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ONDERZOEKSPROGRAMMA | IN-SITU EXPERIMENTEN | KARAKTERISERING VAN KLEI | SELFRAC

SELFRAC

EXPERIMENT'S NAME
Fractures and self-healing within the excavation disturbed zone in clays

LAST MODIFIED
23/02/2004

EXPERIMENT'S ACRONYM
SELFRAC

TYPE OF TEST
Clay characterization

PERSON RESPONSIBLE
Name : Wim Bastiaens
Email : wbastiae@sckcen.be

COLLABORATIONS NAGRA (CH), L3S (FR), G3S (FR), KUL (B), EPFL (CH), SOLEXPERTS (CH)			FINANCING ORGANISM(S) EC, NIRAS/ONDRAF GLOBAL BUDGET (k€) ~350
	Phenomena	Components	Safety (sub) functions concerned (if relevant)
i	Hydro-mechanical	Host rock	SF	C		R		D	L
			SSF	C1	C2	R1	R2
ii	Construction methods	Host rock	SF	C		R		D	L
			SSF	C1	C2	R1	R2

Background

As in all nuclear power generating countries, high and long lived radioactive wastes management is an important environmental issue today. Disposal in deep clay geological formation is one of the promising options to dispose these wastes. An essential item for the long-term safety of High Level Waste underground repositories is the proper evaluation of the Excavation Disturbed Zone and of the evolution with time of both mechanical and transport properties in this zone. The perturbation of the excavation may induce a significant increase of the permeability, related to diffuse and/or localised crack proliferation in the material. Fortunately self-healing properties of clays can in turn reduce the permeability in time. The main objective of the proposed project is to understand and to quantify these processes.

Objective of the experiment

The aim of this experiment is to study the evolution of the EDZ around an underground excavation, including possible (self-)healing and (self-)sealing effects.

Specific explanations (if relevant):
Two in-situ experiments are performed in HADES in the framework of the SELFRAC project:
1) evaluation of the evolution of the EDZ around the connecting gallery using two parallel piezometers (SELFRAC in-situ test III);
2) evaluation of the evolution of the EDZ around a partially cased borehole using seismic and acoustic measurement techniques (SELFRAC in-situ test IV).

Description of the experiment

1. evaluation of the evolution of the EDZ using two parallel piezometers

Design:

Figure 1: Lay-out of the situ test. Two parallel multi-piezometers (porous filters in yellow) are installed in the connecting gallery. Possible fractures are schematically represented in red.

Protocol/explanation :

This test (Figure 1) consists in studying the evolution in function of time of the hydro-mechanical properties within the EDZ around the gallery during a period of three years after its construction. For this purpose two parallel multi piezometers of 5.6m long were installed in the connecting gallery; upwards, in rings 62 and 63 (1m distance between them). Porous filters allow measuring the pore water pressure in the host rock: hollow tubes connect them with pressure transducers in the connecting gallery. During the design, the distance between successive filters was kept as small as possible. This way, almost the entire 5.6m is covered with filters and the possibility that fractures around the gallery intersect the piezometer in between two filters is negligible.

2. evaluation of the evolution of the EDZ using seismic and acoustic measurement techniques

Design:


Figure 2: Lay-out of the seismic and acoustic measurements. TOP LEFT: Overview of the four instrumented boreholes (orange) and the central borehole (yellow), as seen in the ANDRA gallery. TOP RIGHT: Overview of the four instrumented boreholes. BOTTOM: 3D lay-out: four instrumented boreholes (each 2 receivers and 3 transmitters) and central borehole.

Protocol/explanation :

The purpose is to monitor re-consolidation and self-healing of the clay host around a freshly excavated borehole which is (partly) allowed to collapse. The experiment was installed in the western sidewall of the Andra gallery of HADES. Figure 2 shows the borehole arrangement between rings 32 and 36. Around a central borehole (drilling diameter ~157 to 160 mm) four observation boreholes (drilling diameter 117 mm) are located.In a first stage (Dec 2003) the four observation holes were drilled and instrumented. They enable to investigate the re-stabilisation of the host rock after their installation by following up the seismic and acoustic measurements. After this period (~May 2004) the central borehole will be drilled and instrumented.The observation holes are 8.2 m deep and the sensors are located between 5 m and 8 m depth. In each of the holes 2 receivers and 3 transmitters are placed and they are oriented towards the central borehole. The casing diameter is 108 mm. The central hole is 9 m deep and the collapsing part is located between 5 m and 8 m depth. In the collapsing part the diameter is 108 mm, the rest of the central borehole tubes have a diameter of 152.4 mm. Four transmitters and four receivers (not shown on picture) will be installed in liner tube segments in the middle and at both ends of the collapsing borehole part. The closure of the borehole is determined in horizontal and vertical direction in the middle of the collapsing part by measuring the force of a loaded compressional spring due to radial convergence.

