True triaxial testing of rock is a challenging issue in rock mechanics because specialized testing equipment is required. Unlike conventional triaxial tests, true triaxial testing incorporates the effects of the intermediate principal which are now widely recognized as important in high stress environments, such as deep boreholes. Previous studies have shown that the increase in rock brittle strength as a function of the intermediate principal stress can be substantial, as much as 50% or more in some case. Using a historical context of development of methods and techniques, we explore and compare the evolution of true triaxial testing machines spanning several world regions such as Russia, Asia and North America. Current standards of the specimen preparation are described and the problems related to true triaxial rock testing are then discussed. In general we classify these challenges in categories associated either with a machine's material and design or with contact between the machine and specimen. Finally we cover the most popular machines, their importance and benefits, and factors leading to future directions.

Introduction and Background

Brittle rock strength can be expressed in terms of the minor principle stress, s3, intermediate principle stress, s2, and the major principle stress, s1. The role of the intermediate principle stress is commonly ignored given that its effect on rock strength is relatively less than that of the minor principle stress. However the significance of the intermediate principle in extreme earth engineering such as large earth dams, deep underground mining, and in borehole break outs, geothermal and earthquake studies is now widely recognized. True triaxial rock tests unlike conventional triaxial rock tests account for the role of the intermediate principle stress (Figure 1).

True triaxial rock tests are important because they satisfy the general rock testing principle that stress conditions applied to specimens mimic ‘in-situ’ conditions. For example, while useful, Uniaxial Compression Stress (UCS) tests (s1 > s2 = s3=0) are not valid for simulating conditions within the Earth's crust. On the other hand conventional triaxial tests (s1 > s2 = s3) are more reasonable and are widely used in practice. It turns out that, although cumbersome to undertake, it is actually true triaxial (polyaxial) tests (s1 > s2 > s3) which represent insitu conditions very close to reality.

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