Assessment of heterogeneity of an internal structure of an earth-fill embankment with 2-D resistivity survey

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1 Assessment of heterogeneity of an internal structure of an earth-fill embankment with 2-D resistivity survey Peangta Satarugsa, Praiwan Uphatum, Manatchanok Buanark and Sakorn Sangchupoo Department of Geotechnology Khon Kaen University, Thailand Abstract It is no doubt that any earth-fill embankments are subjected to degradation and damage after many decades of use. The present study thus attempted to use 2-D resistivity survey for mapping of an internal structure of six earth-fill embankments (dams) in Thailand that have been constructed and used for the last 47 years or more. Damage to external structure is visible and so its repair is tenable. However, an attempt to protect and restore the utility of earth-fill embankments may be too late if the damage occurring to the hidden internal structure has left undetected. Thus, for the present study, six earth-fill embankments of more than 47 years of age were selected as cases. The resistivity profiles were measured at the top and at the downstream toe of each selected embankment. The results reveal the resistivity anomaly that could be interpreted to suggest heterogeneity/homogeneity of the subsurface earth structure. However, further study is needed to be conducted for a conclusive interpretation of the heterogeneities revealed for the first time from these selected six cases of earth-fill embankments in Thailand. Keywords: 2-D resistivity survey; earth-fill embankment; subsurface structure 1. Introduction The first man-made embankment (dam) in Thailand is almost 59 years old. It was built across the biggest river, the Chao Pha Ya River. Thereafter, many embankments were built across big and small rivers throughout the nation. Those embankments were built primarily for agricultural purposes and they were built as earth-fill embankments. After decades of use, an internal structure of the earth-fill embankment may have undergone erosion, the erosion may lead to seepage and the seepage may lead to piping. In consequences, those aged embankment can no longer hold water, leading to empty (dry) reservoir or embankment failure. It is necessary therefore that the internal erosion is detected in its early stage to make repair and functional restoration tenable. This paper reported and discussed a study using 2-D resistvity survey for detection of heterogeneity/homogeneity of six earth-fill embankments in the Northeastern region, Thailand. The study was conducted as a part of the main research on applied geophysical surveys for geotechnical and environmental practices. The six earth-fill embankments selected for this study were Huai Jorrakaemark, Huai Talad, Phuta Utayan, Haui Sai Kamin, Huai Num Bor and Phu Pet (Figure1). Small seepage of clear (no sediments) water occurring during the level of water storage in the reservoir rising close to maximum level has been reported for Huai Jorrakaemark. 1

2 This suggested potential problem with internal erosion leading to seepage obtained for Huai Jorrakaemark. A 2-D resistivity survey or so called electrical resistivity tomography/image has been used on numerous determinations for internal earth structures (e.g. Sjodahl et al., 2005; Cho and Yeom, 2007; Poisson et al., 2009). Typical homogeneous lateral resistivity anomaly is expected to be mapped for a good intact internal earth structures along the top and downstream toe of embankment profiles. 2. Geological setting of the study areas Surface geology of six studied areas is composed of Quaternary sand silt and clay soils. Outcrops of Quaternary basalts are exposed nearby the Huai Talad and Huai Jorrakaemark embankments. Sandstone and siltstone of outcrops of Khorat Group are exposed nearby the Phuta Utaya, Haui Sai Kamin, Haui Num Bor and Phu Pet embankments. 3. Geophysical Anomaly: Assumption and definition Generally, in the construction of an earth-fill embankment, soil composites (sands silts and clays) are mixed and compacted under specific conditions. Clay layers (beds) are laid out as an impermeable retaining wall. Accordingly, no abrupt lateral resistivity anomaly is expected along the embankment if an earth-fill embankment is properly built and there are no structural defects along the embankment. However, if the embankment has structural defects that might cause the embankment failure, an abrupt lateral resistivity anomaly of the resistivity image is expected to appear in a measurement. Figure 2 shows two assumptions of failure cases for leakage/seepage under a homogeneous earth-fill embankment. Their geoelectrical responses are expected to produce changes in low resistivity zone(s). If a low resistivity zone appears laterally along a profile, it may indicate a steady-stage flow. However, if a few low resistivity zones appear along a profile, it may indicate a non steady-stage flow and heterogeneous material filled during embankment construction. The larger number of low resistivity zones detected under an embankment so it will be the greater risk of failure of the embankment. 4. Method of Study Measurement of field apparent resistivity was conducted with the Syscal R1 Plus resistivitymeter. Electric current was injected into the ground through pairs of current electrodes while potential differences were measured with pairs of potential electrodes. Two profiles were measured for each embankment; the first profile ran on the top of the embankment and the second profile ran parallel the first profile and at the downstream toe of the embankment. Resistivity data were acquired with the Wenner-Wenner electrode configuration and with station of 10 meters. Apparent resistivity data were measured with electrode separations being increase with each successive traverse and they were interpreted with the RES2DINV of Loke (1999). The electrode separation started from 10 with 10 additional increments per separation (10, 20, 30, and up to 100 meters). The greater electrode separation is indicative of the greater penetration depth. 2

