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Institute of Soil Mechanics and Rock Mechanics (IBF)

Engler-Bunte-Ring 14

D-76131 Karlsruhe

Tel.: +49 721 608-42221

Fax: +49 721 69 60 96

E-Mail: info∂ibf kit edu

Large-Scale Triaxial Tests


  Drilling equipment for large-scale specimens

The drilling machine has a diesel hydraulic drive. The the core drill bit is dimensioned for specimens of 600 mm in diameter and 1200 mm in length. The flushing medium is compressed air, sometimes a dusty business...




Triaxial cell for large-scale specimens

Max. specimen diameter: 1000 mm
Max. specimen length: 1900 mm
Max. axial load: 6000 kN
Max. confinement pressure: 2.2 MPa (= 320 psi)

The confinement pressure medium is water. Testing of weak material and bulk solids is also possible with special technical equipment.




  Preparation for the test

Installation of the displacement transducers on a waterproof covered specimen with approx. 600 mm in diameter and 1000 mm in length.



"True" large-scale triaxial tests

Although the term "triaxial test" is well-established on testing cylindrical shaped specimens, the above mentioned testing technique is no "true" triaxial testing because the two smaller principal stress components are equal.

Therefore, the term "polyaxial test" has been chosen to describe a testing technique in which the values of the three principal stresses differ from one another. Such tests are only realisable with cubic shaped specimens. In doing so, a load frame with flat hydraulic jacks on two opposing sides generates the intermediate principal stress whereas the minor principal stress is generated by the confinement pressure. The axial load cylinder applies the maximum principal stress.



  Extraction of a cubic large-scale specimen

with a combinded drilling and sawing method.

Cross sectional area: 620 x 620 mm
Length: 1200 mm




  Installation of the cubic specimen in the triaxial load frame

This load frame is installed in the triaxial cell (on the left in the background). The flat hydraulic jacks on both sides of the red rubber jacket which encloses the specimen generate the intermediate principal stress.




  Cubic specimen after the test

The failure causing shear fracture is obviously.

Accompanying Material Analyses

In addition to the material strength tests, we also perform miscellaneous material analyses. We pass some of it to the Institute of Mineralogy and Geochemistry of the University of Karlsruhe. Among other things, the scope is:

  • Determination of the rock density with hydrometic weighting
  • Determination of the water content of rock
  • Water content at the yield point and the plasticity limit according to Atterberg
  • Ignition loss according to DIN 18128
  • Lime content according to DIN 18129
  • X-ray diffractometric determination of the quartz content
  • X-ray diffractometric determination of the mineral composition
  • X-ray diffraction diagram
  • Microscopic slide analyses
  • Chemical analyses
  • Grain density according to DIN 18124
  • Combined screening and sedimentation
  • Desiccation/moistening tests
  • Determination of the material alteration by the effect of water

Index Tests


  Splitting tension test (Brazilian test)

This test is used for the indirect determination of the tensile strength of rock.If a circular cylindric specimen is compressed along its diameter, failure occurs by an extension fracture in the loaded diametral plane (see image) at some value of the applied load. With this load and the dimensions of the specimen the uniaxial tensile strength can be estimated.

  Point load test

This test is practical for an estimating classification of rock and provides an index for the order of magnitude of the uniaxial compression strength. The point load test is primarily used in the field because of the usability of irregular pieces of rock and the transportable test equipment. The rock specimen ist loaded between two hardened steel cones of predetermined geometry with a hydraulic hand pump. However, useful results are only provided with lagely brittle and isotropic rock material.
     Slake durability test

Many sorts of rock show noticeable signs of disintegration after cyclic drying and wetting. This test provides an index value for the resistance to such alterations. In a predetermined cyclic procedure, broken rock is alternately dried and sieved in a screening drum by the effect of water.

Testing of the Swelling Behaviour

Swelling of materials which are relevant in civil and underground engineering are counted among the most alarming phenomena in geotechnics. The volume increasing of clay by absorption of water is a physical effect (osmosis), whereas the volume increasing during the hydration of anhydrite into gypsum is a chemical effect. Nevertheless, the consequences for the constructions are always the same: destructive stresses, deformations and loss of strength, often occurring and recognised months or even years later.

Common characteristics of both effects are the stress-dependent behaviour, the distinct anisotropy and the long time lapse. A swelling test requires months, even years in hydration of anhydrite. Theoretical, the maximum volume increasing of clay is less then 20%, whereas it's 64% at the anhydrite-gypsum transformation.

 


  Standard swelling pressure test

Number of load frames: 35

Specimen diameter: 60 to 100 mm
Specimen length: 20 to 40 mm
Max. axial load: 50 kN
Testing temperature: 20°C (= 68°F), constant

This test is used for the determination of the swell-caused stress/strain dependency in axial orientation (usually perpendicular to the bedding). The specimen is fitted in a stiff metal ring in order to prevent radial strain.

  Free-swell test

Number of load frames: 14

Specimen edge length: 30 to 150 mm, cubically
Testing temperature: 20°C (= 68°F), constant

This index test is suitable for a comparatively quick (normally 1 or 2 weeks) estimation of the swelling potential and the associated swelling anisotropy factor of rocks.

Tests at high Temperatures


  Compression test system with environmental cabinet

Platen diameter: 310 mm
Max. span: 360 mm
Max. load: 550 kN
Control range of displacement: 0.001 to 500 mm/min
Max. testing temperature: 400°C (= 752°F)

 

Testing of time-dependent Material Properties

 


  Uniaxial creep tests in a environmental chamber

Number of load frames: 32

Max. specimen diameter: 130 mm
Max. specimen length: 260 mm
Max. load: 200 kN
Testing temperature: 20°C (= 68°F), constant

 


  Triaxial creep tests in a environmental cabinet

Number of triaxial cells: 2

Specimen diameter: 70 mm
Specimen length: 140 mm
Max. axial load: 200 kN
Max. confinement pressure: 16 MPa (= 2320 psi)
Max. deviatoric stress (worst case): 37 MPa
Max. testing temperature: 60°C (= 140°F)