Behavior of Returned Lunar Soil in Vacuum

Two oedometer and three direct shear tests have been performed in vacuum on a 200-g sample of lunar soil from Apollo 12 (Sample No. 12001, 119). It had been expected that the vacuum environment and clean particle surfaces would cause the lunar soil to have a lower compressibility and a higher shear strength than that of a ground basalt simulant in air. Instead, it is found that the opposite is true; the comrpessibility of the lunar soil is slightly greater and the shear strength is significantly less. This difference is tentatively attributed to the degradation under stress of fragile lunar soil particles such as agglutinates and breccias. Over the range of densities tested, the typical virgin compression index is 0.06 and the friction angle varies from 28 to 35°. Large-scale particle crushing occurs at normal stresses of less than 70 kN/m² (10 psi), and consequently, the Mohr-Coulomb strength envelope should be extremely curved. The shear strength of the other lunar soils at the same void ratio will probably vary according to the proportion of friable particles.