ABSTRACT

This article compares seven bi-dimensional and seven three-dimensional failure criteria, with the objective of determining the theoretical critical peak failure stress envelope that the laboratory results obtained from a slight anisotropic Feldspar-Quartz-Mica Gneiss represent the most. The Gneiss has a perfect brittle linear elastic behavior. In order to define the failure envelopes, sixteen indirect traction tests, five uniaxial compressive strength and six axisymmetric triaxial compression tests were performed. For the last two test types, a high stiffness servo controlled compression machine was used. Also, the method used for the axisymmetric triaxial compression test was the multiple failure state tests, which permitted to obtain more points at the peak state for each tested sample. For the studied Gneiss, it is concluded that the Non Linear Mohr-Coulomb bi-dimensional and the Non Linear Drucker-Prager three-dimensional theoretical envelopes accommodate better to the laboratory data results obtained for the peak state. The Gneiss belongs to the hoisting rock mass of the Porce III Hydroelectric Powerhouse Chamber, located in Colombia- South America.

1 INTRODUCTION

It is important to determine rock material critical stress envelopes in order to establish the progressive degradation of rock material thresholds, especially in brittle rocks. Knowing the peak strength envelope is important because the failure of a material is often said to occur at the peak strength or to initiated at that state (Jaeger & Cook, 1979) and also because it represents the stress state where a yield process begins, so it can be used as yield function in an associated flow law. Even though studies are concluding that the peak strength, even the uniaxial compressive strength with zero lateral pressure, may not be the characteristic for the rock material, as it is common thought (Martin, 1993).

But in the common practice of rock mechanics, due to the absence, in most cases, of specialized laboratory equipments to determine all the critical states, the peak value are still used as a "characteristic" value and as a failure indicator of rocks.

Under this context, peak failure can be present if a specimen of rock material is loaded under a stress state in which the rock can not sustain it anymore. This stress state is often called limit stress tensor or strength tensor. The strength tensor may be expressed in relation with the three principal stresses (σ1, σ2 and σ3) and strength material parameters (pi), which defines a function called also the criterion of failure (Eq. 1) and a locus in the principal stress space.

  • (Equation in full paper)

Its form may be suggested according to different authors' rules, being mostly empirical rules that should be consistent with experimental data, which for practical use, have a simple mathematical form.

In this article, seven bi-dimensional and seven three-dimensional failure criteria were compared, with the objective of determining the theoretical critical peak failure stress envelope that the laboratory results obtained from a slight anisotropic Feldspar-Quartz-Mica Gneiss represent the most, coming from the hoisting rock mass of the Porce III Hydroelectric Powerhouse Chamber, in Colombia- South America.

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