A COMPARISON OF FOUR GROUND ELECTRICAL RESISTIVITY SURVEY (GERS) ARRAY METHODS USED IN INVESTIGATING INJECTED GROUTING: A CASE STUDY OF THISSA DAM IN SRILANKA
M. D. J. P. Wickramasooriya*
Irrigation Department, Sri Lanka
Session: Technical Session B
Abstract
The Thissa reservoir with a capacity of 4.32 million cubic meters is situated in Thissamaharama in Hambanthota district about 250km South from Colombo, Sri Lanka. Geologically, it lies within the litho-techtonic unit of the Vijayan Complex in Sri Lanka. For several years, the Thissa reservoir has experienced seepage issues, posing potential risk to the earthen dam. To address these problems, a rehabilitation study using the two-dimensional Ground Electrical Resistivity Survey (2D-GERS) method and the clay-cement grouting method was conducted. The 2D-GERS method, a geophysical method for detecting subsurface or rock anomalies, was used to compare four electrode array configurations: Dipole-Dipole, Gradient-XL, Schlumberger and Wenner. These arrays helped identify injected grout and seepage zones beneath the Thissa wewa earthen dam. The objective of this study was to compare the performance of four arrays, assessing seepage after the clay-cement grouting treatment. The analysis of the 2D-GERS profiles at seepage locations along with the grouting results shows that the 2D-GERS method can serve as a cost-effective investigation method for assessing the seepage locations through earthen dams. The areas of heavy cement-clay grouting are clearly delineated by the 2D-GERS profiles from the different array methods. The Dipole-Dipole array recorded the largest dataset (1149 points) and reached greater depths and was faster compared to the others. In contrast, the Wenner array had the smallest dataset (345 points) and captured shallower depths. The Wenner, Gradient-XL (1030 points) and Schlumberger (748 points) arrays produced relatively similar profiles. However, it was noticed that the Dipole-Dipole array was less sensitive compared to the other three arrays. Overall, the Gradient-XL and Schlumberger arrays captured the grout intake at an acceptable level.
Keywords: dipole-dipole, gradient-XL, schlumberger, wenner
DOI: 10.64752/KKCU3519