Application of electrical resistivity method to detect deep cracks in ...

Unsaturated Soils: Theory and Practice 2011
Jotisankasa, Sawangsuriya, Soralump and Mairaing (Editors)
Kasetsart University, Thailand, ISBN 978-616-7522-77-7
Application ofelectrical resistivity method to detect deep cracks in unsaturated
Application
residual
of electrical
soil slope
resistivity method to detect deep cracks in
unsaturated residual soil slope
Eko E.A. Andi Suryo Suryo,
Brawijaya
Brawijaya
University,
University, Malang,
Malang,
Indonesia,
Indonesia
Queensland University of Technology, Brisbane, Australia, eko.suryo@student.qut.edu.au.
Queensland University of Technology, Brisbane, Australia, eko.suryo@student.qut.edu.au.
Chaminda C. Gallage Gallage & B. Trigunarsyah & Bambang Trigunarsyah
Queensland University of Technology of Technology (QUT), (QUT), Brisbane, Brisbane, Australia, Australia, chaminda.gallage@qut.edu.au,
bambang.trigunarsyah@qut.edu.au.
chaminda.gallage@qut.edu.au, bambang.trigunarsyah@qut.edu.au
Indrasurya I.B. Mochtar B. & Mochtar R.A.A. & Soemitro Ria Asih Aryani Soemitro
Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia, indrasurya@ce.its.ac.id, ria@ce.its.ac.id
ABSTRACT: Crack is a significant influential factor in soil slope that could leads to rainfall-induced slope
instability. Existence of cracks at soil surface will decrease the shear strength and increase the hydraulic conductivity
of soil slope. Although previous research has shown the effect of surface-cracks in soil stability, the
influence of deep-cracks on soil stability is still unknown. The limited availability of deep crack data due to
the difficulty of effective investigate methods could be one of the obstacles. Current technology in electrical
resistivity can be used to detect deep-cracks in soil. This paper discusses deep cracks in unsaturated residual
soil slopes in Indonesia using electrical resistivity method. The field investigation such as bore hole and SPT
tests was carried out at multiple locations in the area where the electrical resistivity testing have been conducted.
Subsequently, the results from bore-hole and SPT test were used to verify the results of the electrical
resistivity test. This study demonstrates the benefits and limitations of the electrical resistivity in detecting
deep-cracks in a residual soil slopes.
KEYWORDS: residual soil slope, deep cracks, electrical resistivity method, SPT N-value, bore-hole data.
1 INTRODUCTION
Knowing influential factors and characterizations of
soil behaviour are necessity for investigating soil
slope stability. Crack is a significant influential factor
in soil slope that could lead to rainfall-induced
slope instability. A number of articles show that
cracks influence the stability of natural slopes
(Chowdhury & Zhang, 1991; Yao et al., 2001; Li,
2009). The soil is in unsaturated condition when
cracks are developed due to natural forces such as
soil shrinkages, earthquakes or creeps. Surface water
runoff could fill these cracks with impurities. The
impurities in cracks can change the behaviour of the
soil slope due to differences in characteristics and
strengths. The in-filled cracks materials with its
loose density will saturate faster than the natural soil
of a slope. This condition will build positive porewater
pressure in the soil that affect the slope stability.
Significant researches have been conducted in the
stability of slopes with surface cracks and rain water
infiltration. However, few researchers have examined
the effect of deep cracks in soil slopes. Those
few researchers have not explicitly addressed the effects
of deep cracks on the stability of slopes. Limited
availability of deep crack data, due to the lack of
effective investigation methods, could be one of the
obstacles in this area of research.
Surface cracks in soil can be easily seen. In contrast,
it is difficult to detect deep cracks in soil unless
special equipments for ground investigation, such as
geophysical tools, are used. The application of geophysical
methods may be useful in ground investigation,
especially at the reconnaissance stage. Although
there are limitations to the information that
can be obtained, the geophysical methods can produce
rapid and economic results (Craig, 2004).
Based on different physical principles, there are several
geophysical techniques that can be used as nondestructive
test methods in ground investigation.
Three of these techniques that can be used to identify
soil cracks are Seismic Refraction Surveying,
Ground Penetrating Radar and Electrical Resistivity
Tomography.
Ground investigations using electrical resistivity
method have been used by Samouelian et al. (2003),
Friedel et al. (2006), Oh & Sun (2007), Tabbagh et
al., (2007), Zhu et al. (2009), Sudha et al. (2009).
The electrical resistivity method determines soil type
using electrical resistance difference in different soil
type. The flow of electrical current can move
through soil due to electrolytic action. Water content
and concentration of salts will then be measured the
resistivity of soil. For example, a saturated soil with
high void ratio would be detected as a low resistivity
due to the significant quantity of pore water and free
ions in the water.
