Oxford University
Civil Engineering
Department of Engineering Science
The Influence of Biogenic Gas on the Engineering Behaviour of Soil
Professor Gilliane Sills
Department of Engineering Science
University of Oxford
Parks Rd., Oxford, OX1 3PJ
Tel. (0)1865 273165, Fax (0)1865 273907
Background
Many underwater soils contain gas, largely methane, that has been produced biogenically by the action of bacteria on the organic components of the soil. When it occurs in the seabed, this gas can have a significant effect on the response of seabed structures such as oil rigs, breakwaters and piers. It can also occur in waste slurries and reclaimed land, and it may then provide a serious health hazard.
In order to address the problems posed by gas in different forms and different soils, it is necessary to understand the fundamental processes, and there has been a considerable amount of research at Oxford University with this aim. The earlier programmes addressed specifically the geotechnical behaviour of gassy seabed soil, on the basis that traditional saturated soil mechanics theories did not provide a suitable model for a soil that was compressible under conditions of undrained loading, such as would be caused by storm loading on offshore structures. The research has combined theoretical development with experiments in the laboratory and measurements in the field. For the laboratory aspects, a feature has been the use of reconstituted soil samples made gassy by the addition of methane impregnated zeolite, an inert chemical, to a silty clay. The zeolite takes up water from the soil slurry in preference to the methane. The process of release of methane into the soil occurs over a period of eight hours or so, allowing the sample to be prepared in an oedometer or triaxial sample mould for loading to higher stress levels without much loss of gas. Scanning electron microscopy has been used to compare these laboratory produced samples with those recovered from the seabed, and this has shown an overall similarity of structure.
The results confirmed the significance of the presence of gas, particularly on the undrained strength, the compressibility and the acoustic parameters. Sills et al (1991) have described many of the significant conclusions of this research.
The aims of the present project are to examine the processes by which gas is produced biologically in natural muds, and to compare this naturally gassy mud with that produced artificially using the zeolite technique. The main emphasis will be on the effect of the naturally occurring gas on the consolidation behaviour of the mud.
The programme of work
Mud containing methane producing bacteria will be stored in the laboratory at a temperature around 6C to inhibit the natural gas development. Soil samples will be set up under consolidation loads in acrylic cells at temperatures between 6C and 35C, for different periods of time, to allow biogenic gas production. These samples will be produced in acrylic cells to allow examination by X-rays, and various conditions will be applied, such as different initial densities and periods at different temperatures so that gas production occurs at different stages in the consolidation process
The test conditions will include a back pressure that increases in proportion to the consolidation stress to simulate the increased pore water pressures occurring at greater depths. There is an existing oedometer and load control system, but the latter used an obsolete computer, and will need to be redeveloped. The oedometer was designed to be used with soil introduced as a slurry, with the subsequent gas release taking place inside it. It will be used for some early experiments, particularly to test the new load control system. For the main programme, however, the samples will be produced in separate cells, and a new oedometer (OECC) will be built to incorporate these cells. It will also be provided with a facility for temperature control of the sample in the range 6 to 35. Previous experience has shown that elevated temperatures are likely to be necessary to initiate natural gas production. Temperature control in the oedometer will allow the original temperature of the sample to be maintained throughout its test life, thus reducing the possibility of a significant change in the biogenic gas production during the testing. The development of this new cell and carrying out the test programme will be the main responsibilities of the person appointed, so that experience in design and in laboratory testing are important requirements, along with some familiarity with computers. There will, however, be support available in all these areas.
Typical gas production periods are likely to range from ten days to three or four weeks. It is anticipated that, once the sample was available, a typical test will require up to four weeks, to include setting up, a loading period ranging from three to ten days, an unload and reload cycle requiring another week, and final unloading. In some experiments, the back pressure will be cycled with measurement of volume change to correlate with the gas content, and this will extend the test time. A further period will be required for the analysis of the results.
Reference
Sills, G.C., Wheeler, S.J., Thomas, S.D. and Gardner, T.N. (1991) The behaviour of offshore soils containing gas bubbles. Géotechnique, Vol. 41, No. 2, June 1991, pp. 227-241
