GPR detection of several common subsurface voids inside dikes

Engineering Geology 111 (2010) 31–42
Contents lists available at ScienceDirect
Engineering Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e n g g e o
GPR detection of several common subsurface voids inside dikes and dams
Xingxin Xu a,⁎, Qiaosong Zeng a, Dong Li b, Jin Wu a, Xiangan Wu a, Jinyin Shen c
a
b
c
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, GD 510640, P.R. China
Guangdong Insect Institute, Guangzhou, GD 510260, P.R. China
Water Resources Department of Guangdong Province, Guangzhou, GD 510150, P.R. China
a r t i c l e
i n f o
Article history:
Received 11 July 2009
Received in revised form 2 December 2009
Accepted 4 December 2009
Available online 31 December 2009
Keywords:
Dam safety
Bank stabilization
Subsurface void
Geophysical detection
Ground-penetrating radar
a b s t r a c t
Ground-penetrating radar (GPR) technique has been used in detecting several common subsurface voids
inside dikes and dams in south of China, and the results indicate that GPR can be successfully applied to
uncovering termite nests inside dikes and dams, and the technique proves to be advantageous in real-time
retrieval of detection data, precise positioning and effect of application being basically not affected by locality
and climate, when it is compared to other available methods. GPR is also effective in detecting cracks in the
sloping clay core, and proves to pose less impact on the normal operation of the detected hydraulic projects,
be more efficient, and capable of retrieving more comprehensive detection data, when compared with the
method of artificial observation through holes chiseled out from the ground. Also, GPR is capable of detecting
ferralsol in tropical and subtropical regions to some depth, and shows high value of application in detecting
some hidden troubles such as caves and settlements with low to moderate depths inside dams in karst
regions. Moreover, GPR technique proves to be so capable of detecting carbonate rocks to certain depth and
can yield precise results that it can be applied to analysis and discovery of leakage channels inside reservoirs
in karst regions.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In the 1950s to 1970s, a great number of dikes and dams with low
to moderate heights were built to dam rivers in China. Now many of
these hydraulic projects clearly show hidden troubles or even dangers
(Li, 2005). In flood periods, dangerous situations and even collapse
frequently occur for many dikes and dams; this is simply because
these projects are unable to bear the impact of big flood. According to
statistics, in the period from 1954 when dike collapse data are
available until the end of 2003, 3484 dikes in total collapsed in China,
which means collapse of nearly 70 dikes on an annual average (Zhou
et al., 2007). Risks and breaches of dikes are even more frequent. For
example, in the flood event occurring in the middle to lower reaches
of the Yangtze River in 1998, more than 9000 risky situations occur on
the major dikes along the river, and 1075 dikes collapsed along trunk
streams in the middle to lower reaches of the Yangtze River and
around the Dongting Lake and Poyang Lake, while the majority of
these collapsed dikes were not even inundated at the time of their
collapse (Zhang, 1999). Accidents of dike breach occur in both trunk
streams and tributaries of the Yellow River in the event of flood
occurring in 2003 (Zhao et al., 2004). In the flood event that occurred
in Guangdong in 2005, more than 2000 dike segments were damaged,
more than 1000 dike segments were breached, a great number of
⁎ Corresponding author. Fax: +86 20 85290130.
E-mail address: xuxx@gig.ac.cn (X. Xu).
0013-7952/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.enggeo.2009.12.001
water gates, small hydropower stations and dikes were ruined, and
one dam even collapsed (Yue, 2005).
Unless hydraulic projects were fully inundated or operation of
hydraulic projects exceeds the allowed extreme conditions in case of
very unusual flood event that surpasses the design standard for the
projects, dikes and dams with hidden defects can be periodically
detected with effective means, in an attempt to remove potential risks
or accidents. In this way, defects can be positioned and located, and
measures be taken for reinforcement of these hydraulic projects and
removal of related risks. As a result, with development of effective,
economic, highly efficient technical means for detecting multiple
hidden defects and their application to detection of hidden defects
inside existing hydraulic projects, reliable data can be obtained for
reasonable renovation and safety management of the projects, which
would be significant for controlling the occurrence of risks or breaches
of dikes and dams.