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Electrical Resistivity Tomography (ERT) provides
an electrical image of subsurface soil and it
can be used as an early detection of soil layers. Furthermore,
the result of ERT can be correlated with
soil strength that is derived from geotechnical data.
However, as reported in literatures (Braga et al.,
1999; Giao et al., 2003), there is poor relationship
between resistivity parameter and N-value from
SPT. To gain better understanding of the correlation,
Sudha et al. (2009) used transverse resistivity that
plotted with the N-value.
This paper discusses the results of a soil investigation
to detect deep cracks on unsaturated residual
soil slopes in Jombok village, Ngantang city, Indonesia
using an electrical resistivity tomography
(ERT) method with two-array dipole-dipole and
azimuthal array method. Subsequently, bore-hole
and SPT data were used to verify the results of the
results of ERT. The location of this study along with
profile line of ERT and points of geotechnical investigations
are shown in Figure 1. In addition, this paper
also presents the correlation between measured
SPT N-values and the resistivity of soil.
Figure 1. Map of the study area showing the dipole-dipole ERT profile lines, azimuthal array points
(A) and borehole locations (BH)
2 SITE INVESTIGATION
In order to obtain subsoil characteristics in the study
area, three borehole tests were conducted at BH1,
BH2, and BH3 as shown in Figure 1. At every 2m
depth in each borehole, SPT test was performed following
the procedure of ASTM. The measured SPT
N-values with the depth are shown in Figure 2.
Ground water table (GWT) in three borehole locations
are plotted in this Figure 2 as well. In general,
the N-value increases with depth. However, discrepancies
of N-value were found at some locations. In
BH1 and BH3, low SPT N-values (2 – 7) were recorded
from 0 to 12 m depth.
Soil samples collected at every 1m depth in each
borehole were used to determine water content, specific
gravity, atterberg limits, dry unit weight, grain
size distribution, and shear strength using direct
shear test in the laboratory following ASTM testing
procedures.
Figure 2. Variation of measured SPT N-values with depth in
each borehole.
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3 ELECTRICAL RESISTIVITY TOMOGRAPHY
(ERT) TECHNIQUES FOR SUBSURFACE
EXPLORATION
A residual soil slope with rare vegetations has been
investigated in this research. It has been reported by
local authorities that the downstream of the targeted
slope has experienced a sliding one year before this
research was conducted. Some surface cracks
emerged at the upper side of the soil slope. The
objective of the ERT was to detect deep cracks in
the upper side of the soil slope.
3.1 Dipole-dipole method
The first ERT survey was carried out using dipoledipole
method along the profile line at inter
electrode spacing of 10 m. There were 3 profile lines
of 150 m long each and 5 m distance of spacing. A
direct current (D.C.) was driven into the ground to
initiate electrical responses. These responses will
indicate soil resistivity values that are recorded
using a resistivity meter produced by OYO (type 2
2D, Serie 380275, production year 2006).
Subsequently, soil resistivity data were analysed
using Res2Div licensed software at the Faculty of
Science, Brawijaya University, Indonesia.
Figure 3 presents the soil resistivity distribution
of subsurface soil in the study area. A significant
variation of soil resistivity at different depths along
the profile lines can be observed. The soil resistivity
in the area is ranging from 1 to 2000 m, indicating
wide variation in soil type, clay content of soil, porosity,
and water content. In general, low soil resistivity
has been measured for surface soil layers (5 –
10 m depth). This would be due to high water content
in surface soil as this test was conducted in
rainy season.
Figure 3. The visual results of ERT along 3 profile lines
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Clay content in soil matrix also affects the soil resistivity.
A mobile cloud of additional ions can be
formed around each clay particle by the ion exchange
property of clay. Due to these ions will facilitate
easy flow of electrical current, electrical resistivity
in fine grained soils, such as clay, is always
lower than expected (Zhdanov and Keller, 1994).
As shown in Figure 3, there are some local zones
with very low resistivity (3 – 30 m). The results of
borehole date confirmed that high porosity and high
water content in these zones (The ERT test was undertaken
in rainy season). Therefore, these zones
could be identified as possible locations for cracks.
Soil crack zones have very high porosity and high
water content in rainy season as rain water can easily
seeps into cracks. This hypothesis was justified in
profile line 1 (Figure 3(a)) as the visible surface
crack coincides with the very low resistivity zone in
the subsoil. However, it was unable to perform resistivity
test in the vicinity of the surface crack in profile
line 2 and 3 due to the accessibility issues in the
area.
