THE EFFECTS OF BOTH SAMPLE AND PORE SIZES ON THE MECHANICAL PROPERTIES OF WELDED TUFF

The elastic and strength properties of the welded Topopah Spring Tuff are important to the stability of underground openings for the planned high-level waste repository within Yucca Mountain, Nevada. Two important questions being considered by the Yucca Mountain Project at the lab scale are: “Can we predict the effects of scale on the mechanical properties?” and “We know that the tuff mechanical properties are highly dependent on porosity, but does the distribution of the porosity significantly alter the effect?”

Relative to the first question, two empirical models show a consistent pattern of behavior in the welded tuffs. Samples from the middle non-lithophysal zone of the Topopah Spring, with volumes varying over an order of magnitude, were tested in the mid-1980’s. Recently, a second series of tests have been conducted on another section of the same unit. When the data are plotted as ultimate strength versus sample volume in log-log space, two groupings are apparent. The best-fit curves to the data are parallel, but offset. The offset is believed to be the result of different porosities in the samples tested.

It has been shown that total porosity appears to be the first-order factor in determining the strength of the tuffs; however, the distribution of porosity likely has a secondary effect on strength. The trends in the ultimate strength versus (total) porosity data indicate that, although there is a large scatter in the data, there is a clear correlation between strength and porosity. As a result, knowing the total porosity provides an indirect measure of the intact strength, within uncertainty bounds.

One of the key elements in the design of the potential repository at Yucca Mountain, Nevada, is the stability of the possible emplacement drifts. Mechanical property data collected in the lab are vital to having confidence in the near- and far-field modeling of the facility. Two key aspects of these studies involve the sample size and the size of contained pores, and how these factors affect the mechanical properties. The results from recent studies have shed light on these issues and will be presented here.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.