As a kind of high resolution, non-destructive geophysical detection
technique, ground-penetrating radar (GPR) has been applied extensively in geological prospecting (Walter and Robert, 2000; Hambrey et
al., 2008), underground archeological surveying (Edwards et al., 2000;
Carrozzo et al., 2003.), karst analysis (Walid et al., 2002; Gad et al.,
2005), highway reconnaissance (Saarenketo and Scullion, 2000), etc.
Successful applications have been found in investigation of scour pits
around underwater bridge piers (Chang et al., 2004), detection of
erosion of clay core for dams (Seje et al., 1995), inspection of levees
(Ferguson and Brierley, 1999; Bristow et al., 1999), investigation of
near-surface fault properties (Cai et al., 1996; Rashed et al., 2003), and
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X. Xu et al. / Engineering Geology 111 (2010) 31–42
detection of underwater structures (Dai and Wang, 2003; Xu et al.,
2006).
Subsurface termite nests inside dikes and dams, caves and
settlements inside dikes and dams particularly in karst regions are
all common hidden defects in hydraulic projects in south of China.
Cracks inside sloping clay core are also hidden defects that pose
severe threat to dike safety. However, conventional techniques for
detection of these hidden defects prove to be unsatisfactory. In regard
to this problem, attempts have been made by the authors in
application of GPR means to detection of several hidden defects
inside a number of hydraulic projects in this paper.
When electromagnetic wave reaches the interface between two
kinds of electric materials, its reflection amount depends upon the
difference in dielectric constant between the two materials, while the
reflecting coefficient R for the interface between them can be
expressed as follows:
R=
pffiffiffiffiffiffiffiffiffiffiffiffi
ε2 =ε1
pffiffiffiffiffiffiffiffiffiffiffiffi
1+
ε2 =ε1
1−
where ε1 and ε2 refer respectively to the dielectric constants of the
two materials.
2. Principle of GPR
3. Ferralsol, the major soil type in south of China
GPR generally consists of emitter, receiver and mainframe (Fig. 1).
GPR frequency ranging between 10 MHz and 1 GHz, corresponding to
a pulse width of nanoseconds to dozens of nanoseconds, is used in
geological prospecting. High frequency signal penetrates to smaller
depth beneath ground but shows higher resolution than low
frequency signal. Short pulse is sent into the ground via emission
antenna, while transmission of electromagnetic wave in underground
depends upon the attribute of the media it penetrates; as for the
media its conductivity controls the attenuation of signal, its dielectric
constant controls the rate of signal transmission. Part of the signals
emitted will be reflected at the interface between two materials with
different electric properties, and can be received, magnified, digitized
by the receiver and then sent to the mainframe for storage and
processing. The mainframe functions to coordinate emitter and
receiver and process the data it receives (Xie et al., 1994).
For media like soil, rock and fresh water, the wave velocity of
electromagnetic wave V can be expressed as follows (Flohrer and
Pöpel, 1996):
V = C=
pffiffiffiffiffi
εγ
where C is the rate for light transmission, and εγ is the dielectric
constant of the media.
From the above equation the distance D between the antenna and
a reflection point can be calculated as follows:
Δt
2
D = V⋅
where △t refers to the time required from emitting to receiving of an
electromagnetic wave.
Ferralsol, including red soil, yellow soil and latosol, is a kind of soil
commonly seen in tropical and subtropical regions, and is a major soil
type occurring in south China (Lv and Li, 2006). Distribution of
ferralsol in China is shown in Fig. 2. Ferralsol generally shows intense
weathering of minerals and homogeneous texture, and is comprised
of abundant clay minerals, mainly kaolinite (about 40–50%) (Ma et al.,
1999). For convenience in exploitation, ferralsol is commonly utilized
to construct dikes and dams. Ferralsol generally shows a moisture
content varying between 10% and 30%, with a corresponding
conductivity ranging between 6.1 and 38.5 mS/m (Li et al., 2005).
Moisture content in the range of l5–30% poses the most significant
influence upon the conductivity of ferralsol, while a moisture content
exceeding 30% exerts apparently attenuated influence on its conductivity, since this moisture content is close to the saturation limit of a
soil (Sun, 2000).
