D. McG. Elder

Doctor of Philosophy, Wolfson College, Oxford University, Michaelmas Term, 1985

Stress Strain and Strength Behaviour of Very Soft Soil Sediment


When fine grained natural sediments or artificially produced waste materials are transported and deposited through water, several different phases of behaviour are observed. These have been described variously as suspension, free settling, compression settling, intermediate and consolidating soil phases. Transitions between each are not always distinct in terms of material properties or behaviour and time spent in the early phases before a conventional soil state is attained can be ta significant proportion of the total period of engineering interest. The eventual state attained following procession through these stages may be very different to that if the soil had been remoulded at the same final density.

Standard engineering models exist which can describe soil behaviour well in separate phases under particular conditions, but these are of limited validity when extended to more general conditions and wider volumetric ranges than those for which they were formulated. The number of factors required to describe the entire range of behaviour is consequently larger than that for any one phase, and many of these factors are more familiar in field of chemistry, geology or sedimentology than in classical soil mechanics.

This thesis discusses, in engineering terms, the engineering behaviour observed in a particular soil during the general sedimentation and self weight consolidation process. In the second chapter existing knowledge about behaviour at zero or low stresses is reviewed and evaluated with respect to common assumptions made, often implicitly, in formulating predictive models. It is shown that while these models have been extensively developed to a stage where they can approximate many aspects of soil behaviour, the lack of fundamental investigations carried out in parallel with their development has often led to inadequate appreciation of the causes of discrepancies between modelled and real behaviour. This has occurred particularly where standard geotechnical testing equipment and methods devised for stiff soils have been used to obtain global average relationships between engineering parameters. Even where modified tests have been developed, instrumentation has sometimes been inadequate and measurements too infrequent, so that data available have necessarily been analysed only in terms of constitutive forms assumed already.

In chapter three experimental techniques are proposed which, where possible will allow soil behaviour to be examined under the least restrictive conditions of one dimensional compression so that basic engineering concepts may be analysed. Chapter four describes the testing programme and presents direct results of experiments. Chapter five analyses compression behaviour and establishes some trends which can be observed for particular parameters and relationships, and which exist between experiments under different initial and boundary conditions. Similar analysis of strength behaviour is undertaken in chapter six, where results obtained using different testing methods are compared. In the final chapter the general relevance of these results and their implications for engineering problems are discussed. Some suggestions are made for future work.

Areas of application

Improved knowledge about cohesive waterborne sediments can result in considerable savings for related industries. In the United Kingdom the annual cost of maintenance dredging is œ25m (I.C.E. Costal Engineering Research Panel, 1985). In East Coast ports alone reduction of the distances travelled by each dredger would lead to a saving of œ270,000 per annum, per kilometre reduction. Studies at Rotterdam Europort (Kirby, Parker, van Oostrum, 1979) show that although a channel dredged recently may quickly refill with sediment to a depth which echo-sounding techniques might indicate to be unnavigable, the strength may be so low as to allow passage of vessels virtually unimpeded. A density of 1.2 Mg/m3 is now used by the Rijkswaterstaat to define the "Nautical Depth" of a channel, stated to be "a density within the suspension above whose altitude vessels can safely sail." Dredging control using information from gamma ray densimeters has enabled production increases of up to 50% to be obtained in the Europort area. In the United States $30m was spent in a 5 year period on a dredging research programme aimed at improving disposal methods (Haliburton, 1977). Considerable volumes of waste material are also produced by the mining industry. The phosphate industry in Florida produces 40 million tons by dry mass per annum at an initial 3% solids by mass which even after two years retains void ratios around 10, due to the high content of attapulgite, a clay mineral consisting of long fibrous particles with large specific surface. Disposal areas for these clays occupy over 50,000 acres and are surrounded by 300 miles of dams, posing significant environmental and safety problems (Bromwell, Oxford, 1977).

Failures of underwater slopes have been well documented. In muds deposited recently in the Mississippi Delta area very low shear strengths combine with apparently high excess pore pressures and presence of gas bubbles to cause instability for slope angles less than 1ø. Recent research carried out a Oxford suggests that presence of gas may cause high excess pressures to be deduced where none exist. Duncan and Buchignani (1973) analysed a slope failure in San Francisco Bay which occurred during cutting of a slope from a normally consolidated clayey silt. The importance of accurate determination of an in situ parameters for analysis was shown by the estimated saving of œ200,000 through using a slope of 7:8 rather than 1:1, decreasing the supposed safety factor from 1.26 to 1.17. Analysis of error sources showed that an error of only 4% in the soil density could reduce this safety factor by 10%. Similar problems due to changes in loading or boundary conditions occur where natural changes, such as increase in water current, cause erosion of a sediment layer which might, for example, be supporting an underwater cable or pipeline.

In all these areas in situ property determination in solid of low density provides major problems. Density is often the only quantity that can be measured both accurately and continuously and then only when a stable platform can be maintained. Recovery of high quality samples from these layers is virtually impossible, so that there is a strong need for correlations between density and other properties such as strength and compressibility.

Thesis (7.97Mb, pdf)

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