ABSTRACT:

Twenty seven triaxial compression tests have been conducted on welded tuff at constant and controlled axial piston displacement rates. The piston displacement rates were then converted to axial stress rates according to a nearly linear load-displacement relation. Specimens were tested in three orders of displacement rates or stress rates and at three confining pressures. Results of thirty eight uniaxial compression tests were incorporated into the group of triaxial tests. The specimens for the uniaxial tests were collected from the same location in Yucca Mountain as for the triaxial tests. The stress rates for triaxial and uniaxial tests are the same.

Strain gages were used for thirteen triaxial tests and all the uniaixal tests. Tests in three stress rates of the order of 0.4, 0.04 and 0.004 MPa/s and four confining pressures, 0, 5, 10 and 20 MPa were analyzed. Differential axial stress at failure increases with confining pressure and stress rate, both relations following power laws. Volumetric strain increases with confining pressure following an exponential law and decreases with stress rate following a power law. Young?s modulus decreases with confining pressure and increases with stress rate. Poisson?s ratio increases with confining pressure and stress rate.

INTRODUCTION Results of twenty seven triaxial and thirty eight uniaxial compression tests on welded Topopah Spring tuff are presented. This testing program was a part of an investigation of long-term mechanical behavior of Yucca Mountain tuff in Nevada. Yucca Mountain has been selected as a candidate site for a High Level Nuclear Waste Repository in the United States [1]. The welded Topopah Spring Tuff is where the repository is to be excavated. Units exposed in the Topopah Spring Tuff include the Topopah Spring crystal-poor upper lithophysal zone (Tptpul), the Topopah Spring crystal-poor middle nonlithophysal zone (Tptpmn), the Topopah Spring crystal-poor lower lithophysal zone (Tptpll), and the Topopah Spring crystal-poor lower nonlithophysal zone (Tptpln) [2].

All the test specimens were from the Tptpmn zone. The specimens were prepared from rock drill cores received from the Sample Management Facility (SMF), Yucca Mountain Site Characterization Project. The drill cores were mainly from sidewalls of Alcove 5, which is for drift scale thermal test. The nominal diameter of the cores was 61 mm. For triaxial tests the ratio of length to diameter was controlled in the range of 1.9-2.5 to fit the 61 mm inner diameter Hoek triaxial cell. For uniaxial testing the ratio was between 1.8-2.6. The triaxial tests were conducted at constant and controlled axial piston displacement rates. The piston displacement rates were then converted to axial stress rates according to a nearly linear load-displacement relation. Specimens were tested at three orders of displacement rates or stress rates and three confining pressures. Results of thirty eight uniaxial tests [3] were incorporated into the group of triaxial tests. The stress rates for triaxial and uniaxial tests are the same.

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