In order to attain an approximate understanding of the transmission process of GPR pulse in soil, the authors performed related tests
on a test field, which was established on a ferralsol slope in the suburb
of Guangzhou City in Guangdong Province. An almost upright slope
and a horizontal slope top were first cut on the hillside, then four
cylindrical caves in the size of Ф30 × 150 cm and one cylindrical cave
in the size of Ф7.5 × 150 cm were cut perpendicular to the slope, and
these artificial caves were used as the objects for the study (Fig. 3).
GPR test lines are located on the top of the slope, being vertical to the
axis of the caves. For the tests an antenna with a frequency of
300 MHz and another antenna with a frequency of 500 MHz were
used. As shown by the test results, the 500 MHz antenna can detect
caves to a depth down to 245 cm, while the 300 MHz antenna can
detect caves to a depth down to 345 cm (Fig. 4). Based on the height
difference between the top of each cave and the top of the slope and
the time required for dual transmission of electromagnetic wave, the
wave transmission rate for the soil in the test field (V) can be
calculated as 0.076 m/ns, and the dielectric constant is 17.3.
Numerous tests were carried out in both rainy and dry seasons, and
the test results show only limited differences between the different
seasons. This is because moisture content varies considerably only in
soil layers of thickness varying between 0 and 50 cm, soil layers with
this thickness range are affected by external environmental factors
while soil layers with greater thickness remain less affected by such
factors (Wang et al., 2006; Huang and Zhou, 2003). Therefore, it can
be inferred that seasonal factors pose little influence upon GPR
detection of deeper objects below the ground.
4. GPR detection of subterranean termite nests
Fig. 1. Block diagram showing the principle of ground-penetrating radar.
Approximately 500 species of termites occur in China, and they are
distributed in close to 40% of the total area of China (Fig. 2). With the
Yangtze River as the boundary between north and south of China,
termites show decreasing density of distribution and smaller number
of species toward the north, but increasing density of distribution
and greater number of species toward the south (Li et al., 1989a,b).
X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 2. Distribution of ferralsols and termites in China.
Fig. 3. A simulated GPR test field.
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X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 4. 300 MHz GPR image for caves detected in the simulated test field.
Subterranean termites are basically distributed to the south of the
north latitude 35°, as they simply prefer to build nests below the
ground. Ferralsol used for construction of dikes and dams is
particularly suitable for subterranean termites to build their nests,
and the environmental conditions of dikes and dams also prove to be
suitable for living and subsistence of these subterranean termites,
therefore, dikes and dams become places where subterranean
termites are densely distributed. Termite nests constructed inside
dikes and dams would seriously endanger the safety of these
hydraulic projects, hence becoming one of the commonest hidden
troubles in south of China, as dangerous situations and accidents
posed by termites to dikes and dams occur rather frequently in flood
periods (Li et al., 2004). Fig. 5 is a photo taken for a portion of a dike at
a time just after the flood receded, and the photo vividly reflects
damage to the dike as created by termites.
Taxonomically, termites belong to Isoptera, and occur in thousands
of species. In south China, Vietnam and other South Asian nations,
major termite species that constitute serious threat to earth dikes and
dams include Odontotermes formosanus Shiraki, Macrotermitinae
barneyi Light, Odontotermes hainanensis Light, and so on (Li et al.,
1989a,b). These termite species construct gigantic nest systems below
the ground, consequently endangering the safety of these dikes and
dams. A termite nest system mainly consists of a principal nest (with a
diameter of dozens of centimeters and even 1–2 m), many secondary
nests, and numerous termite channels, which spread out in all
directions, with some termite channels being linked up with the inner
slope and outer slope of the dikes and dams (Fig. 6). Termite nests are
generally built above the soakage line which is located at the normal
water level, so would generally not endanger the safety of the
hydraulic projects in periods of no flood. However, when flood comes
and the water level rises abruptly, water would enter the termite
channels from the upstream slope, which would result in concentrated leakage of water, and consequent collapse and even breach of dikes
or dams in tens of minutes of time. These several species of termite
build their nests at a depth of 0.5–3 m below the ground (generally 1–
2 m), and are active in an area with a radius varying from several
meters to tens of meters. Even though traces of termite activities can
be discovered on ground surface, the principal nest cannot be
precisely located, even though its precise location is a key step to
removal of hidden troubles to dikes and dams. Currently major means
used for nest location cover the following:
(1) Artificial location of group holes and major channels of
termites, followed by excavation of the principal nest by
trekking along the major termite channels (Li et al., 1989a,b).
