Dear Wilf
I agree with John Powell that this behaviour is quite common in over
consolidated soils - and it occurs also in normally consolidated
clays. Danziger, Almeida and Sills (see reference below) describe
the results and analysis of piezocone testing in a very soft normally
consolidated clay in Brazil using a piezocone with four pore pressure
positions, one halfway up the cone face, one at the cone shoulder, one
halfway up the friction sleeve and the fourth at the top of the
friction sleeve. It was apparent that, in some deployments, the
further the measurement was from the cone tip, the greater was the
increase in pore pressure before the decrease started. Like your
results, it didn't *always* occur. The paper discusses the behaviour
and suggests that it's real, not a consequence of poor saturation.
A similar behaviour was observed in large scale laboratory tests at
Oxford. The purpose of this work was to look at the effect of small
degrees of overconsolidation on the piezocone response in clay. This
was published by Nyirenda in his DPhil thesis. Let me know if you'd
like more information about any of this.
Regards
Gilliane
FAB Danziger, MSS Almeida and GC Sills (1997) The signifance of the
strain path analysis in the interpretation of piezocone dissipation
data. Geotechnique, vol 47, no 5, pp 901-914
ZM Nyirenda (1989) Piezocone studies in lightly overconsolidated
clay DPhil thesis Oxford University
-----Original Message-----
From: Wilfred Wrigley [mailto:[log in to unmask]]
Sent: 20 April 2001 21:12
To: [log in to unmask]
Subject: CPTU Dissipation Tests
I have carried out electronic piezocone pore pressure dissipation
tests on several sites in the UK. I have observed that during some
tests, usually in overconsolidated clays, there is an initial period
of pore pressure build up which can last anywhere from a few seconds
to 200 minutes, followed by a dissipation phase.
Has anyone else observed this type of response and does anyone have an
explanation? I have noticed that where a number of tests has been
carried out in one profile some of the the tests do not show the
initial pressure build up phase and hence the response is unlikely to
be due to desaturation of the porous element or system felxibility. I
have not seen any publications describing the phenomenon or any
published analyses to explain it.
I would be greatful for any comments or references.
Wilf
------------------------------------
Dr Gilliane Sills
Department of Engineering Science
University of Oxford
Parks Road
Oxford OX1 3PJ
Tel. (0)1865 273164
Fax (0)1865 273907
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