The Use of Geo-electrical and Geotechnical Techniques to Assess a Small Earth Dam in Remote Areas

It is the hub of more than 200 small earth dams. The location map of the site tests is shown in Figure 1. The field testing included electrical resistivity and dynamic cone penetration testing along with laboratory testing. Two survey lines were conducted on the crest of the earth dam using AGI Supersting R1/IP. A total of 28 electrodes were laid on the crest of the earth dam with 1 m electrode spacing and dipole-dipole array, refer to Figure 2. Data acquired from the field were analyzed with the help of EarthImager 2D. DCP tests were also conducted at various locations and data was recorded up to a depth of 2.5 meters. Soil samples were obtained from the field for laboratory testing in accordance with the ASTM standards. Results showed that the soil was classified as CL according to the Unified Soil Classification System. In addition, the liquid limit obtained was 34%, the plastic limit was 17%, and the plasticity index was 17%. Findings from the standard Proctor test showed that the dry unit weight and optimum moisture content were 19 kN/m3 and 22%, respectively. The angle of friction obtained from the direct shear test was 27°, and the cohesion was 38 kPa. The residual shear strength parameters obtained were c' = 0.5 kPa and φ' = 25°. The 2D electrical resistivity tomography ERTs on the dam are shown in Figures 3 and 4. RMS values of 9.5% and 2.6% for the investigated profiles indicated that the data fit with the computed response. The resistivity for profile 1 ranged between approximately 15 Ω.m and 63 Ω.m, indicating clay-type soil. This was consistent with borehole soil samples collected, which revealed that the soil consists of clay-type material. Profile 2 provided resistivity values ranging from 18 Ω.m to 47 Ω.m. Lower values near the surface represented higher water content due to water infiltration from late spring rainfall. The thickness of the top layer was approximately 2 m along the two profile lines. A layer of drier clay was found between 2 m and 4.5 meters, below which wetter clay was present. The resistivity of clayey formation is represented by a resistivity of less than 100 Ω.m. ERTs also revealed a saturated zone, which may be a probable/possible seepage area. Dynamic cone penetration profile 1 (DCP1) was conducted between electrodes 15 and 16 (i.e., 15 and 16 meters) along profile 1. Figure 5 shows the variation of DCP and resistivity data with depth for DCP1. Results for DCP 2 (along profile 1) with resistivity results are depicted in Figure 6. The DCP shows up to two layers, the top layer had a DCP value of 10.6 cm/blow and the second layer, 4.4. Figure 6 shows that lower DCP values were equivalent to higher resistivity values, which is consistent with the trend shown in Figure 5. A third DCP, DCP 3, was conducted along ERT profile 2, at a distance of 13 m. Results are plotted in Figure 7. DCP and Resistivity results showed the existence of three layers. It is also evident that lower DCP values were associated with high resistivity results. Figure 8 shows a plot of DCP versus inverted resistivity results. Attempts were made to develop a correlation between DCP and resistivity. A linear relationship with a low R-squared value was developed. Results showed no statistically significant correlation in Figure 8. This is consistent with studies in the literature. The study was conducted based on field results and limited to approximately 7 data points. The authors are currently conducting both field and laboratory studies on a wide range of soils (e.g., sand, silt). This study was undertaken to identify areas of anomalous seepage in an earth dam. Results showed zones with low resistivity values, which could be associated with high water content – an indication of probable/possible seepage zones within the embankment. Efforts were also made to establish qualitative and quantitative correlations between electrical resistivity and DCP rates. In general, both approaches revealed the same number of layers and provided consistent depths. Quantitative analysis revealed a linear model with a low R-squared, which is not statistically significant and indicates no correlation between electrical resistivity and DCP rates. Additional laboratory and field testing are being conducted by the authors to increase the database on this topic and attempt to develop quantitative relationships between DCP and other geotechnical parameters.