However, this method shows that some major shortcomings,
for example, termite channels would be frequently covered by
mud in the trekking process of excavation, which means loss of
manual labor and damage of dikes or dams;
(2) Use of insecticides to exterminate the whole nest of termites,
after about a month, followed by finding a species of epiphyte
named Xylaria nigripes (Ki) Saco, which would grow from the
nest up to the ground surface. Based on the distribution of this
epiphyte species on the ground surface, the principal nest can
be located (Li et al., 1990, 2004). If Xylaria nigripes (Ki) Saco
Fig. 5. The photo shows dangerous situations to a dike as caused by termite hazard.
X. Xu et al. / Engineering Geology 111 (2010) 31–42
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Fig. 6. Sketch showing damages to a dike or dam posed by termites.
can grow up, if it can grow up to the ground surface, and how
long it will take for the species to grow up to the ground
surface, these all depend upon the moisture and temperature of
the soil. This method proves to be satisfactory in regard to its
application to dikes and dams in Guangdong and Fujian
Provinces in south of China, but exhibits unsatisfactory results
in Henan and Hubei Provinces in the centre of China;
(3) Detection of termite nests using the method of audio
frequency. First is to insert a detector into the soil, to determine
the termite nest around the detector if sounds due to termite
activities can be heard. This method can only be used to detect
termite nests occurring at a burial depth of tens of centimeters,
as it fails to detect nests which are buried at deeper depths and
pose more serious damage to dikes and dams. Furthermore, the
detection results are subject to influence of environmental
noises.
Because all these conventional methods for termite nest positioning show some shortcomings, new detection means have been
explored and proposed in the past decades.
The upper part of a termite nest is generally a cavum and the
cavum is in contact with the upper soil through air. It is known that air
and soil show a significant difference in relative dielectric constant,
and the difference can be utilized for GPR detection of both nests for
active termite population and empty nests left by termite population
after their deaths. In order to explore if the GPR technique is effective
for detection of termite nests, tests were carried out in six fields
selected in Guangdong Province, where termite activity traces are
displayed but the principal nests cannot be precisely located. Each test
field selected has an approximate area of 100 m2, where the survey
lines were laid out at an interval of 2 m. The test follows the following
procedures: First of all two antennae each with a frequency of
500 MHz and 300 MHz were used respectively to detect each survey
line, then each test field was excavated along sections, finally the
results of excavation are compared with GPR images (Fig. 7). As
demonstrated by the test results, the principal termite nests at the 6
test fields are all displayed vividly on GPR images. Here the GPR
images were taken at both frequencies, with the GPR images taken
with 500 MHz frequency showing better resolution, deeper detection
depth and more satisfactory overall effect. As shown by the 500 MHz
GPR images, the deepest principal termite nest is located at about
3.12 m below the ground surface, while the shallowest one is only
0.35 m below the ground surface. The detection range of nest depth
covers almost all depths for nests of the several species of termites as
discussed above, even some secondary nests and major termite
channels located at a depth less than 1.5 m can be exhibited on the
Fig. 7. Excavation of a termite nest along a vertical section as determined by GPR survey lines for corroboration.
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X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 8. GPR images for termite nests and photos of excavation for corroboration purpose, with 500 MHz antenna.
X. Xu et al. / Engineering Geology 111 (2010) 31–42
GPR images. Fig. 8 is a GPR image showing locations of the nests, flying
waiting rooms and channels as well as a photo taken after the site is
excavated for validation purpose.
The GPR technique for detection of termite nests as developed here
was introduced to Vietnam in 2004. In about one month after the
introduction of the technique, the Center for Termite Control Research
under Vietnam Institute for Water Resources Research applied the
GPR technique to the Red River dike, and discovered more than 500
termite nests. The results of excavation demonstrate that the GPR
judgment of termite nests is 89% correct, as the remaining 11% prove
to be either stones or mice holes. This technique has also been applied
in Henan Province and Guangxi Zhuang Autonomous Region in China,
and proves to be highly effective in locating subterranean termite
nests.
5. GPR detection of hidden troubles in rock-fill dam with sloping
clay core
The wall with sloping clay core is the only hydraulic structure used
to prevent seepage for rock-fill dam, and any hidden troubles in this
wall would pose serious threat to the safety of the entire dam,
therefore, it is necessary to perform periodic survey about its safety.
