Behavior of granular materials in microgravity environment: implication for future exploration missions

Innov. Infrastruct. Solut. (2017)2:22 DOI 10.1007/s41062-017-0082-7 SHORT COMMUNICATION Behavior of granular materials in microgravity environment: implication for future exploration missions Khalid Alshibli1 Received: 3 May 2017 / Accepted: 2 June 2017 Ó Springer International Publishing AG 2017 Abstract The constitutive behavior of soils such as strength, stiffness, and localization of deformations are to a large extent derived from inter-particle friction transmitted between solid particles and particle groups. Inter-particle forces are highly dependent on gravitational body forces. At very low effective confining pressures, the true nature of the Mohr–Coulomb strength envelope, which is the criterion most frequently used, is unclear both with respect to inter-particle friction and cohesion. Because of the impossibility of eliminating gravitational body forces on earth, the weight of soil grains develops inter-particle compressive stresses which mask true soil constitutive behavior even in the smallest samples of models. Therefore, the microgravity environment induced by near-earth orbits of spacecraft provides unique experimental opportunities for testing theories related to the mechanical behavior of soils. A brief summary of the results of triaxial experiments on silica sand that were tested aboard the NASA Space Shuttle are presented in this paper. Keywords Weigtless  Space  Sand friction angle Introduction All aspects of soil stability, bearing capacity, slope stability, the supporting capacity of deep foundations, and penetration resistance depend on soil strength. The stressThis paper was selected from GeoMEast 2017 – Sustainable Civil Infrastructures: Innovative Infrastructure Geotechnology. & Khalid Alshibli 1 Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA deformation and stress-deformation-time behavior of soils are of importance in any problem where ground movements are of interest. In most engineering materials, the strength is derived from internal chemical and physicochemical forces of interaction, which bond the atoms, molecules, and particles together. In cohesionless soils, the constitutive relations are mainly derived from inter-particle friction between particles and particle groups and dilatancy, and to a lesser extent from particle bonding by weak electrostatic, physico-chemical, and Coulomb forces. Under moderate-to-high stress levels, the influence of gravity on the behavior of laboratory test specimens may not be pronounced and, therefore, the test results in terrestrial (1-g) environment may be sufficiently conclusive. However, at low inter-particle stresses, which can result either from low applied (confining) stresses or from excess pore fluid pressures developed within the soil mass without corresponding changes in the applied stresses, the presence of gravitational body forces acting on solid particles and interstitial fluids exerts a pronounced influence on movement of individual particles or particle groups. Such motions, in turn, cause changes in soil fabric which results in significant changes in the inter-particle friction forces contributing to the soil’s strength and deformation characteristics. These experimental limitations on Earth have important implications in geotechnical, planetary, and earthquake engineering. For example, at or near-zero effective stresses, quantitative evaluation of the contribution to soil’s shear strength by particle interlocking cannot be accomplished by direct means at the present time. Yet, this shear strength component may be one of the most important factors affecting the stability of cohesionless earth masses under seismic loading, since it controls arching phenomena and volume changes resulting from dilatancy effects, hence, excess pore fluid pressure build-up 123 22 Page 2 of 4 or dissipation. The microgravity environment induced by near-orbits of spacecraft provides unique experimental opportunities for testing theories related to the mechanical behavior of soils. It eliminates the effects of specimen weight, specimen size, minimizes the effects of boundary conditions and makes it possible to create a uniform nearzero effective stress state throughout a soil test specimen (Fig. 1). Innov. Infrastruct. Solut. (2017)2:22 assembled around the specimen, filled with deionized water, and pressurized to 103.5 kPa. The internal vacuum was then removed and the pore space was vented to atmospheric pressure. The hardware used to perform the MGM experiments was especially designed and built for this purpose. Behavior of sand in microgravity environment Mgm experiments NASA sponsored a project called Mechanics of Granular Materials (MGM) which includes conducting a series of displacement-controlled quasi static triaxial compression experiments in a SPACEHAB module on the Space Shuttle during the STS-79 mission in September, 1996, and the STS-89 mission in January, 1998 [6]. The experiments were conducted on six right cylindrical specimens 75 mm in diameter and 150 mm long at effective confining pressures of 0.05, 0.52 and 1.30 kPa. The specimens, which were tested in the dry condition, consisted of sub-rounded quartz (Ottawa) sand with average grain size of 0.16 mm. The specimens were prepared in a terrestrial laboratory by slow pluviation (raining) of the dry sand into a natural latex membrane having a thickness of 0.30 mm, supported by a removable split cylindrical mold to ensure uniform density at 86.5% (±0.8%) relative density for the STS-79 mission and 65.0% (±1.0%) for the STS-89 mission, based on maximum porosity: 0.446, minimum porosity: 0.327, specific density of the particles: 2.65. A small amount of vacuum, which was typically in the range of 13–35 kPa relative to the outside atmospheric pressure, was applied through one end platen to the pore space within each specimen. The specimen preparation mold was then disassembled and removed. The external test cell was then Fig. 1 NASA Astronaut Jay Apt works on MGM experiment in space shuttle during STS-79 mission 123 The basic and dominant factors responsible for the strength of cohesionless granular materials is frictional resistance between particles at contact, and rearrangement and interlocking between particles’ groups. Angle of internal friction (/) and dilatancy angle (W) are measures of soil’s shearing resistance and volume change, respectively. The results of MGM microgravity experiments show unusually high peak strength friction angles in the range of 47.6°– 70.0° (Figs. 2, 3). The properties for the same material at the same density, tested at 13.8 and 34.5 kPa gives angles of 44.1° [1]. It was observed that the residual strength levels were in the same range as that observed at higher confining stress levels (Fig. 2). The dilatancy angles were unusually high in the range of 30°–31° (Fig. 3b). 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