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
[email protected]
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.
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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.
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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. 2007. Crossline resisitivity tomography for the
delineation of anomalous seepage pathway in an embankment dam. Geophysics.
v. 72, 31-38
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Loke, M. H. 1999. 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. 2009. Geophysical
experiments to image the shallow internal structure and the moisture distribution
of a mine waste rock pile. Journal of Applied Geophysics. v. 67, 179-192.
Sjodahl, P., Dahlin, T., Johanssaon, S., 2005. Using resisitivity measurements for dam
safety evaluatyion at Enemossen tailings dam in suthern Sweden. Environmental
geology. v. 49, 267-273.
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Table 1: General characteristics of the six embankments studied.
Years built
Age
Storage area
Maximum
storage
Water’s depth
Height
Width
Length
Purposes
Haui
Jorrakae
mark
1962-1963
47 yrs
12.60 km2
27.2 x106 m3
7m
8.5 m
7m
1900 m
Agriculture,
water
supply,
recreation
and fish
breeding
Haui Talad
Phuta
Utayan
Haui Sai
Kamin
Haui Num
Bor
Phu Pet
1953-1955
52 yrs
13.44 km2
27.8 x106 m3
1957-1963
47 yrs
5.26 km2
22.0 x106 m3
1953-1957
57 yrs
1.92
2.4 x106 m3
1953-1957
57 yrs
0.78
2.2 x106 m3
1952-1957
58 yrs
0.78
2.7 x106 m3
3m
4.5 m
4m
2300 m
Agriculture,
recreation,
and fish
breeding
8m
13.5 m
6m
1300 m
Agriculture,
water
supply, and
recreation
6m
8.3 m
6m
1300 m
Agriculture
10 m
17 m
6m
950 m
Agriculture
8m
12 m
6m
530 m
Agriculture
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Figure 1: Photographs of six studied embankments.
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Figure 2: Seepage under homogeneous earth-fill embankment with (a)
drainage impermeable and (b) drainage permeable foundations.
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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.
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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).
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Figure 5: Resistivity earth models from the top and downstream toe of six
studied embankments.
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