ABSTRACT:

Laboratory testing has been undertaken to study the variability of the responses of rock specimens prepared with varying length-to-diameter ratios when subjected to uniaxial compressive strength testing. Two rock types were used in the experiment; these are granite and grano diorite, known to be massive and homogenous rocks. Specimens prepared at length-to-diameter ratios ranging from 1.5 to 3 were subjected to uniaxial compression in a Material Testing System (MTS) at a constant stroke rate, to evaluate the effects on the rock strength parameters, such as the peak strength, modulus of elasticity and Poison?s ratio (Í). The results of this testing have demonstrated that the modulus of elasticity of the two rock types did not change while the peak strength also did not vary significantly. The grano diorite and granite rocks indicated slight decreases on peak strength with increasing specimen length-to-diameter ratio. Significant variations were, however, demonstrated in the Poisson?s ratio (Í) achieved by the two rock types: The Poisson?s ratio (Í) decreased significantly with increasing specimen length-to-diameter ratio: a decrease of approximately 75% from the smallest specimen length-to-diameter ratio (1.5-to- 1 ratio) to the largest (3-to-1 ratio) for both rock types was apparent. This observation has a significant effect for determining important parameters such as Bulk Modulus (K) and Shear Modulus (G) of the two rock types tested, which are important parameters when performing numerical modeling. Significant benefits for use of the results to normalize specimen length-to-diameter ratio standards for uniaxial compressive strength testing applications has been assessed and presented

INTRODUCTION

The standard test method for unconfined compressive strength testing of intact rock core specimens and the standard practice for preparing rock core specimens and determining dimensional and shape tolerances are provided in ASTM D 2938-86 and ASTM D4543-01, respectively. Procedures, loading rates, specimen perpendicularity, flatness tolerances, minimum specimen diameter, specimen length-to-diameter ratio, etc. are provided by these standards.

A laboratory test program was set up to investigate the effects on the strength parameters of two different rock types when subjected to unconfined compressive strength testing prepared at varying length-to-diameter ratios. The two massive and homogenous rock types used were grano diorite and granite. Uniaxial compression tests were primarily conducted to obtain the intact rock unconfined compressive strength (UCS), Young?s Modulus (E) and Poisson?s ratio (Í) parameters. Besides providing an important data point for determining a failure locus (i.e. Mohr-Coulomb envelope) they are useful for defining parameters needed for numerical modeling. The loading conditions (stroke rates) during compression were also investigated to understand the effect of this variable on the mechanical behaviour of the rock. All specimens were drilled, cut and trimmed (in conformity with ASTM D4543-01) in the rock mechanics preparation laboratory of the Department of Mining Engineering, Queen?s University at Kingston, Ontario, Canada.

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