Announcement: Award-winning papers in 2011

Retrieval of jack-up rigs at the end of their operations is common practice in the offshore industry, notably to move the rigs to another drilling location. In some cases the process is difficult, time consuming and costly, because the high extraction resistance on the jack-up's spudcan footings can exceed the capacity of the rig to pull. This is particularly the case in soft clay, where significant suction is developed at the spudcan invert. The main option available to operators to ease the spudcan extraction resistance is to use a jetting system at the spudcan invert to attempt to break the suction generated. However, there is a general consensus within the offshore industry that jetting systems, under their current configurations, have a limited efficiency. Centrifuge experiments have been performed at the University of Western Australia in order to understand the mechanisms taking place during jetting extraction, and to provide recommendations to optimise the jetting performance. A reduced-scale spudcan model simulating a 17·11 m diameter prototype spudcan with jets has been tested at 200 times the acceleration of Earth's gravity (known as 200g). It was extracted from penetrations of up to 1·5 diameters in normally consolidated clay at variable extraction rates and variable jetting flow rates. Measurements of the generated suction and the total extraction resistance after a preloading period have provided insight into the extraction mechanisms with jetting. The study has demonstrated that jetting can lead to significant reduction in extraction resistance, provided that the extraction rate is fast enough to ensure undrained extraction, and that a sufficiently high flow rate is applied with respect to the extraction rate.

The George Stephenson Medal, presented to the second best paper published in all the ICE journals, was awarded to Gens et al. (2011).

Marcelo Sánchez, Maarten Van Geet, Maria Victoria Villar, Beatriz Valeján and Antonio Gens, winners of the George Stephenson Medal Medal, with ICE President Richard Coackley

The paper describes a theoretical and experimental study of the coupled hydromechanical behaviour of a compacted mixture of bentonite powder and bentonite pellets intended as sealing material in underground repositories for nuclear waste. One of the main advantages of the use of powder/pellets mixtures is the reduction of the compaction effort required to achieve the value of average dry density necessary to attain the required swelling potential. However, the heterogeneous fabric of the material requires special approaches in order to describe adequately its behaviour during hydration. A double porosity formulation is presented to account for the presence of two distinct structural levels in the material. Hydraulic equilibrium between the two porosities is not assumed; instead a water exchange term between them is postulated. The formulation is applied to the modelling of a number of one-dimensional swelling pressure tests performed in the CEA (Commisariat à l'Énergie Atomique, France) and CIEMAT (Spain) laboratories. A very satisfactory quantitative description of the experimental observations is obtained that includes a number of complex behaviour features such as size effects and non-monotonic development of swelling pressures. Some microfabric observations using X-ray tomography and mercury intrusion porosimetry lend support to the conceptual approach adopted. The formulation is then applied to the analysis of a long-term large-scale sealing test performed at the Hades underground facility in Belgium, using the same set of hydraulic and mechanical parameters employed in the modelling of the laboratory tests. Although the field observations exhibit a much higher degree of scatter, the basic behaviour of the field sealing test is satisfactorily simulated. A formulation that incorporates basic features of the microfabric of the mixture is thus able to span successfully over a large range of space and time scales.