Posted on behalf of Ming-Fang Chang ([log in to unmask])
--------
Dear Angus and others:
The intensive land reclamation activity in Singapore has led us at the
Geotechnical Research Centre, Nanayang Technological University to start
looking into the assessment of in-situ pore pressure and the degree of
consolidation of foundation clays undergoing consolidation under the
influence of the reclaimed fill at several recently reclaimed sites since
1993. Our experience on the matter from Singapore may be useful. We explored
the following two approaches almost in parallel:
(1) Direct interpretation of a redefined OCR (with its value less than one
for partially consolidated clay from piezocone penetration data on the basis
of cavity expansion and modified Cam clay theories.
(2) Interpret the in-situ pore pressure from converging of ut and ubt in a
med-term (3 to 4 hours) dissipation test using a dual filter piezocone.
What are the outcomes ?
(1) A method that allowed for direct interpretation of OCR of a partially
consolidated clay has been developed. The relevant reference is as follows:
Chang, M. F., Choa, V., Cao, L. F. and Arulrajah, A. "Evaluating the State
of Consolidation at A Reclaimed Site", Proc. 1st Int. Conf. on Site
Chatacterization, Atlanta, April 1998, Vol. 2, pp. 1403-1408.
(2) Encouraging results have been obtained at two 15 to 20 year old
reclaimed sites. The relevant reference is as follows:
Chang, M. F. "Cone Penetration Testing in Singapore and Malaysia - National
Report NR29, Proc. Inter. Symposium on Cone Penetration Testing, Linkoping,
Sweden, October 1995,
Vol.1, p.193-200.
Further Findings?
(1) Our very recent tests (just concluded a few days ago) at an on going
reclaimed site in Changi, Singapore, however, indicated some difficulty of
using ut and ubt from a conventional 60* cone at newly reclaimed site, as
the difference in the two values was only 1 or 2 kPa even at some 40 meters
below the ground surface (35 meters below the original seabed). This appears
logical for initially slightly overconsolidated clay which has just yielded
under the influence of the fill load, if you consider the explanation on the
two components of Du given by Paul Mayne.
(2) You may want to consult Andrew Whittle of MIT on his recent experience
with a more sensitive dual filter sharp edge cone in offshore investigation.
Regards.
Ming-Fang Chang
*********************************************
Dr. Ming-Fang Chang
Associate Professor, Nanyang Technological University
School of Civil and Structural Engineering
Blk N1, #1A-29, Nanyang Avenue
Singapore 639798
Tel: (65) 790-5315; Fax: (65) 791-0676/ 792-1650
E-mail: [log in to unmask]
*********************************************
-----Original Message-----
From: Geotechnical Engineering Email List
[mailto:[log in to unmask]]On Behalf Of Angus Maxwell
Sent: 21 April 2001 2:41 AM
To: [log in to unmask]
Subject: Re: CPTU Dissipation Tests
Paul and others,
Is it possible to use piezocone pore pressure dissipation tests to establish
the in-situ pore pressure within a partially consolidated clay? I am looking
for a method of establishing the pore pressure profile across a clay subject
to reclamation load. We have spot measurements from vibrating wire and
pneumatic piezos but the actual shape of the pore pressure profile is still
unknown.
Regards
Angus Maxwell
----- Original Message -----
From: Paul W. Mayne
To: [log in to unmask]
Sent: Saturday, April 21, 2001 4:52 AM
Subject: Re: CPTU Dissipation Tests
Hi Wilf,
Yes, this has been noted quite a bit elsewhere (e.g., Lutenegger at
Penetration Testing in UK 1988; Pane et al. CPT'95; Sully et al. 1999).
We believe that this is a reflection of the two-components of porewater
pressure (octahedral normal stress changes simulated by cavity expansion and
shear-induced porewater pressures that CSSM can cover). The measured du =
duoct + dushear. The octahedral extends out a spherical/cylindrical zone
that is proportional in size D/d = cube root /square root of the Rigidity
index (IR) of the clay (respectively). Say 10 to 20 radii. The shear
zone is a soil-structure interaction of interface, so only a few
millimeters. Thus, the dushear can be postive (low OCR) or negative (high
OCR) and dissipates faster. Leaving the larger octahedral zone always in
monotonic decay of porewater pressure. You can solve by iterative finite
difference solution (Burns & Mayne, CPT'95) or by closed-form analytical
solution (Burns & Mayne, Canadian Geotech J. 1998). We termed this
behavior as "dilatory" which corresponds to two meanings: (1) delayed
response (up then down); (2) dilation effect of OCR dushear porewater.
I can send you a simple document that allows you to approximate the true
soln. if wish that curve-fits your data.
Sincerely,
Paul
At 09:11 PM 4/20/01 +0100, Wilfred Wrigley wrote:
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
Paul W. Mayne, PhD, P.E.
Professor & Group Coordinator
Geosystems Program
School of Civil & Environmental Engineering
Georgia Institute of Technology
790 Atlantic Drive, Room 241
Atlanta, GA 30332-0355
Email: [log in to unmask]
Phone: 404-894-6226 and fax-2281
http://www.ce.gatech.edu/~geosys
|
