Void distributions in samples of Ottawa sand C. HU*†, T.-T. NG†, and S. ALTOBELLIz †Department of Civil Engineering, University of New Mexico, Albuquerque, NM 87131, USA zNew Mexico Resonance, 2301 Yale Blvd SE, Suite C1, Albuquerque, NM 87106, USA (Received 12 May 2006; in final form 17 July 2006) The void distribution of saturated specimens of Ottawa sand is presented. The presence of water inside the sand specimen is detected using the magnetic resonance imaging (MRI) technique. The void distribution of the sample was determined from the image. The specimen was prepared in a non-metallic triaxial cell and was put insie a MRI apparatus to obtain the image. Two sample preparation methods (wet tamping and dry pluviation) were used to illustrate the uniformity of the samples in the initial state. The void distribution along the height of the sample and the three-dimensional orientational void distribution at different locations inside the sample were analysed. The results indicate that the sample generated by the dry- pluviation method is more uniform than the sample generated by the wet-tamping method. When the wet-tamping sample preparation technique is used, the dense sample is more uniform than the loose sample. The development of voids was investigated by a sample loaded inside the MRI device under drained compression condition. The void distribution along the height of the sample at different stages was observed. This work has demonstrated the feasibility of using the MRI technique to examine void distribution in granular material. Keywords: Void distribution; MRI; Ottawa sand; Dry pluviation; Wet tamping; Triaxial test 1. Introduction In order to describe the relationship between stress and strain in granular materials, it is necessary to know how the material changes during loading. This additional internal information is very useful for the development of a micromechanical based constitutive model. Many invasive and non-invasive techniques have been used to investigate void distribution in granular materials and the devel- opment of voids in response to static external loading. These techniques include photo-imaging analysis of assemblies of two- dimensional (2D aluminum or photoelastic rods) (Oda and Konishi 1974, Matsuoka and Geka 1983), thin-section analysis (Oda 1972, Oda et al. 1985, Frost and Kuo 1996, Kuo and Frost 1997), X-ray computed tomography (Lee et al. 1992, Lee and Dass 1993, Desrues et al. 1996), laser-aided tomography (Konagai et al. 1992, Konagai and Rangelow 1994), and indirect measurement of electrical resistance or wave velocity (Arulmoli and Arulanandan 1994, Santamarina and Cascante 1996). Magnetic resonance imaging (MRI), which is truly three- dimensional and non-invasive, has certain advantages over the above-mentioned methods. Unlike X-ray computed tomogra- phy, it does not involve ionizing radiation and does not undergo signal attenuation due to scattering at significant concentra- tions. Because of the sensitivity of MRI to physical and chemi- cal differences, in addition to concentration, it has the advantage that different phases (e.g. fluids and particles or even two fluids) can be distinguished. MRI measures the fluid phase in the voids directly, and thus provides a complete picture of the pore fabric. Therefore the MRI technique is preferred in the study of voids of saturated samples. Ng and Wang (2001) have successfully used the MRI technique to study the internal struc- tures of a granular material in a simple shear test, and Hu et al. (2004) have studied the 2D directional void distribution of samples of Ottawa sand. In the work reported here, void distribution of samples prepared by dry-pluviation and wet-tamping methods was further investigated using 3D MRI test data. 2. Magnetic resonance imaging MRI is an imaging technique used primarily in medical settings to produce high-quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about mole- cules. The nuclei of some atoms have magnetic moments, i.e. they behave like small magnets which line up along a static magnetic field in the same way as compass needles. However, unlike classical compass needles, the atomic nuclei are unable to line up fully with the magnetic field because of the conditions imposed by quantum mechanics. Instead, each nucleus remains at a discrete angle with respect to the field and precesses around the field at a frequency proportional to the magnetic moment and the strength of the magnetic field. MRI measures the dis- tribution of nuclei at the various precession frequencies. Geomechanics and Geoengineering: An International Journal Vol. 1, No. 3, September 2006, 197--206 *Corresponding author. Email: huchhua@unm.edu Geomechanics and Geoengineering: An International Journal ISSN 1748-6025 print=ISSN 1748-6033 online Ó 2006 Taylor & Francis http:==www.tandf.co.uk=journals DOI: 10.1080=17486020600941673