Fawzy M. Ezzein 1 and Richard J. Bathurst 2 ATransparent Sand for Geotechnical Laboratory Modeling ABSTRACT: The paper describes a new transparent granular soil that can be used for laboratory geotechnical modeling purposes. The trans- parent soil consists of fused quartz particles in combination with a mixture of two mineral oils as pore fluid. The solid particles and the matching liquid have the same refractive index. The soil has important advantages with respect to transparency, stability, health safety, and utility over glass and silica gel materials. The transparent soil is also inexpensive compared to silica gel-fluid materials that have been used in the past. Conventional laboratory shear box, triaxial compression, and permeability tests were carried out to demonstrate that the mechanical properties and hydraulic per- meability of the transparent soil are typical of granular soils with angular particles. KEYWORDS: transparent soil, fused quartz, refractive index, granular soil Introduction Transparent soils hold promise for the visualization of a range of model-scale geotechnical soil-structure interaction problems and for laboratory studies of porous media fluid flow and contaminant transport (e.g., Welker et al. 1999; Iskander 2010). Early attempts at transparent soils used crushed glass or glass beads in combina- tion with a matching fluid with the same refractive index (Waka- bayashi 1950; Drescher 1976; Allersma 1982). However, porous media using glass beads are translucent rather than transparent and they do not represent the geotechnical properties of natural granular soil (Mannheimer and Oswald 1993; Sadek et al. 2002). Better success has been achieved by matching transparent solid silica powder or a silica gel medium with a colorless pore fluid having the same refractive index. The use of precipitated and flumed silica powder and silica gel beads to manufacture transpar- ent model clay and sand has been described by Iskander et al. (1994, 2002a, 2002b), Gill and Lehane (2001), Sadek et al. (2002), Liu et al. (2003), Zhao and Ge (2007), and Hird and Sta- nier (2010). However, the use of transparent silica gel beads to manufacture simulated granular soils has the following limitations to different degrees depending on the actual gel material: (a) The particles deform plastically even under low confining pressure (Iskander 1998; Iskander et al. 2003; Zhao and Ge 2007). (b) De-airing the internal pores that are part of the silica gel par- ticles is difficult and impractical for quantities that are required for large scale tests (Iskander et al. 2002b, 2003). (c) Silica gel particles are hydroscopic and thus affected by high humidity and water which can cause the particles to break and become colored. (d) There may be other chemical processes that cause changes in color and hence reduce transparency with time (Iskander 2010). (e) The clear visible depth into the transparent soil is limited to about 50 mm. Sadek et al. (2002) concluded that there is no silica gel material available that can satisfactorily simulate the mechanical properties of fine sand. The writers are currently engaged in research related to granu- lar soil-structure interaction problems using metallic mesh and (polymeric) geogrid soil reinforcement systems. In order to visu- ally observe the reinforcement elements during load transfer a transparent soil was required. The writers initially examined six different candidate granular particle types and 20 different clear liquids having matching refractive indices. The particle materials were glass beads, crushed glass, silica gel beads, fused silica, clear plastic beads, and granulated magnesium fluoride. The limitation of glass materials noted earlier was independently confirmed by the writers as was the breakage of silica gel beads in the presence of water and high humidity. The magnesium fluoride particles available to the writers proved not to be transparent. Fused silica particles were also examined but were found to contain very small air bubbles trapped within the particles which rendered the par- ticles translucent when saturated with a matching fluid with the same refractive index. Candidate fluid materials were eliminated for laboratory modeling for one or more of the following reasons: (a) Excessive fluid volatility leading to changes in composition and thus changes in refractive index with time. (b) Potential health risk due to inhalation of toxic fumes and high flammability rating in excess of safety regulations for unvented laboratory environments. (c) High viscosity of the liquid. (d) High cost. Manuscript received February 10, 2011; accepted for publication June 27, 2011; published online August 2011. 1 Ph.D. Candidate, GeoEngineering Centre at Queen’s-RMC, Dept. of Civil Engineering, Royal Military College of Canada, Kingston, Ontario, K7K 7B4 Canada, e-mail: Fawzy.Ezzein@rmc.ca 2 Professor and Research Director, GeoEngineering Centre at Queen’s-RMC, Dept. of Civil Engineering, Royal Military College of Canada, Kingston, Ontario, K7K 7B4 Canada. (Corresponding author), e-mail: bathurst-r@rmc.ca Copyright V C 2011 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 1 Geotechnical Testing Journal, Vol. 34, No. 6 Paper ID GTJ103808 Available online at www.astm.org Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.