The low resistivity zones, which can be observed
at the horizontal distance (from A) between 60 m to
130 m and at the depth from 0 to 12 m, are consistence
in all three profiles. This will suggest the possible
transverse cracks in this area.
3.2 Azimuthal method
To obtain a more detail identification of the deep
cracks in subsoils, an azimuthal resistivity technique
(ART) was carried out in the possible soil cracks
zone. As shown in Figure 1 and Figure 3 (a), there
are two locations for the ART. They are: the middle
of Profile Line 1 (A1 location) and the nearby location
of the visible surface crack (A2 location).
In general, the nature of anisotropy can be seen
from the existence of cracks in a layer of soil. Azimuthal
resistivity technique can be used to determine
the direction of vertical cracks in the soil (Senos-matias,
2002; Busby & Jackson, 2006 and
Schmutz et al., 2006). A square arrays configuration
was selected to be used in this study to indicate the
existence of anisotropy of the medium. This method
will characterize the soil crack by using minor resistivity
that indicates the angle direction of soil crack
and the influential depth of the crack zones. An incremental
array size (a) from 2m to 12m will be
used. The depth of soil crack (D) will be determined
using this equation below:
From the results of ART in the selected locations
as shown in Figure 4, it was found that:
(a) A1 location results (b) A2 location results
Figure 4. Result of Azimuthal Resistivity Technique
At location A1, cracks in soil were detected in a
direction of 135 ° from the north, 0 to 5.65 m
deep.
At location A2, a non linear crack direction is
found. From surface to a depth of 1.41 m, the
crack begins at an angle of 165 from the North
(N 165 E). From the depth of 1.41 m to 4.24 m,
direction of the crack changed to an angle of
180 from the North (N 180 E). Then from the
depth of 4.24 m to 5.65 m, the crack direction
lies between 180-195 angles from the North
(N 180-195 E).
The results of the ART conducted at A2 will confirm
the existence of the crack that is visible on the
surface (Figure 1). The results of dipole-dipole and
ART at A1 are consistence and that will suggest a
possible crack at this location.
4 DISCUSSION ON SOIL RESISITIVITY AND
GEOTECHNICAL DATA
In this study, the geotechnical investigation (SPT
and soil sampling) was carried out in the study area
in order to verify the subsurface cracks detected using
the ERT result. Further, the results of geotechnical
investigation and ERT can be used to develop
useful correlations that can predict soil parameters
from the soil resistivity data.
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4.1 The soil resistivity and SPT N-values
The measurement of soil resistivity using ERT
method is non-destructive, fast, and economical.
Therefore, it is great advantage if the soil resistivity
can be used to predict the soil parameters such as
shear strength. Sudha et al. (2009) presented a linear
correlation between SPT N-value and the transverse
resistance (T) that is given by equation 2.
where i and h i is the resistivity and thickness of i th
layer.
In this study, the measured SPT N-values at BH1,
BH2, and BH3 were plotted with the corresponding
transverse resistance values. As shown in Figure 5, a
linear relation between these two parameters was
found. This is consistence with the finding of Braga
et al., 1999; Giao et al., 2003; Sudha, 2009. However,
it was not possible to obtain a unique relationship
between SPT N-value and the transverse resistance
because the soil resistivity depends not only on
SPT N-value but also on water content and clay content
in soil.
Figure 5. Relationship between measured N-values and the transverse resistance
Figure 6. Density parameter, volumetric water content and grainsize distribution at BH3
4.2 Detection of possible crack location based on
resistivity and geotechnical data:
Since the soil resistivity is affected by clay content
and soil density in addition to soil water content, it is
important to use the measured soil parameters such
as density, grainsize distribution, water content of
the soil in the site with soil resistivity measurement
to detect location of cracks in the subsoil. The existence
of the cracks could be determined by presence
of high porosity and water content in wet seasons.
Figure 6 shows some soil investigation results at
BH3.
As shown in Figure 6, low soil resistivity zone
was found at the depth of 2 m to 9 m in this location.
At 2 m to 5 m depth, an average volumetric water
content of 70%, an average clay content of 18%,
were measured. At 6 m to 9 m depth, an average
volumetric water content of 50% and an average
clay content of 35% were measured. Therefore, the
low resistivity at 2 – 5m depth could be mainly due
to high water content while the clay content could be
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an influential factor to register low resistivity at 6 –
9m depth. Based on the above information and arguments,
a crack could be located at 2 – 5 m depth.
This can be further confirmed by high porosity
(70%) and low dry unit weight (7 kN/m 3 ) measured
at this depth. By direct observation of ERT results
obtained in a wet season (Figure 3(a)), it could be
possible see the crack location (at 2 -5 m depth in
BH3) that is confirmed by the results of detail soil
investigation at BH3.
