basis of design, including installation, capacity, serviceability, and retrieval; design considerations, including various modes and origin of loading, choice of design parameters for capacity and settlement prediction, and scour; design solutions, including skirts, drainage blankets and tolerable foundation mobility; factors of safety; and industry design guidance including current trends in design. The wide range of issues that need to be considered for shallow foundations is discussed in this article; however, no attempt is made to provide detailed solutions for the different aspects that need to be considered, since these are well documented in industry design guidelines.