Evaluation of Grouting Performance Using Electrical Resistivity Tomography Nguyen Duc Thanh 1,a , Gye-Chun Cho 1,b , Young-Jong Sim 1,c , and Seok-Won Lee 2,d 1 Dept. of Civil & Env. Eng., Korea Advanced Institute of Science and Technology (KAIST) 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea 2 Dept. of Civil and Env. System Eng., Konkuk University, Mojin-Dong, Kwangjin-gu, Seoul, Korea a nguyenthanh@kaist.ac.kr, b gyechun@kaist.ac.kr, c yjsim@kaist.ac.kr, d swlee@konkuk.ac.kr Keywords: electrical resistivity tomography, grouting performance, inversion. Abstract. This study proposes a tomography-based method for evaluating grouting performance after injection. Tomography is a convenient approach for solving the boundary measurement inverse problem of capturing discrete pixels and synthesizing these pixels into a unified image. Four arrays of eight electrodes are installed into large triaxial cell specimens to simulate in-situ crosshole resistivity testing. Sand is used as a base material and a wet cement mixture is grouted into the specimens. Electromagnetic waves are used as a means of accumulating the physical properties of the specimens. Each measured electrical resistivity is considered as a discrete signal. The electrical resistivity distribution is calculated and optimized through an iterative modified least-squares inversion based on a forward solution of Coulomb and Gauss’s law equations. Results show that the electrical properties of an injected grout material and its location and size can be effectively estimated from a series of resistance measurements. Introduction Electrical resistivity tomography is a useful tool in a wide range of geotechnical problems such as tunneling [1], mineral exploration [2], environmental monitoring [3] and grouting performance [4]. For the specification of anomalies, electromagnetic techniques become especially important because an experiment can be conducted in a small or large scale and can be relevant to non-destructive testing. Cross-borehole resistivity tomography using pole-pole data is a basic method used to investigate a medium [5], and it could be used quite effectively in laboratory tests. A regression inversion technique, which is also known as ‘iterative inversion’, has been devised for the solution of a simple two-dimensional resistivity [6]. The iterative method obtains the convergent values of parameters from an initial arbitrary guess. In some other researches, the Levenberg-Marquart scheme is used; as well, and the finite element method is a viable way to solve the problem [7]. Electric currents flow in a 3-D space instead of a straight path; therefore, it is necessary to deal with the problem of integration over the zone of interest [8]. In this paper, the pole-pole configuration is adopted as it can be used effectively in laboratory testing. Integration equations for electric field analyses are derived to obtain the transfer resistance of the system. By using an iterative modified least-squares inversion method, a method is suggested to reconstruct the image of an anomaly: a material injected in grouting performance. Theoretical Analyses A large triaxial cell specimen of 1.2 meters in height and with a radius of 0.3 meter is considered. Sand is used for the base material of the specimen. A grouting material is injected into the base material and its shape can be approximated as a spherical shape with the radius of r . A current is applied into the specimen and the induced potential is measured at a pair of electrodes with a radius . a