Triaxial compression

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. 

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. 

Density of soil in place by the sleeve method Unconsolidated undrained compressive strength of cohesive soils in triaxial compression... 

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 transformation of closed cell polymethacryUmide and LDPE foams are described in an attempt to impose reentrant structures on them. New methods of achieving permanent, triaxial compression were developed. 9 refs. 

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. 

Mehldahl (65) depicts several failure surfaces by photographs of various three-dimensional models. Figure 23 illustrates three such surfaces taken from Ref. 110, which shows geometries which are symmetrical about the space diagonal, oi = hydrostatic compression cannot lead to failure in the ordinary sense ). 

It is actually the creep strength of a grouted soil (either in unconfined compression or triaxial compression, depending on the specific application) that should be used for design purposes, not the unconfined compressive strength. In the absence of specific creep data, the value of one-fourth to one-half of the unconfined strength may be used for applications. A suitable safety factor must be applied to these suggested values. 

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). 

Figure 11.23 Comparison between experimental and predicted data in triaxial compression.

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. 

In most unsaturated rocks and soils, elastic properties and plastic flow depend on the capillary pressure which is related to water saturation degree through water retention curve (Fredlund and Rahardjo 1993). In this work, for simplicity, we neglect the variation of elastic constants with capillary pressure. However, we intend to account for the influence of capillary pressure on plastic behaviour of argillites. Only a small number of triaxial compression tests with different water saturation degrees are available. We can only provide a first approximation of such a influence. We consider that the failure parameter A (see Equation 13) linearly increases with capillary pressure ... 

In this section, we present some numerical simulations of triaxial compression tests performed on argillites in natural saturation condition and in different water saturation degree. 

Figure la Simulation of a triaxial compression test with 2MPa confining pressure (thin lines are model s simulations)... 

On Figures la and lb, we present simulations of two triaxial compression tests performed on... 

On Figure 4, we present simulations of triaxial compression tests performed with different water saturation degrees. Even if constant elastic parameters have been used, there is a qualitatively good agreement between simulations and data. Mechanical strength of material increases when saturation degree decreases due to effect of capillary pressure. 

Figure 4 Simulation of triaxial compression tests with different water saturation degrees...

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). 

The occurrence condition of shear fracture is examined on the basis of the Coulomb criterion. The averaged shear stress across the fracture plane in the simulated hydraulic stimulation tests is plotted in Fig. 5, as a function of the effective normal stress across the fracture plane. The steady-stale pore pressure distribution given from Equation (1) is averaged over the fracture plane and is used to compute the effective normal stress. Triaxial compression tests have been performed on the granite using the same apparatus shown in Fig. 

I (Takahashi, 2000). The peak shear stresses obtained from the triaxial compression tests are also plotted in Fig. A2. In the plot of Fig. 5, there is general agreement between the two types of the experimental results. Thus, it is thought that the occurrence of the shear fracture in the simulated hydraulic stimulation tests can be approximately predicted by the Coulomb criterion. Based on the comparison, the critical condition for the shear fracture due to hydraulic stimulation was estimated using the experimental results of the triaxial compression tests, as given in Fig. A2, and the averaged value of pore pressure. The detailed discussion of the triaxial compression tests can be found elsewhere (Takahashi, 2000.). 

Figure 5. Comparison of the shear strength data obtained from simulated hydraulic stimulation tests and triaxial compression tests. Those specimens which were pre-heated up to 500°C are designated (h).

Takahashi, 2000) are plotted in Fig. A2, as a function of the temperature. In order to eliminate the effect of thermally induced microcracking, some specimens were pre-heated up to 500 °C in air environment before the triaxial compression tests. 

Figure A2. Temperature dependence of shear fracture strength determined by triaxial compression tests...

Figure 2. Stress-Strain curves obtained by triaxial compression test. Confining pressure is lOOMPa and pore pressure is 25MPa.

Figure 7. Polarized micro-photograph of a quartz grain which has undergone triaxial compression test at condition of lOOMPa confining pressure and (a) room temperature, (b) 30(fC. (c) dSlFC and (d) 60O C.

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