Research and Development on Geological Disposal of High-Level Radioactive Waste

3-3

Long-Term Investigation of an Excavation Damaged Zone around a Gallery－ Seismic Tomography Survey for Investigation of an EDZ －（a）（b）

Excavation 1（c）

Excavation 2

East access shaft

Excavation 3
Excavation 4
Excavation 5
Excavation 6
Excavation 7

Bo
se rehoism leic forsourceExcavation 8

Start sectionabout
4.0 mBorec reho
eiv leer forSeismic source
Receiver

Seismic tomographyareaExcavation1

Excavation

2

Excavation

3

Excavation

4

Excavation

5Excavation

6Excavation

7Excavation

8Excavation

905m

23m

Excavation 7Excavation 8Excavation 9Excavation 9Excavation 1001Pre-excavation

250 m W-Gallery

23m

Excavation 1001Excavation 6（d）

（e）

23m

Excavation 12（f）

12.0 mExcavation 10Excavation 1101

Driving direction

Ventilation shaft

Excavation 1201203m

150 days after excavation

123m

0350 days after excavation

123m

550 days after excavation
Pre-excavation area

1.1

West access shaft

Fig.3-10 Layout of the seismic tomography
This figure shows a plan view of the 250 m gallery off thewest shaft (250 m W-Gallery).

In response to the excavation of an underground opening,
the host rock properties such as the rock strength andpermeability are expected to change. The resulting zone iscalled the excavation damaged zone (EDZ). In geologicaldisposal of high-level radioactive waste, an EDZ is expected
to provide critical pathways for radionuclides. Therefore, it isimportant to develop a technique for quantitative assessmentof the EDZ.
To investigate the EDZ, seismic tomography surveys havebeen widely conducted in the field. This technique investigatesthe seismic velocity in rock by measuring the travel time ofseismic waves. When a seismic wave passes through fracturesin rock, its velocity decreases. In this study, we estimate theextent of the EDZ on the basis of the decrease in seismic wave
velocity at the Horonobe Underground Research Laboratory.
Fig.3-10 shows a plan view of the layout of the seismic
tomography in a gallery at a depth of 250 m. Seismic
tomography was conducted at the 250 m W-Gallery during

1.3

1.5

1.7

1.9Seismic velocity (km/s)

2.1

Excavated area

Fig.3-11 Results of seismic tomography survey
These figures show the velocity contour maps at each investigation
stage.

excavation (Excavations 1 to 12) and after the excavationof the gallery was complete. Fig.3-11 shows the results ofthe seismic tomography survey. In the pre-excavation andExcavation 6 stages, shown in figures (a) and (b), the seismic
velocity did not change remarkably. In Excavation 12,
shown in figure (c), a low-velocity area in which the velocity
decreased to around 1.3 km/s developed about 1.0 m into thegallery wall. After the gallery was completely excavated, thelow-velocity area was smaller at 150 days after excavation,
as shown in figure (d), but had expanded again at 350 and
550 days after excavation, as shown in figures (e) and (f),respectively. Consequently, it was found that the EDZ can beexpected to develop about 1.0 m into the gallery wall and thatthe extent of the low-velocity area changes with time.
We aim to examine the long-term change in the low-velocity
area in detail. In addition, we will conduct permeability tests andmeasure the saturation around the tomography area to investigate
the change in the hydrogeological properties in the EDZ.

ReferenceAoyagi, K. et al., Geomechanical Assessment of Excavation Damaged Zone in the Horonobe Underground Research Laboratory, Japan, Proceedings of the13th Japan Symposium on Rock Mechanics & 6th Japan-Korea Joint Symposium on Rock Engineering, Okinawa, Japan, 2013, p.905-910, in CD-ROM.
JAEA R&D Review 201343