Following the above discussion on resistivity and
geotechnical data measured BH1 and BH2. A possible
crack location in BH1 and BH2 can be identified
at the depth of 7 – 9 m as evidenced by Figure 3(a)
and (c).
5 CONCLUSIONS
The following conclusions can be drawn from
this study:
The soil resistivity can be affected by water
content, density, and clay content of the soil.
The results of ERT could be used to detect
the deep crack in the subsoil if ERT test is
conducted in wet seasons.
The linear relationship between SPT N-
values and the transverse soil resistance can
be obtained.
ACKNOWLEDGEMENTS
Authors are grateful to Trihanyndio Rendy Satrya,
Munthoha, Dwadesa Wanana and Umar Cs. from
ITS Surabaya for their assistances in Geotechnical
interpretation; Dr. Arief Rachmansyah and Fajar
Rakhmanto from Brawijaya University Malang for
their assistances in the geophysical application.
Braga, A., Malagutti, W., Dourado, J., Chang, H.. 1999. Correlation
of electrical resistivity and induced polarization data
with geotechnical survey standard penetration test measurements.
Journal of Environmental and Engineering Geophysics
4, 123–130.
Busby, J. and Jackson, P. 2006. The Application of Time-lapse
Azimuthal Apparent Resistivity Measurement for the Prediction
of Coastal Cliff Failure. Journal of Applied Geophysics,
59, 261 – 272.
Chowdhury, R.N. and Zhang, S. 1991. Tension Cracks and
Slope Failure. Slope Stability Engineering, Proc. International
Conference, Isle of Wright, 27-32.
Craig, R.F. 2004. Craig’s Soil Mechanics, 7th edition. Spon
Press. London.
Friedel, S., Thielen, A. and Springman, S.M. 2006. Investigation
of a Slope Endangered by Rainfall-induced Landslides
using 3D Resistivity Tomography and Geotechnical Testing.
Journal of Applied Geophysics, 60, 100 – 114.
Giao, P.H., Chung, S.G., Kim, D.Y., Tanaka, H., 2003. Electrical
imaging and laboratory resistivity testing for geotechnical
investigation of Pusan clay deposits. Journal of Applied
Geophysics 52, 157–175.
Li, J. 2009. Field Experimental Study and Numerical Simulation
of Seepage in Saturated/Unsaturated Cracked Soil.
The Hong Kong University of Science and Technology.
Hong Kong. PhD Thesis.
Oh, A. and Sun, C. 2008. Combined Analysis of Electrical Resistivity
and Geotechnical SPT blow Counts for the Safety
Assessment of Fill Dam. Environ Geol, 54, 31 -42.
Samouelian, A., Cousin, I., Richard, G., Tabbagh, A. and Bruand,
A. 2003. Electrical Resistivity Imaging for Detecting
Soil Cracking at the Centimetric Scale. Soil Science Society
of America Journal, 67(5), p. 1319.
Schmutz, M., Andrieux, P., Bobachev, A., Montoroi, J.P. and
Nasri, S. 2006. Azimuthal Resistivity Sounding over a
Steeply Dipping Anisotropic Formation: A Case History in
Central Tunisia. Journal of Applied Geophysics, 60, 213 –
224.
Senos Matias, M.J. 2002. Square Array Anisotropy Measurement
and Resistivity Sounding Interpretation. Journal of
Applied Geophysics, 49, 185 – 194.
Sudha, K., Israil, M., Mittal, S. and Rai, J. 2009. Soil Characterization
using Electrical Resistivity Tomography and
Geotechnical Investigations. Journal of Applied Geophysics,
67, 74 – 79.
Tabbagh, J., Samouelian, A., Tabbagh, A. and Cousin, I. 2007.
Numerical Modelling of Direct Current Electrical Resistivity
for the Characterisation of Cracks in Soils. Journal of
Applied Geophysics, 62, 313 – 323.
Yao, H.L., Zheng, S.H., and Chen, S.Y. 2001. Analysis of the
Slope Stability of Expansive Soil considering Cracks and
Infiltration of Rainwater. Chinese Journal of Geotechnical
Engineering, 23(5): 606-609.
Zhdanov, M.S., Keller, G.V. 1994. The Geoelectrical methods
in geophysical exploration. Elsevier, Amsterdam.
Zhu, T., Feng, R., Hao, J., Zhou, J., Wang, H., Wang, S. 2009.
The Application of Electrical Resistivity Tomography to
Detecting a Buried Fault: A Case Study. JEEG, 14(3), 145
– 151.
REFERENCES
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