Triaxial Testing

Failure modes in triaxial tests consisted of the following typical failures (Lama and Vutukuri, 1978d):

1. Sliding along joints.

In biaxial compression, sliding along joints occurred at low principal stress ratios σ1/σ3

and with joints at low angles to the direction of the major principal stress. As σ1/σ3 is

increased, a greater proportion of failure takes place though the intact material.

2. Shear failure through blocks

This failure mode appeared to be dependent upon the orientation of the block to the applied stress field. The principal stress at failure of the joints is dependent upon the shear strength of an element and the confining pressure (Lama and Vutukuri, 1978d). The failure surface of this failure mode occurred along a persistent failure surface inclined to the joint sets in the model.

3. Rotation of blocks

Rotation of the blocks within the shear plane appeared to occur when there was a high normal stress on the shear plane and a low normal stress perpendicular to the shear plane. This failure mode leads to high dilation, crushing and a wider shear zone. This phenomenon was also observed by Ladanyi and Archambault (1972).

Singh et al. (2002) and Yang et al. (1998) separated failures through intact material into those that involved shearing and those that involved splitting. Figure 2.10 shows the various failure modes of the jointed model rock mass as observed in Singh et al. (2002).

Figure 2.10: Modes of failure in regularly jointed rock masses (from Singh, 2002).

These mechanisms are not mutually exclusive and both Yang et al. (1998) and Singh et al. (2002) report combinations of more than one mechanism occurring together.

Singh et al. (2002) found the failure mechanisms depended greatly upon the inclination of the joints and the interlocking between the blocks. Obviously, the effects of interlocking are reduced at low joint inclinations where the failure mechanism is largely of the first types, splitting (a) and shearing (b) of the intact material. These types of failure determined the upper bound failure envelopes of the rock mass. The lowest values for failure were during sliding failure (d).

Figure 2.11 shows load-deformation curves through plaster tests shown in Lama and Vutukuri (1978d). The peak strength is maximum when failure involves shearing through plaster blocks (angle of joints to applied load, β = 67°) than when failure is more through the joints (β = 0). Failure along a single joint plane results in no dilation, whereas failure along multiple joint planes results in volumetric changes. The highest change in volume (β = 22°) was associated with rotation of the blocks.

Figure 2.11: Load displacement curves for different values of ββββ (Lama and Vutukuri, 1978d).

Ladanyi and Archambault (1970 and 1972) conducted triaxial tests on orthogonally jointed concrete block masses. Three separate failure modes were observed in these tests with continuous joints.

• Shear failure through intact material along a persistent failure surface inclined to both joint sets.

• Shear failure along a narrow zone accompanied with block rotation and sliding

• Formation of a ‘kink band’ three to five blocks wide, whereby columns of rocks rotate and separate.

The phenomenon of “kink bands” was further investigated by Ladanyi and Archambault (1993). The kink band failure was found to significantly reduce the strength of the rock mass even below that of a polished discontinuity surface. This was therefore considered to be a very dangerous mode of failure.

Similar results were found by Reik and Zacas (1978). On tests with low persistence, or discontinuous joints, failure occurred along a well defined failure plane or involved the formation of a shear zone involving several blocks. Shearing of the intact blocks proceeded in a progressive manner with the intact blocks being sheared off one after each other along the interface during failure. Interlocking of the blocks was largely significant in the failure behaviour of the specimen.

Figure 2.12: Triaxial test specimens used by Brown (1970).

Brown (1970) carried out triaxial tests on specimens constructed from parallelpipedal and hexagonal blocks as shown in Figure 2.12 and recognised the following additional failure types.

• At low confining pressures, axial cleavage fractures occur splitting the elements constituting the test body.

• At low pressures collapse of the specimen occurred due to block movement and opening of the joints. This type of failure was called dilational failure.

Despite the useful insights into jointed rock mass behaviour from these model studies, the ability to predict rock mass strengths from the results is severely limited by the assumptions and simplifications required for construction and testing.

2.2.5.4

Summary

It is clear that the failure mechanisms are mainly controlled by the number of weakness planes, their strength and orientation in relation to each other and the applied stress field. Unfortunately most model studies are conducted upon regularly shaped blocks in highly ordered assemblies. They are also designed to ensure that shearing failure occurs through the intact material during testing. While these tests are useful in examining the properties of good quality rock masses, the strength reduction associated with more irregularly shaped blocks and more naturally fractured rock masses is even less well understood although we can make relative qualitative guesses as to the magnitude. This is clearly an area of further interest as fractured rock masses become increasingly encountered in practical rock engineering. However, the effect of changes to the factors affecting rock mass strength discussed above are still likely to have a similar effect upon the strength of an irregularly jointed rock mass, although the relative change may be significantly different.

2.3 Estimating failure strength and deformability of rock