Alam, A.K.M.B., Niioka, M., Fukuda, D. and Fujii, Y. (2013), Confining Pressure Effect on Structual Changes in Rock
Types under Consolidation and Compression, Proc. MMIJ Fall Meeting 2013 Sapporo, PY-7, p. 571, Hokkaido University,Sapporo, Sep. 3Confining Pressure Effect on Structural Changes in Rock Types
under Consolidation and Compression
Alam AKM B., Niioka M., Fukuda D., Fujii Y., Hokkaido Univ.
Triaxial tests were carried out at 1 15 MPa confining

For Kimachi sandstone, the 0.39 mm average thickness ofpressures (CP) at 295 K. The samples of 30 mm diameter andcementing materials by TSIA for 1 MPa CP consolidation was

60 mm in length were held for 24 h at the target consolidationmuch thicker than the 0.25 mm average by compaction atpressure, and then constant strain-rate compression at 10−5 s−1

15 MPa CP. A distinct main rupture plane was observed in thewas applied until the stroke-based strain reached 10 % for theCT images after axial compression at 1 MPa CP (Fig. 2a).

Shikotsu welded tuff or 7 % for the Kimachi sandstone andSeveral subrupture planes were observed and the thickness of

Inada granite. After the tests, microstructure was analyzed bycementing materials was 0.27 mm. At 7 MPa CP, only one

thin-section image analysis (TSIA) with a resolution of 8.8 µm

main rupture plane was found. At 15 MPa CP, macroscopicof the blue resin-impregnated specimens, and macrostructure

rupture planes were observed neither in CT images (Fig. 2b)(the number, orientation, and geometry of the rupture planes of

nor in TSIA. This occurred because large plastic deformation

the specimens) was also analyzed by micro-focus X-ray

took place in the cementing materials. The thickness of thecomputed tomography (CT) with a resolution of 37 µm.

cementing materials was 0.20 mm.

For Shikotsu welded tuff, the porosity decreased by
41.23 % due to the consolidation at 15 MPa CP by TSIA. Thisoccurred because the CP was close to the unconfinedcompressive strength and was sufficiently high to cause pore(a)collapse. The equivalent diameter of pores (dpore) in the intact(b)

specimen was 0.04 0.16 mm. The frequency of dpore ≥ 0.08mm decreased, and dpore ≤ 0.08 mm became dominant at

Fig. 2 Sample and CT images of Kimachi sandstone under

15 MPa CP (Fig. 1). Pores with a smaller aspect ratio increasedcompression (a) at 1 MPa CP (b) at 15 MPa CP.

at 15 MPa CP, but pores with a small angle to the horizontal
flow layer remained dominant in the 15 MPa-consolidated

For Inada granite, no consolidation effect was observed inspecimen. A rupture plane was observed in the CT images in

TSIA. A main rupture plane in the axial plane, with subrupture

axial plane of the specimen after axial compression at 1 MPa

planes, was observed in the CT image at 1 MPa CP (Fig. 3a).

CP. However, macroscopic fractures were not observed for 15

The main rupture plane consisted of a network of microcracks.

MPa CP. The porosity by TSIA near the rupture plane after

Numerous axial cracks from biotite grains were also observed.

axial compression at 1 MPa CP increased by 16.99% and was

The distinct single rupture plane in the CT image was also

greater than that far from the rupture plane. The frequency

observed in TSIA at 9 MPa CP. This rupture plane also

value of dpore = 0.10 mm increased near the rupture plane, andconsisted of a network of microcracks; however, it had athe frequency value of dpore = 0.06 mm was dominant far fromsmaller width than that at 1 MPa CP, and axial cracks fromthe rupture plane. The frequency of a pore aspect ratio ofbiotite were not observed. Multiple rupture planes werearound 0.5 decreased near the rupture plane.

observed at 15 MPa CP (Fig. 3b). The subrupture planes
formed, as there were no soft cementing materials between
mineral grains, so significant stress concentration occurred on
the stiff and rough main rupture plane.

Consolidation

80

6040

no loading
15 MPa20

Frequency (n)Frequency (n)8000.1 0.2 0.3 0.4
Equivalent diameter (mm)1MPa (far)60
1MPa (near)
40
20
00

After Compression

00.1 0.2 0.3 0.4
Equivalent diameter (mm)

(a)

(b)

Fig. 3 Sample and CT images of Inada granite underFig. 1 Equivalent diameter of pores of Shikotsu welded tuff.

compression (a) at 1 MPa CP (b) at 15 MPa CP.