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Repository tunnel construction in deep clay formations

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136 pages
Nuclear energy and safety
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Commission of the European Cor
Repository tunnel construction
in deep clay formations
Report
EUR 13964 EN s
Commission of the European Communities
nuclear science
and technology
Repository tunnel construction
in deep clay formations
R. J. Mair,1 R. N. Taylor,2 B. G. Clarke3
1 Geotechnical Consulting Group (contractor)
1A Queensbury Place
London SW7 2DL
United Kingdom
2 The City University
United Kingdom
3 University of Newcastle-upon-Tyne
United Kingdom
Contract FI 1W/0200
Final report
Directorate-General PARI EUROP. Bibliolh.
Science, Research and Developmen
1992 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, 1992
ISBN 92-826-3738-7
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1992
Printed in Belgium CONTENTS
1. INTRODUCTION 1
1.1 Project Outline
1.2 Aims and Objectives 2
1.3 Description of the Underground Construction 3
at Mol
1.4 Publications 5
2. DATA ANALYSIS AND INTERPRETATION 7
2.1 Advance of a Deep Tunnel in a Clay Formation
2.2 Plasticity solutions applied to shafts and 8
tunnels
2.2.1 General 8
2.2.2 Experimental shaft : Measurements and
Interpretation 9
2.2.3l Drift:s and n 12
2.3 Test Drift: Measurements and Interpretation 14
2.3.1 Measurements
2.3.2 Plasticity Calculations 16
2.3.3 Finite Element Analyses 20
2.3.4 Results from Finite Element Analyses
and Plasticity Calculations2
2.4 Summary 29
3. INTERCLAY EXERCISE 33
4. STRESS PATH TESTING OF BOOM CLAY FROM MOL, BELGIUM 35
4.1 Introduction
4.2 Apparatus and Test Procedure
4.3 Results 4Q
4.4 Conclusions
5. DEVELOPMENT OF A SELF-BORING RETRACTING
PRESSUREMETER5
5.1 Introduction
4
5.2 Reason for Development
4
5.3 Self-Boring Retracting Pressuremeter 46
5.3.1 Retracting Mechanisms 46
5.3.2 A Mechanical Retracting System8
5.4 The Drilling System 50
5.5 Calibration 50
5.6 The Prototype SBRP 51
5.7 A Retraction Test 52
5.8 Initial Field Trials 53
5.9 Current Developments 5
5.10 Conclusions and Further Work6. IMPLICATIONS ON DEEP REPOSITORY CONSTRUCTION IN CLAY
FORMATIONS 57
7. CONCLUSIONS 61
7.1 Analysis and Interpretation
7.2 Special Laboratory Testing3
7.3 Development of a Self-boring Retracting
Pressureraeter4
7.4 Implications on Deep Repository Construction in
Clay Formations
Acknowledgements6
References
Tables 71
Figures5
Appendix A Derivation of Plasticity Solution for Application to a Tunnel
Face 11
IV-REPOSITORY.TUNNEL CONSTRUCTION IN DEEP CLAY FORMATIONS
FINAL REPORT
July 1991
EXECUTIVE SUMMARY
This Report describes the work undertaken on a three year project concerning
the tunnel construction at Mol, Belgium. The project was funded by the
Commission of the European Communities and by the UK Department of the
Environment.
The objectives of the work described in this Report were to monitor, analyse
and interpret the extensive geotechnical measurements obtained from the shaft
and tunnel excavations at Mol. An additional objective was a programme of
special laboratory testing on samples of Boom clay to be undertaken at City
University. Also a self-boring retracting pressuremeter was to be conceived
and developed by the University of Newcastle-upon-Tyne as an in situ testing
device for use in strong clays and weak mudrocks.
The Report describes the underground construction for the HADES project at Mol
and summarizes measurements of ground movements, lining pressure and
convergence, and pore pressures. The purposes of the HADES project are to
investigate the technical feasibility of construction of a repository facility
in the Boom clay formation at a depth of about 225 m, and evaluate the safety
of disposal of radioactive waste in such a facility. Ground freezing
techniques were used to construct the access shaft and underground research
laboratory in Boom clay. However, of most interest are the observations and
measurements made during and after construction of a small experimental shaft
and drift (each of 1.4 m internal diameter) and the test drift of 3.5 m
internal diameter. These were successfully constructed at depths of 225-240
m in unfrozen clay.
Plasticity calculations, based on simplified assumptions, have been used to
interpret the measurements of ground movements made during construction of the
small experimental shaft and drift. The measurements have been shown to be
consistent with idealized spherical and cylindrical cavity models. In-situ
values of soil stiffness have been inferred from these analyses. The same
- V type of calculation has been performed to analyse construction of the test
drift. Prediction of the immediate build-up of stress on the test drift
linings is in good agreement with the measurements: about 30% of the total
overburden stress was observed. Use of these simple calculations allows the
sensitivity of predicted lining pressure and convergence to assumed parameters
to be explored, and results are given in the Report.
A limited number of finite element analyses have been undertaken of the test
drift construction, assuming axisymmetric conditions. The results, presented
in the Report, are generally in good agreement with the plasticity
calculations and with the in situ measurements. Use of an effective stress
model to characterize the Boom clay has allowed long term pressures on the
test drift lining to be predicted for the cases of the lining being permeable
or impermeable. Only small increases in pressure are predicted for the actual
lining, which is permeable. If it were impermeable, significantly higher
pressures are predicted, corresponding to about 60% of the total overburden .
The finite element analyses have been used to explore the detailed influence
of assumed soil model on the predicted behaviour of the test drift at Mol.
A non-linear elastic-plastic soil model, based on the results of the special
laboratory testing, was compared with a more conventional linear elastic -
plastic soil model. Much better agreement is obtained between predicted and
observed pore pressures with the more sophisticated model, but poorer
agreement was obtained in respect of ground movements.
Linked to the analysis and interpretation was a pilot benchmark exercise
(INTERCLAY) initiated by the CEC to compare different approaches of analysing
the deep excavations and other observed in-situ behaviour in the Boom clay at
Mói. Different models for characterizing the Boom clay were assumed by some
of the participants. Details of the exercise, and the results of the
comparison of the various methods, are summarized in this Report.
The Report presents the results of a comprehensive programme of triaxial tests
on Boom clay undertaken at City University. A high pressure stress path
apparatus was used, and local strain measuring devices were used for a number
of tests. Vertically and horizontally orientated specimens were tested in
compression and extension. The results indicate that the Boom clay behaviour
is essentially similar to other stiff clays at shallower depths, such as
VI London clay. The tests have demonstrated a significant difference in soil
behaviour between compression and extension paths, with shear strengths in
extension being 10-20% lower than in compression. Detailed measurements of
stress-strain behaviour on the high quality block samples obtained from the
test drift indicate a marked non-linearity in stiffness at small strain
levels.
The Report describes the concept of the self-boring retracting pressuremeter,
developed at the University of Newcastle-upon-Tyne. The two prototype
devices, their performance in the laboratory, and initial field tests in stiff
clays at shallow depths are described.
Conclusions are presented discussing the implications of the measurements at
Mol on possible deep repository construction in clay formations. The Report
concludes that the feasibility of excavation in plastic clay formations has
been well demonstrated by the experiments at Mol, and rapid progress rates
would be achievable for tunnels of considerable length constructed with
tunnelling machines. The improved understanding of ground and tunnel lining
behaviour resulting from the measurements at Mol should lead to significant
economy in lining design for deep repositories or other tunnels in clay
formations.
VII -