Transparent Amorphous Silica to Model Clay Magued G. Iskander, P.E., M.ASCE 1 ; Jinyuan Liu, S.M.ASCE 2 ; and Samer Sadek, S.M.ASCE 3 Abstract: The fundamental premise of this research is that transparent synthetic materials with geotechnical properties similar to natural soils can be used in model tests to study three-dimensional deformation and flow problems using nonintrusive optical visualization techniques. The transparent ‘‘clays’’ discussed in this paper were made by consolidating suspensions of amorphous silica in a pore fluid with a matching refractive index. Conventional triaxial compression, consolidation, and permeability tests were performed to study the behavior of the material under normally consolidated and overconsolidated conditions. This paper demonstrates the feasibility of produc- ing a family of transparent materials, which can be customized to exhibit shear-strength, pore-pressure, volume-change, and permeability characteristics that are consistent with the behavior of natural clays. DOI: 10.1061/ASCE1090-02412002128:3262 CE Database keywords: Shear strength; Clay; Imaging techniques; Model tests; Silica. Introduction Imaging technologies are increasingly being used to study many geotechnical problems. For example, a system has been devel- oped to study the distribution of voids in soil specimens Frost and Kuo 1996. A vision cone has also been developed for in situ soil exploration Raschke and Hryciw 1997, and techniques were developed to use its captured images in soil characterization Hry- ciw et al. 1997. Optical techniques have also been used in model studies to measure the response of transparent materials, which model specific soil properties. For example, translucent media made of crushed glass and a matched refractive index fluid have been used to study stress distribution under simple shear Al- lersma 1982, as well as the response of saturated embankments under seismic loading Konagai et al. 1992. However, the study of soil response using optical techniques is limited by the lack of a transparent material that can accurately model the properties of natural soils. The development of transparent materials that can be custom- ized to meet model test requirements is the necessary foundation for utilizing optical techniques to study spatial deformation pat- terns and flow characteristics in three-dimensional 3D transpar- ent soil models. Two families of transparent materials are being developed for modeling sand and clay. The first family, which models sand, is made of transparent silica gels and matched re- fractive index pore fluid Sadek et al. 2002. The second family, discussed in this paper, is made by consolidating suspensions of amorphous silica and liquids with matched refractive indices and was found to model the geotechnical properties of natural clays Iskander et al. 1994; Iskander 1997, 1998. Both families have the same refractive index and use the same pore fluids, thus per- mitting their use in the same model Fig. 1. The material described in this paper has been used, with simple optical techniques, to measure the response of transparent soils in model tests. For example, Welker et al. 1999 used color saturation to study the flow of contaminants into perforated ver- tical wick drains and Gill 1999 used target tracking to study pile penetration in clays. At this time, techniques for measuring spatial deformations in transparent soils are being developed. Transparent soil models are sliced optically using laser sheets. Digital image correlation of successive images, taken before and after movement, is being developed to provide a two-dimensional image of the deformed shape within each slice. Multiple slices can be used to construct the complete spatial picture of the deformed mass. This paper is a comprehensive study of the geotechnical be- havior of transparent amorphous silica under both normally con- 1 Associate Professor, Polytechnic Univ., 6 Metrotech Center, Brook- lyn, NY 11201. 2 Graduate Research Assistant, Polytechnic Univ., 6 Metrotech Center, Brooklyn, NY 11201. 3 Graduate Research Assistant, Polytechnic Univ., 6 Metrotech Center, Brooklyn, NY 11201. Note. Discussion open until August 1, 2002. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on December 11, 2000; approved on August 1, 2001. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 128, No. 3, March 1, 2002. ©ASCE, ISSN 1090-0241/ 2002/3-262–273/$8.00+$.50 per page. Fig. 1. Target grid viewed through 2-in.-thick layered transparent soil model. The bottom three rows are viewed through amorphous silica discussed in this paper and the middle 4.5 rows are viewed through silica gel represents sand discussed in Sadek et al. 2002. The top 2.5 rows are viewed through pore fluid. 262 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING / MARCH 2002