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

2-10 Tunneling Underground to Investigate Initial Stress Conditions–3-D Initial Underground Stress Measurement by Hydraulic Fracturing–

Borehole ԙ
Borehole Ԛ
Borehole Ԙ
Borehole to measureinitial stress(20m×3)
Niche
Ventilation shaft

5q
30q

Borehole Ԛ

Horizontal drift
depth 140m

West shaft

Fig.2-19 Measured
initial stress(Schmidt net lower
hemisphere of theprincipal stress)

Borehole Ԙԙ
Niche
Ventilation shaft

Horizontal drift
Borehole to measure depth 140m
initial stress(20m×3)

East shaft

BoreholeԘ N60W, Upward 5°
Boreholeԙ NS, Upward 5°
BoreholeԚ N30W, Downward 30°

stress evaluated●，▲，■；Principalfrom borehole breakout

The inside of theparenthesis in a figuremeans strike and dip.

Initial stress measured
140m Niche:Borehole breakout (Max)underground(b) 140m deep level tunnel(niche) side view

140m Niche:Borehole breakout (M i n)140m Niche:Hydraulic fracturing (Max)140m Niche:Hydraulic fracturing (M i n)

Depth from surface(m)(a) 140m deep horizontal tunnel(niche) plane view

Dug part
Undug partstress evaluated by●，▲，■；Principal
hydraulic fracturing technique

Initial stress measured
underground

Overburdenpressure(HDB-6)

Overburdenpressure(HDB-11)

140m Niche:Borehole breakout
140m Niche:Hydraulic fracturing

Max・Min of explanatory notes shows themagnitudes of the maximum and theminimum horizontal principal stress

Magnitudes of the maximum andminimum horizontal principal stess(c) Borehole ① drilling situation(d) Relation between the
maximum/minimum horizontal
principal stress and depth

Directions of the maximum horizontal
principal stress(e) Relation between the directionof the maximum horizontal
principal stress and depth

Fig.2-18 Borehole drilling at initial stress measurement
positionFig.2-20 Comparison with the initial stress estimated inPhase1The Underground Research Laboratory (URL) has been
conducting a research project in Horonobe-cho, Hokkaido. This
project aims at improvement of the reliability of technologies
for geological disposal of high level radioactive wastes. It
started in 2000, and the surface-based investigation phase
was completed in 2005. Now it has shifted to the construction
phase2 (investigation during construction of the undergroundfacilities).
In the surface-based investigation phase1, the area of about
3 km×3 km around URL in the Hokushin district, Horonobecho was selected as the main research area, and geophysicalexploration, geological survey, borehole survey, etc. werecarried out in this area. In general, initial stress due to crustal
movement and overburden pressure has been taking place
underground here. In designing underground structures, itis necessary to measure initial stress around undergroundstructures. Therefore, estimation of initial stress distributedin the URL area has been attempted during surface-based
investigations in order to design URL.
In investigation during construction of the undergroundfacilities, the updating and check of the validity of initial stressestimations made during surface-based investigations is oneof the main issues. Thus, three boreholes were made from theniche (GL-140 m) of ventilation shaft shown in Fig.2-18, anda three-dimensional evaluation of the initial stress in Koetoi
Formation was done by the method called hydraulic fracturing.
Breakout (spalling) of the walls of the boreholes due to thestress concentration was detected on plastic films wrapped on
an impression packer inserted in the borehole. Then, analysisof the borehole breakout was performed to estimate in situ theorientation and magnitude of stress.
The measurement result of initial stress is shown in Fig.2-19.
Initial stress was broken down into principal stress σ1 (majorprincipal stress), σ2 (intermediate principal stress), and σ3(minor principal stress), and comparatively good coincidence indistribution of the direction of the principal stress evaluated by
the two above methods was observed. Although the magnitudes
of principal stress σ1, σ2, and σ3 found by the surface based
investigation and the impression packer were 2.3, 1.8, 1.2
MPa and 2.4, 2.3, 1.3 MPa, respectively and σ2 values were
somewhat different, general agreement was obtained. Fig.2-20
shows the result of the horizontal initial stress obtained fromthe borehole investigation (HDB-1, 3, 6, 9, 11 holes) carriedout in the past, and the result obtained from the present
underground test. Fig.2-20(d) shows the relation between themagnitudes of the horizontal principal stress, and depth. The
Fig.2-20(e) shows the relation between the direction of themaximum horizontal principal stress, and depth. From thisresult, although the magnitudes of the maximum horizontal
principal stress obtained here are mostly in agreement withthe overburden pressure, they are slightly smaller than the pastresults. Moreover, the direction of the maximum horizontal
principal stress had been found to be east/west in general.
However, the present test showed that the direction was 25° or
43° counterclockwise from east/west. To gain data to explain
this difference, initial stress measurements in the pressure at thedepth 140 m (not counting the niche) in Koetoi Formation, andpressure at the still deeper depth for the Wakkanai Formation
will be carried out.

Reference
Nakamura, T., Sanada, H. et al., Initial Stress Estimation of Diatomaceous Mudstone using Hydraulic Fracturing Test”, The 12th Japan Rock Mechanics
Symposium, 2008, p.297-303, in CD-ROM (in Japanese).
JAEA R&D Review 200831