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Society requires increasingly that the hazard and risk associated with engineered constructions be quantified. The paper presents ge-otechnical hazard assessment in the context of a risk framework. Concepts of uncertainties, reliability, safety and risk are briefly re-viewed. The use of the approach is exemplified for offshore facilities, including a piled foundation, a gravity foundation, a jack-up structure and underwater slopes. The applications demonstrate that probabilistic analyses complement the conventional deterministic safety factor and deformation-based analyses, and contribute to achieving a safer and optimum design. The conclusions emphasize the usefulness of a risk assessment, the importance of engineering judgment in the assessment and the need for involving multi-disciplinary competences to achieve reliable estimates of hazard and risk. The profession can only gain by implementing more sys-tematically than before probabilistic-based thinking and risk-based approaches.

A case history of the foundation behaviour of an offshore gravity base structure (GBS) is presented. The platform rests on an overconsolidated fissured clay, bounded, top and bottom, by pervious sand layers. Sixteen piezometers have been placed within this 30 m layer. Based on one-dimensional consolidation theory, independent analyses using both settlement and pore pressure measurements indicated a high degree of consolidation had occurred much sooner than was estimated in the initial design phase. These analyses indicated that laboratory oedometer tests underpredicted the coefficient of consolidation by one to two orders of magnitude. Updated settlements and stability analyses yielded 50% of the initially anticipated settlement and a 20% increase in the available safety factor. In addition, the certainty that the theory relating pore pressure to settlements was appropriate, led to confidence in the piezometer performance, and in turn the procedure used to install them.

The geological profile of submerged slopes on the continental shelf typically includes soft cohesive soils with layer thicknesses ranging from a few meters to tens or hundreds of meters. The response of these soils in simple shear tests is largely influenced by the presence of an initial consolidation shear stress, inducing anisotropic stress-strain-strength properties which depend also on the direction of shear. In this paper, a new simplified effective-stress-based model describing the behavior of normally to lightly overconsolidated cohesive soils is used in conjunction with a one-dimensional seismic site response analysis computer code to illustrate the importance of accounting for anisotropy, small strain nonlinearity and pore pressure development. In particular, a simple example is carried out to compare results for level ground conditions and a 10° slope. Depth profiling of the maximum shear strains and permanent deformations provide insight into the mechanisms of deformation during a seismic event, and the effects of sloping ground conditions.

The parameters determined in laboratory tests for use in dynamic analyses are usually the secant modulus and the area of the hysteresis loop. The accuracy of this approach was studied for highly non-linear behavior through analytical solutions and numerical simulations of a single degree of freedom system. Only the effect of the shape of the hysteresis loop was studied, i.e. the stiffness and the area of the loop were kept constant independent of amplitude. The study shows that irregular hysteresis may cause a significant increase in the response of the system in higher frequencies compared to what would be expected from visco-elastic and standard non-linear models. The area of the hysteresis loop provides a good measure of the damping also for high degrees of non-linearity.

The paper makes a brief review of the state-of-the-art in marine geotechnics. The design problems for different offshore foundation types, from traditional piled and gravity base foundations to the new lightweight skirted foundation concepts, are described. Geotechnical breakthroughs have enabled new and daring constructions offshore and opened the way to cost-effective solutions. At the same time, the requirements and conditions imposed by the offshore industry also greatly contributed to an improved understanding of the behavior of soils under new loading conditions. The paper looks into the development of enhanced site investigations and soil characterization, model testing, improved design methods and new foundation solutions. The challenges facing the geotechnical engineering profession when moving into deeper waters and the steps needed to meet these challenges are outlined.

Statistics, reliability analyses and risk estimates can be very useful decision-making tools in geotechnical problems. Yet the methods are little used in practice. The offshore and mining industry are at the forefront for the use of these approaches, having encouraged their use and sponsored research that has enabled the methods to be well-documented and of proven usefulness in the study of alternatives for design and decision-making in face of uncertainties. The paper presents a few case studies in different areas of geotechnical engineering and discusses the results that would have been obtained without the use of the risk approach. Special emphasis is given to dams and offshore structures, both piled and shallow foundations. The authors take a look at the reasons why the methods are not used to a greater extent in practice and make recommendations as to when and how one should uses such methods.