Principal Research Results

Development of In-situ Triaxial Test for Discontinuous Rock Masses

Background
Conventional in-situ test methods based on standards or guidelines of scientific societies are performed for determining themechanical properties of foundations of nuclear power plant and dam sites. They have several problems in which deformation andstrength characteristics are investigated separately, test results are significantly affected by disturbance of the loading surfaces and soon. A new field test method (in-situ triaxial test for rock masses) was developed to provide the solution to the above the problems.
Proof tests were carried out at sites of homogeneous soft rock and inhomogeneous rudaceous rock. However the application of the test
method at site of discontinuous rock was a very difficult problem because self-supporting large cylindrical specimens were not madeeasily for the site.

Objectives
The purpose of this study is to develop a new effective method of making large cylindrical specimens, improve the in-situ
triaxial test equipment, and then investigate the applicability of the test method to discontinuous rock masses.

Principal Results(1) Cylindrical specimens of weathered discontinuous sandstone were successfully made by a new drilling equipment developed inCRIEPI. The size of the specimen is about 38cm in diameter and about 100cm high. In the drilling, the drilling rate and the amountof drilling water were controlled based on the results of the measured bit weight, torque and so on. After the drilling, a triaxial cell
was inserted into the slit and hydraulic pressure was provided to the cell while lifting a core pack made of acrylic. As a consequence, the making of the cylindrical specimen for the in-situ triaxial test could be possible for discontinuous rock for which themaking of one has so far been difficult (Figure 1, Figure2).
(2) In-situ triaxial compression tests were carried out under four confining pressures with the in-situ triaxial test equipment which hasbeen improved in the strain measuring system. All the specimens were retrieved to the ground surface after the tests so that thevalidity of the test results was ascertained from any information as to discontinuity, shear bands and so on. Consequently, the highprecision measurement of stress-strain relationship and Poisson's ratio could be possible for discontinuous rock for which themeasurement has so far been difficult (Figure 3).
(3) The dependency of deformation modulus and Poisson's ratio on shear stress level, and the dependency of deformation modulus onconfining pressure were quantitatively-measured from the result of in-situ triaxial test for the weathered discontinuous sandstone.
As a result, the investigation of detailed mechanical property which has not been investigated before was possible (Figure 4).
The development of the testing equipment was carried out in CRIEPI and the application for in-situ rock mass was carriedout as a funded research with Kyushu Electric Power Company.

Future Developments
In order to put this test method to practical use, further examinations are needed to enhance applicability to discontinuousrocks, and thus it is important to improve the in-situ triaxial test equipment to investigate the dynamic mechanical properties andtensile strength characteristics for discontinuous rock masses.
Main Researcher: Tetsuji Okada,Research Engineer, Nuclear Fuel Cycle Backend Research Center, Civil Engineering Research Laboratory
ReferenceT. Okada, et.al., 2008, “Applicability of in-situ triaxial test for discontinuous rock masses - A case of discontinuous sandstone -”,CRIEPI Report N07513 (in Japanese)1189. Construction and Preservation of Electric Facilities

The drilling equipment is composed of a large steel casing
equipped with small motors which rotate drill bits. Thesteel casing is rotated and moved up-and-down by rubber
wheels equipped with two servomotors. The Specimen isinserted into core pack made of acrylic.

motorSteel casing
Core pack38cm

rubber wheel
Triaxial cellDrill bit

Specimen

Specimen

Specimen

100cm
①②④

③

①The drilling equipment is prepared on the ground surface.
②The specimen is made by drilling with a suitable width and then is covered with core pack.
③The rubber wheels and the upper part of the drilling equipment is taken away.
④The triaxial cell is inserted into the slit.

Hydraulic jack

Shear bands

Shear bands

Specimen

Specimen

Specimen⑤Specimen⑦⑥⑧⑤Hydraulic pressure is provided to the cell while lifting a core pack.
⑥The cap equipped with a load cell and the hydraulic jack are placed on the top of the specimen.
Triaxial test is conducted in the field.
⑦After the test, the cap and the hydraulic jack are removed.
⑧The specimen is cut at its bottom and lifted to the ground surface for further observation.

Poisson's ratio νFig.1 Test procedure of in-situ triaxial test
1.6

1.4

1
Test No.T- 4
Test No.T- 5
Test No.T- 9①
Test No.T- 9②

0.8
0.6
0.4
0.2

1.2

0
Axial force F

00.2

0.4

Axial force F10.6

0.8

1shear stress level q/qf
3500.8dd'ιElastic modulus E50 (MPa)

Devia tor stress q （MPa）

Fig.2 Specimen retrieved after tests
The specimen exhibited several shear
planes mainly along the discontinuities.
The brown lines in the photograph show
the discontinuities.

ι'0.60.4

Radial strain εr

Axial strain εa＝（d - d ' ）/ d＝（ι - ι '）/ιAxial strainεa

Test No.4
Confined pressure
σc：0.2MPa

0.2

Radial strain εr

-101

2

250
200150
Test No.T- 4
Test No.T- 5
Test No.T-9①
Test No.T-9②

100
5000-2300

00.1

0.2

0.3

0.4

0.50.60.7

90.8

Confining pressure σc (MPa)3Fig.4 Mechanical properties of the site
The dependency of Poisson's ratio on shear stress level(top figure) and elastic modulus on confining pressure(bottom figure) which has not investigated before were
measured from the result of the test .

strain εa ，εr（%）

Fig.3 Relationship between stress and strain
Stress-strain relationship of discontinuous rock is
obtained accurately under confined pressure.

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