A cylindrical test specimen is subjected to an axially acting compressive load by our extremely rigid test frame. The load can be applied monotonically increasing with time at an arbitrary rate, with loading-unloading cycles or with time intervals of constant force or strain level. The test may be stopped just after the ultimate stress of the specimen or continued into the post-failure state until complete disintegration of the specimen. The specimen can be equipped with axial and circumferential strain gages and the relevant data is continuously recorded during the test. On the one hand, this ensures that the influences from the flexibility of test frame are eliminated, on the other hand, very brittle rocks can be stressed in a controlled manner beyond their peak strength. Another advantage of this technique is the chance to record the precise data on the axial and lateral deformation of test over the entire testing procedure. Our testing system can be equipped with specialized equipment such as an acoustic emission monitoring system to gain additional information on the specimen behavior.

The main values that can be obtained from this test are the unconfined compressive strength (UCS), the modulus of deformation, the modulus of elasticity (Young’s modulus), the corresponding Poisson’s ratio and the specific destruction energy.

In the triaxial test, right-cylindrical specimens are subjected to simple compression in the presence of a superimposed hydrostatic pressure. Theoretically, this results in a rotationally symmetric stress state where one principal stress is oriented parallel to the axis of the cylinder and two equal principal stresses lie within the plane that is normal to the axis of the cylinder. These two (horizontal) principal stresses have the same magnitude as the superimposed hydrostatic pressure, while the third (vertical) principal stress departs from it. In practice however, the principal stresses will locally depart from the specimen axis because of inhomogeneities within the rock material and because of boundary effects.

The triaxial cell used in the rock mechanics laboratory allows the installation of samples up to 250 mm in length and 100 mm in diameter. Like uniaxial compression test, special transducers for the measurement of longitudinal and circumferential strain are attached directly to the specimen in the triaxial test, which provide measured values for controlling the tests. By performing a multi-failure-state triaxial test, the progressive stress history of a single specimen can be observed instead of using different sample which might be inhomogeneous in the term of their microstructure. The advantage of this method is obvious, a complete data set (Mohr‘s envelope) is obtained for every single specimen that can be tested by the triaxial machine. The results have a significantly higher data density and provide more information in comparison to conventional single-failure-state triaxial tests. Testing brittle rock material with such a regime always is a big challenge, since failure tends to occur very quickly even under confined conditions. Therefore, it is practically impossible to manually increase the confining pressure in time of failure. Thus, the tests are controlled with an automatic failure detection routine, which ensures to perform a multi-stage test regardless of the condition of samples.

Shear strength of rock along discontinuities plays a significant role in the behavior of rock masses and can be assessed by laboratory direct shear tests. These tests can be conducted as multi-stage tests with several stages (levels) of a constant normal load applied to the discontinuity plane. This boundary condition is appropriate for certain rock engineering problems which involve sliding of rock blocks near the ground surface, such as rock slope stability, where normal stress remains constant while sliding occurs. However, when the dilation of the discontinuity while sliding is constrained, e.g. by the surrounding rock mass in a tunnel excavation, the normal stress on the sliding surface is supposed to vary with rock stiffness and sliding plane morphology. In such cases, constant normal stiffness boundary conditions are more appropriate.

With the direct shear device developed by our institute in cooperation with MTS Systems both classes of boundary conditions can be implemented in the experiments. Even new testing procedure that deviate from standardized tests can be carried out due to the automated control options. Examples are: Shear tests with constant normal stiffness, dilation-controlled tests, multi-stage normal stress-controlled tests, etc. Our testing system can also be used to perform shear tests on intact rock samples (size of shear plane up to 20 × 20 cm) without any problem. With these testing procedures all necessary parameters for shear failure criteria such as cohesion, peak and residual friction angle, peak and residual dilation angle, etc. can be derived from a minimum number of tests.

For weak rock samples where intact cores cannot be drilled and extracted due to their nature, but are still blocky material (e.g. Phyllite), the shear parameters such as cohesion, friction or residual angle can be determined through shear tests in blocky samples.

This triaxial testing equipment, which is specially designed for testing of a type of transition rock between hard soil and soft rock (HSSR), can properly duplicate field conditions in these very challenging formations. In weak and low permeable materials, core extraction, handling, storage, and sample preparation can easily reduce the degree of saturation of the sample material. Thus, also the effective stress state is influenced prior to testing. The triaxial machine is able to perform sophisticated testing procedure on this very sensitive material including back-saturation, consolidation and shearing under hydro-mechanical coupled boundary conditions. The valuable advantage of this machine is the availability of a high-precision pore water pressure system, which provides a chance to measure and control pore water pressure, permeability coefficient, and the saturation level through any stage of the tests. The whole testing system is situated in a climate-controlled room which minimizes measurement errors due to temperature fluctuation and enables long-term tests to be conducted. Equipped with a high-speed digital controller operated in a closed-loop the testing procedure can be programmed in a very flexible way on three control circuits. Therefore, novel testing procedures such as multi-state tests in drained or undrained conditions can be conducted to determine the shear parameters. Axial and radial deformation is monitored directly at the specimen (i.e. within the confining pressure cell) allowing for a high-precision evaluation of the deformational properties of the challenging HSSR-rock samples.

The preparation of specimens which comply with the requirements and tolerances defined by testing standards or recommendations is a prerequisite for any kind of rock mechanical testing. This work takes up much and skilled craftsmanship. Most mechanical property test specimens are cylindrical. They are obtained by secondary drilling in drill cores from exploration drillings or block samples. The specimens are prepared by cutting the cores into required lengths by disc saws and finished on a surface grinder. After preparation, every specimen is measured for its dimensions, weighted, photographed, and finally sealed to preserve its water content.

The rock mechanics laboratory is equipped with all necessary technical equipment and machines to prepare specimens out of any kind of rock. Typically, diamond tools with tap water used for cooling and flushing are used. However, for very (water-) sensitive rock material other flushing media such ethanol, brine or compressed air can be used as well. This is mandatory for HSSR-material (at the transition of hard soil to soft rock) which typically is inhomogeneous, having low cohesion and finest silty-clay parts alongside with coarse rock components. For this reason, we are working with a special diamond band saw to prepare specimens without crumbling.

In addition to standard laboratory tests, a wide variety of special test can also be performed with the available laboratory equipment. We offer tailor-made tests either on rock material or tunnel support elements. Examples are: Large-oedometer tests (diameter 600 mm), compression tests on tunnel lining elements, four-point-bending tests of pipe roof umbrella elements and compression tests on lining stress controllers. The tests can be carried out in force or strain control. Creep test (constant stress), stress relaxation tests (constant strain), rapid load tests as well as static and dynamic loading are possible.