[HTT18-05] A study of evaluation method for chemical soil improvement using 2-D and 3-D electrical resistivity tomography.
Keywords:electrical resisitivity tomography, chemical grouting, model experiment
Electrical resistivity tomography (ERT) is a one of the geophysical techniques to conduct the electrical resistivity of the underground by installing electrodes to surround the target using boreholes and is able to visualize underground structure with higher accuracy than a survey from the ground surface.
ERT is very effective in evaluating the ground improvement effect because the resistivity changes dramatically before and after ground improvement. However, since many boreholes are required for measurement, ERT is not still used as confirmation technique of the ground improvement effect widely. Therefore, we examined the optimal and feasible measurement and analysis method on the evaluation of the chemical grouting through the small-scale model experiments.
Before small-scale model experiments, we conducted the numerical simulation to decide the optimal electrode layout for ERT. The numerical simulations show that arranging the borehole electrodes as close as possible to the improvement object is important for imaging the form of improvement body. In addition, it was found that the accuracy of inversion increased as the number of boreholes increased.
Since it is not preferable to excavate many boreholes after improvement construction, we considered to use the boreholes of chemical injection for resistivity tomography. By using this idea, it is expected that the measurement cost for ERT will be greatly reduced.
Based on the numerical simulation results, several small-scale tank models were constructed in the laboratory, and 2-D ERT measurements were conducted. The model was made using a PVC column tank (diameter 600 mm, height of 400 mm), filled with Iide silica sand No.7. By preparing small block-shaped improved blocks in advance and arranging it in the tank, soil improved models were constructed in various patterns. The electrodes (2cm spacing) for ERT were installed to penetrate the improved blocks just like a grouting tube, the evaluation target was almost surrounded by the electrodes.
2-D measurement with cross-hole dipole-dipole array was conducted with and without improved block section, the resistivity change between two was considered as a change before and after improvement. These measurement data were inverted, and resistivity distribution was obtained for each section. Compared with these sections, resistivity decreasing zones were obtained and these zones were well matched with improved blocks.
Considering the actual soil-improvement site, there will be many other improved bodies around the improved target to evaluate. If 2-D ERT is applied in this situation, it can be predicted that a correct resistivity distribution cannot be obtained because of 3-D effect of other improvement bodies. Therefore, we applied 3-D ERT and verified the 3-D effect in same tank model experiment. As a result, it was found that the boundary of the improved body could be obtained more clearly than 2-D ERT and it was confirmed 3-D ERT had to be conducted when the structure was complicated. Currently, 3-D ERT require much more cost than 2-D ERT, but it is necessary to develop more efficient 3-D ERT technique in a shorter time.