3 5. Results and Recommendations Table 1 provides a summary of general characteristics of the six embankments selected in the present study. Figures 3 and 4 show an example of resistvity images along the top and close to the downstream toe of the Haui Jorrakaemark and Huai Talad. Figure 5 shows only resistivity earth model interpreted on the basis of field apparent resistivity data from the six embankments. Comparison of the results from resistivity earth models as shown in Figure 5 can be drawn as the following. (1) Internal structures of the Haui Talad and Phu Pet embankments and subsurface beneath the downstream toe of the embankments resistivity images had the least heterogeneity whereas the internal structures of the Phuta Utayan, Haui Sai Kamin and Huai Num Bor and Haui Jorrakaemark showed great variation in heterogeneities. Thus, a re-survey along the embankments of Phuta Utayan, Haui Sai Kamin, Huai Num Bor and Haui Jorrakaemark is required for verification of the results revealed in this study. Additional embankments construction details should be obtained, including sources of material filled and any re-structures made after completion of its construction. (2) Subsurface image along the downstream toe of the Haui Jorrakaemark exhibits low resistivity values between locations 230m and 300m, and this requires a double check. Because seepage of water close to the downstream toe of the embankment in this area was reported, acquiring resistivity data should be attempted during the period when the water level in the reservoir rising close to the maximum level. (3) Of the six earth-fill embankment studied, the Phu Pet embankment appears to have the best intact internal structure. High resistivity profiles along the top and downstream toe of the embankment were mapped. For Haui Talad case, the low lateral resistivity zone outlined may indicate seepage under its embankment with a steady-stage flow through the sections despite the presence of homogeneity of lateral resistivity at the top and downstream toes. This suggests degradation occurs similarly throughout the internal structure of Haui Talad s embankment. For Huai Jorrakaemark and Phuta Utayan, a re-survey is needed before any conclusive interpretation can be drawn. Repeated measurements of resistivity at the same profiles and at the same season of every 3-5 years will help in an evaluation of an internal degradation of the earth structures. 6. Conclusions Results of the present study illustrate that the 2-D resistivity imaging survey successfully detects heterogeneity/homogeneity of the man-made structures such as earth-fill embankments. The resistivity images can be used as a basis for mapping and also for monitoring the internal erosion/degradation of earth structures. Acknowledgements This study was made possible by the Research Funds of Khon Kaen University and of the Department of Geotechnology, Faculty of Technology, Khon Kaen University. References Cho, I. K., and Yeom, J. Y Crossline resisitivity tomography for the delineation of anomalous seepage pathway in an embankment dam. Geophysics. v. 72,

4 Loke, M. H Electrical imaging survey environment and engineering studies: a practical guide to 2-D and 3D surveys: Geometrics, San Jose. Poisson, J., Chouteau, M., Aubertin, M., and Campos, D Geophysical experiments to image the shallow internal structure and the moisture distribution of a mine waste rock pile. Journal of Applied Geophysics. v. 67, Sjodahl, P., Dahlin, T., Johanssaon, S., Using resisitivity measurements for dam safety evaluatyion at Enemossen tailings dam in suthern Sweden. Environmental geology. v. 49,

5 Table 1: General characteristics of the six embankments studied. Haui Jorrakae mark Haui Talad Phuta Utayan Haui Sai Kamin Haui Num Bor Phu Pet Years built Age 47 yrs 52 yrs 47 yrs 57 yrs 57 yrs 58 yrs Storage area km km km Maximum 27.2 x10 6 m x10 6 m x10 6 m x10 6 m x10 6 m x10 6 m 3 storage Water s depth 7 m 3 m 8 m 6 m 10 m 8 m Height 8.5 m 4.5 m 13.5 m 8.3 m 17 m 12 m Width 7 m 4 m 6 m 6 m 6 m 6 m Length 1900 m 2300 m 1300 m 1300 m 950 m 530 m Purposes Agriculture, water supply, recreation and fish breeding Agriculture, recreation, and fish breeding Agriculture, water supply, and recreation Agriculture Agriculture Agriculture 5

6 Figure 1: Photographs of six studied embankments. 6

7 Figure 2: Seepage under homogeneous earth-fill embankment with (a) drainage impermeable and (b) drainage permeable foundations. 7

8 Figure 3: Photograph of Huai Jorrakaemark s embankment showing line locations and pseudosections of resistivity along Lines 11 and 12 as shown on the photograph above. Note: Line 12 between locations 230m and 300m shows a deep low resistivity zone whereas Line 11 illustrates results of low lateral resistivity zones as expected. 8

9 Figure 4: Photograph of Huai Talad s embankment showing line locations and pseudosections of resistivity along Lines 21 and 22 as shown on the photograph above. Note: resistivity images from Lines 21 and 22 appear as the results that can be expected from intact internal earth structures. A low lateral resistivity zone shown both Lines 21 and 22 suggests seepage under embankment with drainage permeable foundation (Figure 2 b). 9

10 Figure 5: Resistivity earth models from the top and downstream toe of six studied embankments. 10