Laboratory evaluation of grouted bulb region using cross-hole electrical resistivity tomography

Ground improvement by permeation grouting is widely utilized for improving engineering properties of soil. The accurate evaluation of a region improved by grouting is a critical issue to ensure the performance of grouting; nevertheless, current methods to evaluate the area of grouted bulbs under the ground are not well-documented. This study aims to use cross-hole electrical resistivity tomography (CHERT) to evaluate a grouted bulb region by considering the effect of electrode configuration at laboratory scale using a cylindrical container filled with saturated sand. Curing time monitoring was also conducted on grouted bulbs with various recovered Carbon Black (rCB) concentrations. Based on the inverted result, the predicted area of the grouted bulb was addressed, and then the error value between the actual and predicted areas was examined. The results of this study show that CHERT can be employed to assess the location and area of a grouted bulb. Also, it was observed that electrode spacing, damping factor, and curing time had significant influences on the image resolution and error value. Unlike the above-mentioned factors, change of rCB concentration ratio had only a slight effect on the image resolution. Thus, the use of a small amount of rCB as an additive material to enhance the grout strength can also provide a distinct contrast of measured resistivity between the grouted bulb and surrounding soil. In this study, a grouted bulb containing 3% of rCB is recommended based on the test results on strength characteristics. In addition, CHERT measuring can be conducted even in early stages of curing, during which it exhibits a better electrical resistivity contrast between the objective area and the surrounding soil.