However, wall with sloping clay core is generally covered by concrete
or masonry, which adds difficulty to its safety inspection. Generally
the upper cover of the wall is chiseled to allow artificial observation
through holes, each having a size of several square meters, only this
method demands huge amount of work and obviously affects the
normal operation of the hydraulic project, and moreover, the test data
thus acquired is not comprehensive. In order to test the capability of
the GPR technique for detection of wall with sloping clay core, the
authors carried out a GPR test toward the dam for Nanshui reservoir in
northern Guangdong Province.
As a hydraulic project for hydropower generation, the Nanshui
reservoir was constructed and started water storage in 1969, and has a
total storage capacity of 13 × 108 m3. The principal dam for the
reservoir is a rock-fill dam solidified with a sloping clay core, with a
maximum crest height of 81.3 m. The dam structure consists of
concrete cover, sand cushion, sloping core wall, anti-filter layer, rockfill and loose-stone, from its top to bottom (Fig. 9). The sloping clay
core is generally constructed with screened ferralsol with high
content of clay minerals. According to the data supplied by the
reservoir management authority, the sloping clay core had been
inspected several times by chiseling out part of the concrete cover for
evaluating safety of the dam, and multiple cracks parallel to the river
flow had been discovered. These cracks are 1–30 mm wide, several
meters to tens of meters long, and are associated with fracture zones
being several meters wide and less than 1 m deep in areas around the
principal cracks. These cracks had been excavated, backfilled, and
grouted under pressure.
37
GPR inspection was performed on the sloping clay core, which was
divided into two parts, i.e., the part above the water level and the part
below the water level, and antennae with two kinds of frequency
were used respectively.
In regard to the part above the water level, 300 MHz shield
antenna was utilized to satisfy requirements for detection depth and
resolution, while GPR survey lines were laid out parallel to the dam
axis at an interval of 2 m. A work photo was shown in Fig. 10. In the
process of detection, if there appears any anomaly in GPR images for
the sloping clay core , then repeated detection would be performed on
the anomalous site, so as to remove false phenomena resulted from
bumping of GPR antenna or interference from ground objects. The
results of repeated detection prove to be generally consistent with
those acquired from previous detection. Fig. 11 is a GPR image for two
scans performed on one profile, and shows that the differences
between the two scans are minimal. Through the GPR detection, it
was discovered from the GPR images that numerous linear anomalies
being vertical to the dam surface and strong reflection areas at a depth
less than 1.5 m occur in the sloping clay core. In half a month
thereafter, the reservoir management authority sampled three sites
showing linear anomaly (A, B and C in Fig. 12), with one area showing
strong reflection (D in Fig. 12). The concrete cover of the area was
subsequently chiseled out for validation, with each spot being
excavated with a size of 2 × 2 m2. As shown by the results of validation,
cracks were discovered at the three sites showing the linear anomaly,
while a fracture zone was discovered in the area showing strong
reflection, indicating that the linear anomaly and strong reflection
were resulted respectively from the cracks and fracture zone.
Excavation for the purpose of validation of GPR detection results
was carried out in rainy season characterized by extremely high
humidity. At the excavation sites, the authors found that cracks were
closed due to soil expansion as affected by very high humidity at the
fracture zone and one crack spot, while at the remaining two crack
spots, cracks can be vividly seen inside the sloping clay core (Fig. 13).
Obviously, insufficient number of sites was excavated for validation of the test results; this is simply because excavation was carried
out right in the flood period when the reservoir showed rapid changes
in water level, and the reservoir management authority proposed to
limit the number of sites to be excavated for the sake of the reservoir
safety. Fortunately, the results of validation are all consistent with the
analytical results shown by the GPR images. Moreover, the sites
showing anomaly were inspected repeatedly, so the anomalies as
shown on the images would not be possibly resulted from antenna
bumping or interference from ground objects. Therefore, it can be
inferred that GPR technique is capable of detecting cracks in the
sloping clay core. More explanations can be detailed as follows:
(1) GPR technique is capable of detecting cracks. Numerous reports
have been published in regard to detection of faults or cracks in
Fig. 9. Structure of the major dam of the Nanshui reservoir in Guangdong Province.