Instrumentation :

The observation holes are 8.2 m deep and the sensors are located between 5 m and 8 m depth. In each of the holes 2 receivers and 3 transmitters are placed and they are oriented towards the central borehole. The casing diameter is 108 mm.The central hole is 9 m deep and the collapsing part is located between 5 m and 8 m depth. In the collapsing part the diameter is 108 mm, the rest of the central borehole tubes have a diameter of 152.4 mm. Four transmitters and four receivers (not shown on picture) will be installed in liner tube segments in the middle and at both ends of the collapsing borehole part. The closure of the borehole is determined in horizontal and vertical direction in the middle of the collapsing part by measuring the force of a loaded compressional spring due to radial convergence.

Measurement details:

Every day: seismic measurements (1 à 2 hours). Transmitters emit acoustic signal, receiver register the response. Preferable at night.

Rest of the day: detection and logging of acoustic emission features.
Coordinated and logged by an underground PC.
Duration: 2 à 2.5 years

Status/timing/planning :

Dec 2001: Start of the SELFRAC project.
March 2002 End of the construction of the connecting gallery.
May 2002: Installation of the two parallel multi-piezometers.
Dec 2003: First installation phase of the seismic and acoustic measurements: four instrumented boreholes.
May 2004: Installation of the central borehole.
Dec 2004: Official end of the SELFRAC project.

Results of the experiment :

EXPERIMENT 1: No preferential pathway has been observed between the piezometers. Macro-fractures range up of about 1m into the host rock. From other piezometer measurements around the gallery, we have observed a reduction of the extent of the EDZ with time, it has been shown that the open fractures close progressively. One and a half year after the excavation of the gallery the extent of open fractures do not extend beyond a zone of about 0.6 m around the gallery.
EXPERIMENT 2: In progress.

A modification of the water content during the first heating phase of CACTUS 1;
Displacements in the massif of low amplitude;
A slight increase of the density.

The most interesting observation concerns the water content variation: the irreversibilities established during the first heating phase do not appear during the next cycle of similar amplitude.

Conclusions :

The impact of the EDZ on the performance of radioactive waste geological repositories is an important issue. The first experimental results of the SELFRAC project clearly show That healing and sealing processes occur in-situ in Boom Clay. This means that the extent of the EDZ reduces with time. Therefore, the EDZ will probably not serve as a preferential pathways for the migration of radionuclides in a HLW storage.

Supplementary (new) experiment(s) expected.
Drilling and installation of the central borehole in the seismic and acoustic test will be performed May 2004.A supplementary test will also be installed May 2004. A fractured clay core will be instrumented with acoustic and piezometric sensors and re-installed in another borehole. The evolution of pore water pressure, seismic and acoustic parameters will be studied. The clay core will be retrieved by coring after the experiment to allow visual inspection.

Figure 3: Lay-out of the supplementary test.

Bibliography :

SELFRAC Progress Report N°1, period 01/12/01 to 01/12/02.
C. Coll, S. Escoffier, X. L. Li, R.W. Hamza, A. Vervoort, P. Blümling, W. Bastiaens, P. Berest, B. Bazargan, B. Frieg, J. Desrues, G. Viaggiani, V. Labiouse, B. Dehandschutter, L. Wouters, Y. Vanbrabant, J. Mertens, F. Bernier, "State of the art on Fracturation and Self-Healing Processes and Characterisation”, EC SELFRAC Deliverable 1, EURIDICE 04-114.
W. Bastiaens and J. Mertens, "EDZ around an industrial excavation in Boom Clay", Proceedings of a EC CLUSTER Conference held in Luxemburg 3-5 Nov 2003: Impact of the Excavation Disturbed or Damaged Zone (EDZ) on the Performance of Radioactive Waste Geological Repositories.