Triaxial device
The triaxial device (triaxial device) is an experimental device in a soil mechanical laboratory with which the shear parameters ( cohesion and angle of friction ) of a soil sample can be determined. The execution of the test is precisely defined in DIN 18137, Part 2.
It is a cylindrical device into which a soil sample (which is taken from a drill core as undisturbed as possible) is inserted. At the top the device has a pressure stamp that presses on the soil sample, and at the side it is enveloped by a larger cylinder that is filled with a liquid (or compressed air). The soil sample is sealed with a rubber sock so that the liquid cannot penetrate. At the top and bottom are filter stones that allow water to escape from the sample.
During the test, the specimen is subjected to pressure from the sides and from above until it breaks. First a radial pressure is applied, then a vertical pressure is generated from above with the pressure stamp. Both voltages are measured and can also be varied. The compression is also measured. It is typical of the fracture state that the sample shears off from above on one or more inclined surfaces when overloaded, i.e. sample parts slide obliquely to the side.
The shear parameters are then calculated from the stress conditions at break. Mohr's circles of tension are used for this purpose , of which each experiment provides a circle. The results of several tests are entered in a diagram with compressive stresses and shear stresses and the enveloping straight line of the fracture circles (the shear line) is drawn from which the shear parameters can be read off directly.
The shear parameters are needed when calculating the stability of embankments , foundations and structures that are based on soils.
There are variants for the shear tests with the triaxial device depending on whether you allow the water to escape or not during the test and during the fracture and how quickly you load the sample:
- Consolidated, drained experiment (CD experiment)
- Consolidated, undrained attempt (CU attempt)
- Unconsolidated, undrained attempt (UU attempt)
All these conditions can occur in nature, and therefore the experiment must be carried out in a way that best reflects nature and the state of construction. The resulting shear parameters are accordingly different.
The triaxial device is so named because pressure is exerted on the sample in the direction of all three coordinate axes that are at right angles to one another. (Or between all 6 sides of a cube.) The test is therefore also called a three-axial pressure test or a triaxial compression test. If the side pressure is completely omitted, it is a uniaxial pressure test with unhindered side expansion, the so-called cylinder pressure test .
Theodore von Kármán had already used the triaxial device in 1910 to check the Mohr-Coulomb stress conditions. In 1928 Ehrenberg (an employee of Hans-Detlef Krey in Berlin) designed the first triaxial device for floor mechanics . Triaxial devices were used especially in the 1930s by Karl von Terzaghi and Leo Rendulic in Vienna (and then by Rendulic and Arthur Casagrande in Berlin at Degebo ) to experimentally substantiate the concept of effective voltages. The device was further developed in the 1940s by Terzaghi student Arthur Casagrande at Harvard (who was already working on it around 1930 at MIT) and, for example, in England in the 1950s by Alan W. Bishop . The latter wrote a standard work with Henkel on the triaxial device.
Web links
- TU Munich, see page 11, chap. I.4.1 (PDF file; 1.41 MB)
See also: direct shear device , box shear tester , circular ring shear tester , soil mechanics
Individual evidence
- ↑ e.g. B. Vasarhelyi Tribute to the first triaxial test performed in 1910 , Acta Geod. Geophys. Hung., Volume 45, 2010, p. 227. Alec Skempton already pointed this out in the 1950s.
- ↑ Klaus Weiß 50 years Degebo , Degebo Mitteilungen No. 33, 1978, p. 25.
- ^ Bishop, Henkel The measurement of soil properties in the triaxial test , 2nd edition, Edward Arnold, London 1962.