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X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 10. Photo showing GPR detection of sloping clay core in the Nanshui reservoir in Guangdong Province.
rocks. A systematic test for detection of tiny cracks in rocks was
completed by Tetsuma Toshioka et al., who once used antennae
with frequencies at 100 MHz, 300 MHz, 500 MHz and 900 MHz
to detect 26 tiny cracks, each 1–5 mm wide, in tuff, respectively, and the test results indicate that the detection depths are
up to 4 m, 4 m, 2.7 m and 1.5 m respectively, with more than
50% of the cracks being successfully detected (Toshioka et al.,
1995);
(2) Soil used for constructing the sloping clay core underwent
strict screening processes and generally shows homogeneous
textures and small particle sizes, and the background shows
little interference with electromagnetic reflection, which is all
favorable for detection of cracks in the sloping clay core.
In regard to the underwater part, the antenna with low frequency
(50 MHz) was used for detection from water surface down to the
underwater part, and several anomalies characteristic of collapse of
the sloping clay core in underwater environment were discovered
(Fig. 14). The effect of GPR detection underwater hydraulic structures
had been detailedly introduced in another article (Xu et al., 2006).
6. GPR detection for common hidden troubles inside dams in
karst regions
Carbonate rocks (mainly limestone) are extensively distributed in
China and the distribution area is as high as 3,440,000 km2, including
a distribution area of about 910,000 km2 for exposed carbonate rocks
(He and Zou, 1996). In south of China which is located in tropical
and subtropical regions, dissolving of carbonate rocks is intense,
so karst areas are extensively distributed, and this is particularly the
case in southwest of China, where the area of the karst area is up to
430,000 km2, and the population in the karst regions exceeds
100,000,000 (Zhang et al., 2001). Most of the precipitation in the
karst regions would flow away through underground leakage
Fig. 11. Two GPR images for two detections at the same profile for comparison, with 300 MHz antenna.
X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 12. GPR images showing cracks and a fracture zone in sloping clay core, as corroborated through excavation, with 300 MHz antenna.
Fig. 13. Photos showing excavation of cracks inside a sloping clay core for corroboration.
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X. Xu et al. / Engineering Geology 111 (2010) 31–42
Fig. 14. Anomalies discovered in underwater sloping clay core, with 50 MHz antenna.
channels, which results in shortage of water resources on the ground
surface. Despite the unfavorable geological conditions in the karst
regions, a great number of reservoirs had been built in order to supply
water to the local industry, agriculture and living by the residents.
However, after these reservoirs were put into operation, leakage and
associated mud removal would commonly occur and lead to
occurrence of dangerous situations such as dam collapse, inhomogeneous settlement and cracking, and these dangerous situations would
develop very fast if without control (Feng and Shen, 2005). Because
the hydrogeological conditions in the karst regions are rather
complicated, many reservoirs would show leakage and other
dangerous situations even after repeated artificial treatment. As a
result, timely discovery of hidden troubles buried inside dams proves
to be necessary for effective measures to be taken, and for occurrence
and development of dangerous situations to be prevented or
controlled, which is significant for the safety operation of these
reservoirs. GPR technique is good at detecting underground caves and
settlements, so finding extensive applications in this field. In regard to
the depth of GPR detection of homogeneous earth dam constructed
with ferralsol, 50 MHz antenna capable of greater depth detection was
used for several tests. Fig. 15 is a 50 MHz GPR image showing caves (or
internal settlements) detected in the principal dam for the Heshui
Fig. 15. GPR image showing caves or settlements (shown by arrows) inside a dam, with
