Grain Density Measurements of Ash Flow Tuffs: An Experimental Comparison of Water Immersion and Gas Intrusion Pycnometer Techniques

摘要

The Nevada Nuclear Waste Storage Investigations (NNWSI) Project is currently investigating tuff formations at the Nevada Test Site for feasibility as a repository site. Grain density measurements are routinely made on tuff samples obtained from cored exploratory holes and are used in conjunction with mineralogical data, downhole density logs, and lithologies to define a thermal/mechanical stratigraphy. Grain densities are used directly in the calculation of porosity and in the interpretation of the variation seen in thermal and mechanical properties. Standardized grain density procedures such as ANSI/ASTM or API do not address the problems of testing hygroscopic minerals such as clays and zeolites that commonly occur in silicic tuffs. This report compares two widely used techniques for measuring grain density: water immersion and gas intrusion. It also describes sample-handling and operating procedures necessary for repeatable grain density measurements of zeolitized and claybearing tuffaceous rocks. Laboratory tests included in this report show the importance of careful sample-handling on the acquisition of accurate and repeatable data. Without consistent thermal pretreatment of hygroscopic tuff samples, grain densitites determined by either method can vary by as much as 10% due to the loss or gain of adsorbed water. Repeatable data are obtained only when pretest sample-handling procedures are both defined and rigorously followed. These data indicate that both techniques are probably sufficiently accurate and precise for most project needs. However, water pycnometer data have a higher level of precision for both zeolitized and nonzeolitized tuff samples than do gas pycnometer data. 10 refs., 4 figs., 5 tabs. (ERA citation 10:043871)