Julie Q. Shang, University of Western Ontario
Electrokinetic Strengthening of Marine Sediments Around Foundations
El Naggar, M.H.
Local strengthening of sediments around deepwater foundation elements would be extremely beneficial: hastening "setup" time of clays around driven piles, increasing uplift capacity of suction anchors, and rapid regain of sediment strength loss resulting from cyclic loading. One possible means of strengthening sediments is the application of a high voltage electric field which generates electrophoretic and dielectrophoretic forces on the sediment. This project is aimed at evaluating the effects of high voltage electrical treatment of marine clays on clay shear strength, pile bearing capacity and dynamic response of single piles and pile groups.
One aspect of the strategic plan concerns research on innovative deepwater foundations. High voltage electrokinetics is an innovative approach to strengthening marine sediments around deepwater foundations.
The theory of dielectrophoresis in clay-water-electrolyte systems was developed recently and supported by encouraging experimental evidence on fresh water soils. The method was then tried on marine clay samples with a simple electrode arrangement, measuring the changes in shear strength and moisture content after treatment. Metal plates and cylinders were then embedded in the sediment and pullout forces were measured after treatment. The theory was extended to predict optimum electrode arrangements. To date, experiments show significant increases in sediment strength and pullout resistance of the plates. As a continuation to this research, a larger (1 m. x 1 m. x 1 m.) test tank has been obtained in which model piles can be tested after high voltages obtain the optimum benefits of the treatment. As a continuation to this research, two larger (54" diameter and 60" deep)tanks have been built at the University of Western Ontario in which model piles will be tested for their static and dynamic responses after high voltage treatment. The experimental setup has completed and the trial test will be conducted in January 1997.
The application of dielectrophoresis to soils was pioneered in Canada in the late 1980's. Consequently, the method is too new to have gained popularity. It is presently being studied in Canada as a method for dewatering and strengthening mine tailings. There is some local research on use of the method for removing contaminants from soils.
If the method works on a large scale, it can be used to economically strengthen sediments around all types of marine foundations. Its most effective use should be for suction caissons owing to the simplicity of electrode installation, but it will also work for driven piles and drag embedment anchors.
January 1996 to December 1997
The research is aiming on the development of an innovative technology for strengthening marine sediment around foundations of offshore platforms for oil exploration and production. Up to date, four papers have been published/accepted in technical journals and conferences. The results of the large tank tests obtained from this project will be of major interests to practising geotechnical engineers, especially those specializing in deep foundations in marine environment.
The preliminary nature of the research has precluded strong industry involvement. There has been discussion with two industry affiliates regarding potential uses of the method, but no field trials are forthcoming.