50 MHz antenna.
reservoir in Yangjiang City, Guangdong Province. Here strong signal is
reflected from a cave located at a depth of around 15 m. Obviously, the
maximum detection depth is greater than 15 m, and is estimated at
around 20 m. Even though the GPR technique appears to be less
capable of detecting deeply hidden troubles in earth dam with great
height, it proves to be highly effective in detecting defects in
numerous dams and dikes with low to moderate heights. In regard
to reconnaissance of carbonate rocks, GPR detection depth can be
remarkably increased as this kind of rocks generally show smaller
conductivity, which is favorable for GPR application in geological
reconnaissance in karst regions. An example of such application is
detailed as follows:
The Fengshuping reservoir is located in northern Guangdong
Province and had been constructed and started water storage in the
1960s. The principal dam for this reservoir is a homogeneous earth
dam with a maximum crest length of 194 m and a maximum crest
height of 31 m. The dam foundation consists of Devonian strata, the
left dam base and left bank consists of sandstone, while the right
dam base and right bank consists of limestone (Fig. 16). Ever since
formal operation of the reservoir, multiple accidents of collapse or
settlement have occurred at the dam body, bank slope and reservoir
base, yellow water mixed with mud frequently leaked at the foot of
the anti-filter layer at the downstream slope of the dam, sometimes
gushing out unsteadily, with a flux being up to 0.07–0.15 m3/s. Ever
since the reservoir started water storage, dam body and dam base
have been grouted for many times, only the problems of leakage and
settlement cannot be completely solved. Based on the geological
conditions of the dam, the authors laid out multiple profiles parallel
to the dam axis, in an area extending from the middle of the dam to
the slope base at the right side of the dam, for GPR detection with
a 50 MHz antenna. Based on the GPR detection results, a strong
reflection zone being more than 10 m wide and dozens of meters
deep was discovered at the right side of the dam body, and extends
from inside the reservoir to the outlet at the base of the downstream
slope. Based on drilling and shallow seismic data, it can be inferred
that this is a significant erosion zone, and could be the major channel
for leakage of the reservoir. Based on our analytical results, the
erosion zone was grouted with more than 870 tons of cement and
clay (in dry weight). After the treatment was completed, gushing at
the foot of the anti-filter layer disappeared quickly and the leakage
was controlled for the first time. As shown in Fig. 17 for one of the
GPR profiles, the scope of the erosion zone, the inclination of the
rocks and the development of karst along the rock layers are clearly
demonstrated. In addition, even though no noise treatment has been
X. Xu et al. / Engineering Geology 111 (2010) 31–42
41
Fig. 16. Plan of the dam for the Fengshuping reservoir in Guangdong Province.
performed on the data acquired, effective signals emitted at a depth
of 30–40 m can still be recognized, illustrating that the 50 MHz
antenna is capable of detecting limestone at a depth of 40 m or even
greater.
7. Conclusions
GPR technique was applied to detect termite nests inside dikes and
dams, and satisfactory detection results were obtained in a real-time
manner for nests populated by living termites and empty nests left
by dead termites. Also GPR application seems to be less affected by
terrain location or climatic features, so is preferred to other conventional methods for termite nest location.
GPR technique proves to be effective in detecting cracks and
fracture zones in the sloping clay core for dams, and is advantageous
in posing less impact on the operation of hydraulic project, requiring
only low detection cost, and in retrieving comprehensive detection
data, when compared to the method of artificial sampling by chiseling
out part of the cover.
The low frequency (50 MHz) antenna is capable of detecting caves
or settlements in ferralsol dams to a maximum depth of around 20 m,
so can basically meet the requirements for detection of caves and
Fig. 17. GPR image for a karst development area (see Fig. 16 for location), with 50 MHz antenna.
42
X. Xu et al. / Engineering Geology 111 (2010) 31–42
settlements in dams with low to moderate height in karst regions. The
GPR technique using the 50 MHz antenna shows depth of detection
up to tens of meters in carbonate rocks, and can yield high-resolution
detection data, so proves to be applicable to geological reconnaissance
of karst regions where dams with low to moderate height are to be
built, and to locating leakage channels in completed reservoirs.
Fast speed and high resolution of GPR detection enable it to be
used for locating, monitoring and assessment of multiple hidden
troubles inside hydraulic projects, no matter if they are above the
water level or located in underwater areas. GPR technique is showing
favorable prospect in safety monitoring of hydraulic projects, so
deserving further research and development.
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (Project number 49171051), the Guangdong
Natural Science Foundation (Project number 8151064004000006)
and the Guangzhou Department of Science and Technology (Project
number 2006Z3-D0331). This is contribution No.IS-1138 from
GIGCAS. Thanks are due to Mr. Yuguo Dong of Geochemistry Institute
of Guangzhou, Chinese Academy of Sciences and Ms. Qizhen Rao of
Guangdong Entomological Institute for their generous assistance to
this study, and to two anonymous reviewers whose critical comments
and suggestive advices help to greatly improve the quality of the
manuscript. Finally, we would like to thank Dr. Chunyun Wang for his
help in proofreading the manuscript.
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