The CEC benchmark Interclay on rheological models for clays

European Commission Directorate-General For The Information Society And Media Directorate-General For Research And Innovation

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Results of pilot phase (January-June 1989) about the boom clay at Mol (B)
Nuclear energy and safety
Energy research

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 2 Mo

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Commission of the European Communities
nuclear science
and technology
The CEC benchmark Interclay
on rheological models for clays
Results of pilot phase (January-June 1989)
about the boom clay at Mol (B)
j3Ë jM
Report
EUR 12791 EN Commission of the European Communities
nuclear science
and technology
The CEC benchmark Interclay
on rheological models for clays
Results of pilot phase (January-June 1989)
about the boom clay at Mol (B)
Edited by
B. Come
Commission of the European Communities
200, rue de la Loi
B-1049 Brussels
Work carried out in the framework of
the Community programme on radioactive waste management
(1985-89)
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Science, Research and Development
A/EÜFH2791-EN 1990
LCL
«NÜSZOI Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1990
ISBN 92-826-1484-0 Catalogue number: CD-NA-12791 -EN-C
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1990
Printed in Belgium Pilot phase for INTERCLAY Benchmark
Background data
The exercise is part of the 3rd Community R&D Programme on Radioactive
Waste Management and Storage (1985-1989), Part A, Task 4 "Geological
Disposal Studies", and Part B, Project B2 "HADES", pilot underground
facility in the Boom clay at Mol (B).
Contributors are listed below (calculating teams underlined) :
BELGIUM : CEN/SCK, Mol
FRANCE : CEA-ANDRA, Paris
LMS (Laboratoire de Mécanique des Solides),
Palaiseau
ITALY . : ISMES, Bergamo
UNITED KINGDOM : GCG (Geotechnical Consulting Group), together with
City University, London
Building Research Station (UK-DOE), Garston
The present summary document was compiled by the CEC with the assistance
of the CEN/SCK.
- Ill -CONTENTS
Page
Summary and conclusions VII
General description 1
Problem specifications 3
Summary of rheological models 7
Some comparative results - Tunnel excavation problem 23
Somee results - Di latometer test 3
Appendix 1 - Complete results by LMS 45
Appendix 2 - Complete results by GCG 77
Appendix 3 - Complete results by ISMES 101
- V -SUMMARY AND CONCLUSIONS
Background
A pilot phase of a benchmark exercise for rheological models for Boom
clay, called INTERCLAY, was launched by the CEC in January 1989. The
purpose of the benchmark is to compare predictions of calculations made
about well-defined rock-mechanical problems, similar to real cases of
the Mol facilities, using existing data from laboratory tests on
samples. Basically, two approaches were to be compared : one
considering clay as an elasto-visco-plastic medium (rock-mechanics
approach), and one isolating the role of pore pressure dissipation
(soil-mechanics approach). The former was used by LMS, whereas the
latter was adopted by GCG and ISMES. Both approaches were based on test
data on good quality samples of clay taken from excavations at a depth
of 240 m at Mol.
It must be emphasized that the plastic Boom clay at Mol is probably the
only existing case of a geological medium on which such a large testing
programme has been performed, that enough laboratory sample tests of
notably different nature (e.g. undrained creep tests, effective stress
triaxial tests and oedometer tests) are available for meaningful
comparisons.
Two "hypothetical" problems were drafted by the CEC in consultation with
the participants : a dilatometer test, and a tunnel excavation problem.
Both were solved "blind" and independently by the 3 teams involved,
between March and June 1989. The CEC ensured a preliminary comparison
of the results, which was presented at a technical session held in
Brussels on June 8, 1989.
Main conclusions of the pilot phase
It must be stressed first that the results of calculations, using
different rheological models and different sets of parameter, showed
nevertheless an "acceptable" degree of agreement between themselves, and
also with measurements carried out at Mol in reasonably similar
experiments. This, in a way, is reassuring, because it gives some
credibility to predictions using rheological data derived from
laboratory tests on clay samples.
- VII -However, there was no unanimity in the calculated results ; it was shown
that different Theological models using different soil parameters, each
one claiming to replicate adequately the behaviour of the same material,
yield different (although not too divergent) results. This must lead to
an attitude of caution when it comes to comparing results of
measurements and results of calculations about the same situation in
plastic clays.
Furthermore, the pilot phase showed that relatively small changes in
parameter values, for the same rheological model (the modified CAM-CLAY
in this case) and for the same material, could lead to significantly
different results for calculation of the same problems. Within the
limited scope of the exercise, it was not possible to disentangle the
sensitivities of :
(a) the specified problems to the assumed parameters ;
(b) the adopted rheological models to the assumed parameters ;
(c) the numerical aspects, such as the finite element mesh density.
Additionally, it must be borne in mind that the problems posed were
idealizations of real cases, so that no straigthforward comparison can
be made with the actual in-situ measurements. Therefore, at the present
stage of the exercise, no calculated solution can be qualified as being
either correct or wrong.
Finally it was observed that, mainly as regards pore-pressure
distribution around tunnels in the Boom clay, some discrepancies between
calculations and measurements made in similar conditions remain to be
explained. This would anyway point to the desirability of performing
analyses using an improved constitutive representation of soil behaviour
which includes coupled mechanical-hydraulic calculations for the
modelling of groundwater flow in such conditions of low-permeability,
plastic media.
VIM