Compression triaxial tests

The hot mixes are designed by using a standard laboratory compaction procedure to develop a composition reflecting estabUshed criteria for volume percent air voids, total volume percent voids between aggregate particles, flow and stabdity, or compressive strength. Tests such as the Marshall, Unconfined Compression, Hubbard-Field, Triaxial Procedure, or the Hveem stabdometer method are used (109). 

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. 

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.)... 

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. 

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.

The results of the drained triaxial compression shear tests with the same initial states and different temperatures (22 °C and 90 °C) are compared for several overconsolidation ratios (OCR) (Figure 2). The initial confining pressure was 600 kPa. 

Similarly, the group of logarithmic curves of cohesion vs. time of the rock are obtained shown in Figure 3, by a series of triaxial compressive creep tests at seven temperatures from 20°C to 300°C. In the same way as mentioned above we can also obtain the main curve (Figure 4) at the reference temperature at 20°C as well as the corresponding shift factor parameters (Table 2). 

MV—miniature vane shear UC—unconfined compression test ICU— isotropic consolidated triaxial test. 

AnhDan, L., Tatsuoka, R, and Koseki, J. 2006. Viscous effects on the stress-strain behavior of gravelly soil in drained triaxial compression. Geotechnical Testing Journal, 29(4) 330-340. 

Considerable numbers of laboratory triaxial tests have been performed worldwide on a wide variety of clays and silts over the past four decades and these are documented elsewhere (Mayne, 1988 Kulhawy Mayne 1990).A summary plot of the triaxial compression data (both CIUC and CKqUC) are presented in Figure 9 and indicate a rather nice corroboration of equation (lb)... 

Janbu, N. (1963). Soil compressibility as determined by oedometer and triaxial tests. In Proceedings of 3rd European Conference on Soil Mechanics and Foundation Engineering, Volume 1, Wiesbaden, Germany, pp. 19-25. 

The elastic-plastic constitutive model for triaxial simulations is introduced to illustrate the Mohr-Coulomb strength criterion and dilatancy. Figure 11 shows the soil model for the simulation of a displaeement-controlled drained triaxial test, where total stresses from the previous isotropic compression are taken as initial conditions. A vertical displacement... 

Rock strength (which is typically measured using unconfined triaxial compression [TX] test per ASTM guidelines) is used to evaluate bearing capacity, excavatability, and so on. 

Undrained shear strength testing of cohesive material (e.g. clay) can be realised in the laboratory by means of the vane shear test or the triaxial test, type UU (Unconsolidated Undrained) or UCS (Unconfined Compression Strength) on undisturbed samples. In such triaxial tests, sample preparation and sample size may influence the results. 

Where the state parameter approach substantially improves on the NCEER method is if the effects of soil properties are included by measurement/calibration rather than correlation. Extension to directly include the effect of soil properties requires measurement of in situ (which is frequently carried out anyway for the site response analysis) and measurement of the soil compressibility A (defined on Figure 8.46, and typically requiring a program of 5 10 triaxial tests). 

Strength Laboratory vane Unconfined compressive strength test Triaxial test Direct shear test In very soft clays only Usually only for slope design 1... 

Strength Unconfmed compressive strength Triaxial test Indirect tensile strength test (such as Brazilian splitting test) Point load test (Zs50) Most widely used measure of strength Unusual, weak rock only Must be supported by compressive strengths 1... 

Deformation Under Loa.d. The mechanical behavior of coal is strongly affected by the presence of cracks, as shown by the lack of proportionahty between stress and strain in compression tests or between strength and rank. However, tests in triaxial compression indicate that as the confirming pressure is increased different coals tend to exhibit similar values of compressive strength perpendicular to the directions of these confining pressures. Except for anthracites, different coals exhibit small amounts of recoverable and irrecoverable strain underload. 

Direct shear test of soils under consolidated drained conditions Consolidated-undrained triaxial compression test on cohesive soils One-dimensional consolidation properties of soils One-dimensional consolidation properties of soils using controlled-strain loading... 

The tensile strength of compacts [30] also provides useful information. Excellent specimens of square compacts are necessary to conduct the tensile testing. For this reason, a split die [31 ] (Fig. 2) is used to make compacts that are not flawed. The split die permits triaxial decompression, which relieves the stresses in the compact more uniformly in three dimensions and minimizes cracking. These specimens are then compressed with platens 0.4 times the width of the square compacts in the tensile testing apparatus. (Fig. 3). Occasionally nylon platens and side supports are used to reduce the tendency to fail in shear rather than tension. The force necessary to cause tensile failure (tensile forces are a maximum... 

Uniaxial tension testing with superposed hydrostatic pressure has been described by Vernon (111) and Surland et al. (103). Such tests provide response and failure measurements in the triaxial compression or tension-compression-compression octants. 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

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