I agree with Paul McCombie's suggestion. I have tried to make use of the
term "consolidating" to describe these soils, although I must honestly admit
there have been occasional lapses.
Another consideration, although this may be a stretch, is that the weight of
the drill/sampling rig can result in an increase in porewater pressure,
which exists only during the sampling.
The magnitude would be small, and limited to shallow depths, however we have
had the experience of observing unexplained rises in porewater pressure in
instrumented construction sites which seems, after some investigation, to
have been linked to passage of heavy construction machinery (dragline)
nearby.
In this same light, I have struggled with the best way to compensate for
tidal action in trying to evaluate the excess porewater pressure. We are
frequently confronted with the problem of stage construction of embankments
in tidal marshes, which are, presumably normally consolidated, silty and low
plasticity clays with thin layers of fine sand, of unknown horizontal
extent. These sites are subjected to twice daily tidal ranges of 10 to 15
metres (Bay of Fundy - Eastern Canada). The problem is also pertinent to
the stability of excavations in the marsh, although, for us, this is less
frequent.
At advanced stages of embankment construction we may be increasing the total
stress in increments of 20 to 40 kPa, while this tidal phenomenon has the
potential to alter the effective stress by as much as 100 kPa. Even at the
very initial stages of construction we seldom apply an embankment load in
excess of 120 kPa. Attempts to exceed this have generally lead to failures.
This corresponds generally well with measured undrained shear strengths of
about 30 kPa in the upper soil, and about 15 kPa at greater depths. The
strength in the upper layers is attributed to some overconsolidation by
solar drying and suction due to vegetation.
? What is a "steady state" porewater pressure under such conditions?
The marshes occasionally (roughly two to four times per month) are flooded
due to spring tides, but generally the tidal action is restricted to the
face of stream banks located within the marsh.
? Does the "static" porewater pressure vary directly with the tides?
? Is there any simple way to calculate the lag between the tidal water
height and the "static" porewater pressure at a location perhaps 200 or 300
metres form the shore?
The length of these horizontal sand layers is unknown, and probably quite
variable. Neither the porewater pressure at the piezometer nor at the
shoreline is static, due to the effects of consolidation and tides.
I suspect that the effect could be measured, however there is not a lot of
funding available these days for measurements of parameters or phenomena
that might be considered as incidental to the primary task, which is to
build
Without much to back myself up, it appears that the best approximation is
that the "steady state" porewater pressure, at some distance from the shore
is something like the maximum annual tide height.
? How have others dealt with this problem? I understand there are
places in the UK (Bristol?), as well as the Nova Scotia shore of Fundy, that
have similar ranges of tides.
Any insight, suggestions, pointers to papers, or anecdotal experience would
be appreciated.
Mike Bleakney,
New Brunswick Department of Transportation
-----Original Message-----
From: P F McCombie [SMTP:[log in to unmask]]
Sent: Tuesday, July 04, 2000 5:59 AM
To: Keith Tovey
Cc: Bruce L. Kutter; Shiwakoti; Gilliane Sills;
[log in to unmask]; [log in to unmask]; shbm msnbsh;
[log in to unmask]; [log in to unmask]
Subject: Re: more on underconsoldation and ??
overconsolidation
Surely if we replace the word 'underconsolidated' with
'consolidating'
then we cover all the situations where there is (even a small)
excess pore
pressure which is dissipating in a normally consolidated sediment?
Or
would this be missing something important?
Paul McCombie
University of Bath
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