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					          Holocene Marine Deposits: modelling self-weight consolidation
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N. Keith Tovey      ,   Michael A. Paul , Yap Chui-Wah , and Simon Tovey
  School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
  School of Life Sciences, Heriot Watt University, Edinburgh, EH14 4AS, UK
  Singapore Government, Singapore
  101 Media Ltd, Keswick Hall, NR4 6TJ, Norwich, UK

        It is widely recognised that Continental Shelf deposits hold critical
information regarding recent geological sequences and provide opportunities for
palaeo-climatic reconstruction. However, relatively little attention has been paid
to the consequences of self-weight consolidation of the recent clayey marine
sediments, and recent studies (e.g. Tovey and Paul, 2002) have demonstrated
that errors in the determination of sedimentation rates of Holcene sediments
based on simple linear models can be significant and often may be in error by a
factor of two or more.
        This paper has two functions. Firstly it extends the work of Tovey and
Paul, by incorporating further data from other locations around the world which
support a more general relationship between the consolidation parameters (e 1
the void ratio at unity stress, and the compression index Cc). This is
demonstrated in Fig, 1 where the original data of Tovey and Paul is
supplemented by data from elsewhere. This relationship which has a high
coefficient of correlation at 0.98, extends over a wider range of compression
index from 0.2 to 2 and may be represent by:-
                        e 1  0.8483  2.7226 C c

        Tovey and Paul emphasised the importance of this relationship as e1 is
rarely reported for sediments, and yet is needed in decompaction routines. This
relationship provides a convenient route to estimate these values with a high
degree of confidence.
        Furthermore, Tovey and Paul suggested, contrary to established ideas,
that excess pore pressures may develop in deep Holocene sediments. This
paper also attempts to explore the situation further to examine whether or not
such excess pore pressures could remain undissipated and thereby account for
the higher than projected moisture content seen at depth within such samples.
While there is no doubt that full dissipation of excess pore pressure will occur at
the surface as each new layer is laid down, similar pressures will also develop
at depth, and because of the relatively long path lengths to a free drainage
surface it is probable that full dissipation may not be achieved before the next
layer is sedimented.

     Fig.1 Data from additional sites appears to confirm a unique relationship
between e1 and Cc.

     Models using annually incremented sediment layers indicate that for
marine Holocene sequences thicker than around 2m excess pore pressures do
indeed persist and continue to increase with each annual increment. Evidence
from Hong Kong suggests that the sequences are draining only in an upward
direction, while the sequence from Osaka in Japan suggests double drainage,
and this difference in behaviour appears to be linked with the borehole records .

Tovey, N.K. and Paul, M.A. (2002). Modelling self-weight consolidation in Holocene
sediments. Bulletin of Engineering Geology and The Environment, 61: 21-33.

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