The effect of porosity on shearing resistance and thermal conductivity for amorphous soils in vacuum

Abstract

In support of the ensuing requirements for basic knowledge of the engineering properties of lunar surface materials, a study was made of the relationship between thermal conductivity, porosity, and shearing strength for amorphous silicate materials in a high vacuum (to 10⁻⁸ torr) environment. The basic objective of the study was to evaluate the "potential" of utilizing thermal conductivity measurements to delineate a soil's porosity or shear strength in a high-vacuum environment. Amorphous granular packings composed of glass beads and granules of volcanic glass were selected as laboratory soil models in order to minimize variables and coordinate with pertinent related research. The glass beads formed three test soils in the 20-40% porosity range and provided a proving media for the shear and conductivity apparatus. The granules of volcanic glass, on the other hand, formed four test soils in the 56-96% porosity range and were the object of the major effort to correlate thermal conductivity measurements to porosity and shear strength. The vacuum thermal conductivity of the test materials was determined by a "transient state" thermal probe constructed as a part of this research. The thermal probe was found to be stable in a vacuum of 10⁻⁸ torr in a temperature range of a -100 to +150°C. The data from the thermal probe evaluations was compared to that of a guarded cold plate reported in recent literature and was found to agree within 15% for comparable test materials and vacuum levels. The results of the direct shear investigation show that the shearing characteristics of the glass soils are not significantly affected by a 150° bake-out and a 10⁻⁸ torr vacuum test environment. The results of the vacuum correlation of thermal conductivity, porosity and shear characteristics for the 16-30 mesh sieve size volcanic granules show that thermal conductivity measurements are capable of discerning porosity to within ±5% and the angle of shearing resistance to ±3°. This finding points to the possibility of utilizing the thermal properties of the lunar surface materials to indicate in situ engineering characteristics.