Triaxial compression tests
Specimens from drill cores are prepared by cutting them to the specified length and are thereafter grinded and measured. There are high requirements on the flatness of the endsurfaces in order to obtain an even load distribution. Recommended ratio of height/diameter of the specimens is between 2 and 3.
A membrane is mounted on the envelope surface of the specimen in order to seal the specimen from the surrounding pressure media. Deformation measurement equipment are mounded on the specimen and the specimen is inserted into the pressure cell whereupon the cell is closed and filled with oil. A hydrostatic pressure is applied in the first step. The specimen is then further loaded by increasing the axial load under constant or increasing cell pressure up to failure or any other pre-defined load level.
- Cell pressure up to 100 MPa
- Air-hole pressure up to 100 MPa
- Breaking stress
- Angle of friction
- Parameter of elasticity: E, ν
Stress state and deformations during the test.
Specimen inserted in between the loading platens, with equipment for deformation measurement attached on the specimen, before the cell is closed.
The pictures below show a specimen which has a shear failure.
The results and pictures are taken from
Jacobsson, L. Oskarshamn site investigation, Borehole KLX03A, Triaxial compression test of intact rock. SKB-P05-96, Svensk Kärnbränslehantering AB, 2005.
This and many other reports on triaxial compression tests, conducted at SP, are published on SKB
Example of standards
ISRM 1978, Suggested methods for determining the strength of rock materials in triaxial compression. Int J Rock Mech Min Sci Geomech Abs 15(3, sid 99-103.