Triaxial tests

Triaxial compression tests are another means of determining shearing strength of a soil. A complex device is used to apply pressure along the sides of a cylindrical specimen and axially down the axis of the cylindrical specimen. In general, triaxial tests are superior to direct shear tests since there is better control over intake and discharge of water from the specimen. 

Triaxial tests usually involve more elaborate testing equipment and more precise measuring techniques than those normally used for uniaxial and biaxial measurements. In the poker chip test (33, 59) the faces of a thin circular disc are bonded to rigid plates (Figure 18). The speci-... 

Repeated Load Triaxial Tests for Permanent Strain Evaluation. In accordance with the procedure in the VESYS IIM Users Manual, the same sample used in the creep tests was also used to measure permanent strain, an important indicator of the rutting potential of a pavement. Repetitive loads of the haversine type were applied to the specimen at a magnitude equal to the applied vertical stress used in the creep tests. The load duration was 0.1 sec followed by a rest period of 0.9 sec. 

Data from the repeated load triaxial test are used to calculate permanent strain which can be plotted vs. load repetitions on log-log scale. Figure 5 shows a typical plot of accumulated axial strain vs. number of load repetitions. The confining stress was zero for all triaxial tests. 

Figure 5. Typical accumulated strain curve from triaxial test data...

Luong, M.P. Reflections on Triaxial Testing, Rheology and Flowability, Silos, Fundamentals of Theory, Behaviour and Design Brown, C.J., Nielson, J., Eds. E. FN Spon London and New York, 1998 65-75Ch. 2.4. 

Kolymbas, D. Wu, W. Recent results of triaxial tests with granular materials. Powder Technol. 19 60 (2), 99-119. 

Laboratory tests to determine shear strength are the direct shear test, the unconfined compression test, and the triaxial test. ASTM Standards give complete details of these tests. Triaxial tests yield the most reliable results. The manner in which these tests define the failure line as shown in Figure 2.4. 

Figure 10.7 (a) Failure lines for grouted and ungrouted granular soils, (b) Drained triaxial test results for silicate grouted coarse and medium sands. (From Ref. 11.15.) (c) Typical stress-strain curve from unconfined compression test on chemically grouted sand, (d) Compression versus time data for creep test on chemically grouted sand, under constant load, (e) Failure time versus percent of unconfined compression failure load. (+) indicates unconfined compression tests, and ( ) indicates triaxial tests with S3 = 25% of Si. 

Creep tests are run by subjecting a grouted soil sample to a sustained load less than the short-term Unconfined Compression strength, until failure occurs. As the sustained load decreases in value, the time to failure increases. Data from such tests plot as shown in Fig. 11.22, and indicate an asymptote at what may be called a creep endurance limit. For acrylamides these values will range from as low as 20% Unconfined Compression for triaxial tests at low lateral pressure to as high as 40% for triaxial tests at at rest lateral pressures. (For silicates, values may be taken from Fig. 11.11.)... 

Figure 11.22 Acrylamide creep studies. Triaxial tests with 83 = 25% Si. Acrylate Grouts...

Since many grouts and grouted soils are subject to creep phenomena, the tests must be long-term. Because strength increase due to confinement within a soil mass is an important factor, triaxial tests are indicated. 

The triaxial test apparatus was first developed for testing soils16 but this is unsuitable for powders because it is designed for a relatively high stress range. Many different modifications of the... 

A flexible boundary cubical triaxial test is another commonly used test for compression studies (Kamath et al., 1993 Li and Puri, 1996). A picture of a triaxial compression tester is shown in Figure 8. It allows not only the application of the three principal stresses independently, but also constant monitoring of the volumetric deformation and deformations in three principal directions. In a triaxial compression test, the specimen is at an initial isotropic state of stress then the three pressure lines apply the same pressure at the same rate to all six faces thus pressure is the same in all three directions (i.e., cti = 02 = 03). 

Different commonly used compression characterization methods have been demonstrated the Brazilian test, the confined uniaxial and unconfined compression test, and the flexible boundary cubical triaxial test. The HHP method appears as a new and promising possibility within the traditional compression methods. In HHP, higher pressure gives higher bulk density however, beyond a critical given pressure, the final compressed bulk density remains constant (ultimate bulk density). Further studies will determine whether this concept can be included as a quality descriptor in the specification data sheets of commercial food agglomerates. 

The triaxial testing data and other data (e.g., from... 

Figure 2. Argillites mechanical behaviour under classic tr iaxial compression tests, 2a) Triaxial test with unloading paths for determining damage evolution, 2b) Triaxial test with increasing step by step the confining pressure on the post-failure phase.

In order to study the effects of desiccation on the mechanical behaviour of argillites, several types of laboratory hydromechanical coupling tests were performed uniaxial and triaxial tests on the samples with different water content, desaturation under isotropic confining pressure with monitoring associated displacement. In most cases, the samples were previously submitted to a cure under a given humidity level and at constant temperature. 

Figure 3. Triaxial tests on argillites samples with different water content under 10 MPa of confining pressure...

Figure 2. Drained triaxial tests at ambient (22°C) and high temperatures consolidation pressure 600 kPa, 22 °C - dashed lines, 90 °C - solid lines.

To study hydromechanical coupling in deep stiff clays we need to supply mean stress relatively high linked to the in situ stress state. Thus, to perform triaxial tests on stiff clays with permeability measurement we use a special triaxial apparatus (high pressure cell) with two independent drainage line. We have developed also a system for local strain measurements composed of 7 LVDTs which are suitable for tests in oil (confining fluid) under high level of stress. Tests are performed on cylindrical specimens (up to 80 mm in height and 40 mm in diameter). 

Experiment results of triaxial tests on saturated samples allow us to define the yield surface of oillike and water-like plugs. The transition between these two cases is characterized by results of suction controlled oedometer tests. 

Janbu, N. 1963. Soil copresibility as determined by oedometer and triaxial tests. Proc. European. Conf. on SMFE Vol. 1 420-431. 

High temperature triaxial tests of rock salt... 

Figure 2. Sketch of the integrated apparatus for gas hydrate coal syntheses and triaxial test.

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