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REPOSITORY TUNNEL CONSTRUCTION IN DEEP CLAY FORMATIONS

Objective

AT MOL, BELGIUM, THE FEASIBILITY OF CONSTRUCTING A REPOSITORY AT LARGE DEPTH IN A CLAY FORMATION IS BEING INVESTIGATED (HADES PROJECT). THE MOST RECENT CONSTRUCTION HAS BEEN A TEST DRIFT OF A LENGTH OF 40 M AND AN INTERNAL DIAMETER OF 3.5 M: 60 CM THICK CONCRETE BLOCK SEGMENTS WERE USED FOR LININGS. AS AN EXTENSION TO THE TEST DRIFT, A 12 M LONG GALLERY HAS BEEN CONSTRUCTED USING SLIDING STEEL RIBS AS LININGS. EXTENSIVE MEASUREMENTS OF LINING LOADS, PORE-PRESSURE AND GROUND MOVEMENTS HAVE BEEN MADE IN THE TEST DRIFT.
THE OBJECTIVES OF THIS PROJECT ARE AS FOLLOWS:
1. ANALYSE AND INTERPRET AVAILABLE MEASUREMENTS MADE AT THE HADES PROJECT AT MOL IN RESPECT OF TUNNEL CONSTRUCTION.
2. CARRY OUT LABORATORY STRESS PATH TESTING ON SAMPLES OF BOOM CLAY TAKEN DURING THE CONSTRUCTION OF THE TEST DRIFT AT MOL.
3. RESEARCH AND DEVELOP A SELF-BORING RETRACTING PRESSUREMETER (SBRP) AS AN IN-SITU TESTING DEVICE FOR USE IN HARD CLAYS, WITH SPECIFIC REFERENCE TO TUNNEL LINIG DESIGN.
Excavations in the Boom clay at Mol have been undertaken. The measurements and observations have provided a unique opportunity to study the behaviour of excavations at full scale, during and after construction, in a clay formation at great depth. Of most relevance to any future repository construction in deep clay formations are the observations of the small experimental shaft and drift (each of 1.4 m internal diameter) and the test drift (3.5 m internal diameter), because these were successfully constructed in unfrozen clay. Of principal interest are the measurements of ground movements, lining pressure, lining convergence and pore pressure changes.

Plasticity solutions, based on simplified assumptions, have been used to interpret measurements of ground movements made during construction of the 1.4 m experimental shaft, and also to analyse the construction of the test drift. A limited number of finite element analyses has been undertaken. Prediction of the immediate build up of lining stress was in good agreement with the measurements; about 30% of the total overburden stress was observed.

Use of an effective stress model in the finite element analyses to characterize the Boom clay has allowed long term pressures on the test drift lining to be predicted. The laboratory testing of the Boom clay has shown it to exhibit marked nonlinear characteristics. The measured distribution of ground movements around the test drift are indicative of the ground behaving as a 'blocky' material. This may be associated with cracks in the clay. Two fundamentally different approaches of modelling of the Boom clay were compared. The elastic viscoplastic and effective stress rheological models produced significantly different results.

High quality strain measurements have shown a marked variation of elastic stiffness with strain level, prior to yield. The results indicate that the Boom clay behaviour is similar to that observed in clays at shallow depths, provided due allowance is made for stress level.

A self boring retracting pressuremeter has been conceived, manufactured and tested. Two devices have been manufactured. The instruments have been found to perform satisfactorily in the laboratory but only limited field trials have been undertaken.

The improved understanding of lining behaviour resulting from the measurements at Mol should lead to significant economy in lining design for repositories in deep clay formations. The results from monitoring the test drift suggest that in such stiff clay formations, a tunnel lining would be subjected to no more than half the total overburden stress.
THE WORK PROGRAMME CONSISTS OF THE FOLLOWING ACTIVITIES:
1. DATA ANALYSIS AND INTERPRETATION: (A) SYNTHESIS OF DATA, (B) PLASTICITY CALCULATIONS, (C) FINITE ELEMENT ANALYSES, (D) INTERPRETATION AND GENERALIZATION OF DATA IN RESPECT OF POSSIBLE DEEP REPOSITORY CONSTRUCTION IN CLAY FORMATIONS.
2. LABORATORY TESTING: (A) DEVELOPMENT OF A HIGH PRESSURE COMPUTER CONTROLLED STRESS PATH TRIAXIAL TESTING APPARATUS, (B) TESTING OF SAMPLES OF BOOM CLAY.
3. IN-SITU TESTING:
3.1. DEVELOP A PROTOTYPE SELF-BORING RETRACTING PRESSUREMETER.
3.2. CARRY OUT FIELD TESTS IN THE UK WITH THE SBRP IN APPROPRIATE GEOLOGICAL MATERIALS SUCH AS MUDSTONES.
3.3. DEPENDING ON RESULTS, CARRY OUT TESTS FROM THE GALLERY OR TEST DRIFT AT MOL.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Geotechnical Consulting Group Ltd
Address
1A Queensbury Place
SW7 2DL London
United Kingdom

Participants (1)

UNIVERSITY OF NEWCASTLE UPON TYNE
United Kingdom
Address
6 Kensington Terrace
NE1 7RU Newcastle Upon Tyne