Commission internationale
DES GRANDS BARRAGES
VINGT TROISIEME CONGRES
Brasilia, juin, 2009
A STUDY ON THE WEAK ZONE TRACING SURVEY OF FILL DAM
HYEK KEE KWON1, HAE SANG JEONG2, DONG HEE CHOI3, HYUNG JOON PARK4
P.E., Manager, Korea Infrastructure Safety and Technology Cooperation,
/ Ph. D, Candidate, University of Incheon, Korea1
Manager, Korea Infrastructure Safety and Technology Cooperation, Korea2
Manager, Korea Hydro & Nuclear Power Co., Korea3
Assistant Manager, Korea Hydro & Nuclear Power Co., Korea4
1.
INTRODUCTION
The major defects in embankment dams are the leakage, clacks, unbalanced
displacement, settlement, internal erosion. These major defects need to be
investigated and characterized during the site investigation, and then the proper
measures including rehabilitation are to be suggested based on the analysis results
of the site investigation and characterization.
The site investigation methods include the detailed eye-watching, underwater
inspection, displacement survey, three dimensional laser scanning, soil investigation,
seepage measurement, geo-physical investigation. The choice of the site
investigation methods are based on the current status of the embankment dam,
scope and purpose of the study, and analysis method because the each
investigation method has their own characteristics.
In this study, at first, the characteristics of the investigation methods were
analysed, and then proper investigation methods to detect the weak zone in
embankment dams were studied.
2.
SITE INVESTIGATION
The frequently used site investigation methods are shown in the Table 1.
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Table 1
Feature of site investigation methods
Type
Merit
Detailed
appearance
examination
Demerit
․Investigation net using and
data management possibility
․Appearance investigation of dams
․Detailed survey possibility
․Evaluation by displacement
point criteria
․Investigation possibility about
underwater
․Under turbidity influence of water
3-D Scanning
․Total displacement grasp
․External displacement grasp
Seepage
measurement
․Channel formation judgment
inside dams
․Distinction difficulty with surface
water
․Grasp possibility of weak zone
․Uncertainty in representation of
investigation location
․Total weak zone grasp
․Quantitative reliability deterioration
of result
Displacement
survey
Underwater
inspection
Borehole
test
Geo-Physical
investigation
As shown in the Table 1, many of the site investigation methods are to obtain
the information for the out-surface defects, and this causes the difficulties to
characterize the internal issues of the embankment dams. In case, no out-standing
surface defects are investigated, there is a doubt that the safety of the dam is clearly
confirmed.
This study tried to present and verify the effectiveness of the geo-physical
investigation methods concurrently used with borehole tests in order to characterize
the weak zone in embankment dam. This approach is effective to evaluate the
overall qualitative stability and helpful for the quantitative analysis for the specific part
of the embankment.
3.
3.1.
INVESTIGATION METHODS FOR THE WEAK ZONE
STUDY CASE OF THE EMBANKMENT DAM
The study case shows the properties as shown in the Table 2 and Fig. 1. This
case presents no out-standing defects during the surface inspection.
During the borehole test, the SPT N-value showed weak zone in the part of the
central core. There was a need to introduce more detailed and overall methods to
investigate these weak zone inside of the central core zone.
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Table 2
Dimension of the case study
Division
Facility Status
Total reservoir water quantity
47,100,000 Ton
Effect reservoir water
quantity
46,070,000 Ton
Flood water level
EL. 22.50m
Operation water level in flood
EL. 21.50m
Type
Fill dam of zone type
Length × Level
314.5m × 13.3m
Remark
Fig. 1
Typical Cross Section
3.2.
OVERVIEW OF THE SITE INVESTIGATION
In order to inspect the overall status of the embankment, the specific resistivity
test were conducted at the center line of the embankment and the downstream berm.
Based on the results of the specific resistivity test, the three borehole (BH-1, 2, 3)
tests were decided to compare with the specific resistivity results. The location and
the survey lines are presented in Fig. 2 and Table 3.
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Fig. 2
Site Investigation Status drawing
Table 3
Site Investigation Status
Division
Section (Sta.No.)
Contents
Dam crest
centerline
1+02~15+10
(288m)
Dam crest
Up,down
stream
1+14~7+14,
9+00~15+00
(120m, 120m)
Down stream
berm
2+00~15+10
(270m)
MASW
Dam crest
centerline
10+00~13+00
(60m)
Seismic
tomography
Dam crest
centerline
BH2~BH3
Borehole image
processing
system
Dam crest
centerline
BH-2,3
Borehole
test
Dam crest
BH-1,2,3
Specific
resistivity
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- 2-D Image processing investigation
- Dipole arrangement
- Electrode space(a)
: Dam crest up․down stream 3m
: Dam crest centerline 4m
: Down stream berm 5m
- Electrode number(N) : 8
- Geophone space 1m
- 2.5 Hz geophone
- Borehole space : 12m
- Seismic start : Spaker
- 1m space 18ch. Hydrophone
- Depth BH-2 : 22.50m
BH-3 : 18.50m
- 360° Rotation photography
- Photography space 2mm
- SPT
- Sampling
- Site test
3.2.1
Specific resistivity
The results of the specific resistivity conducted on the centerline and the
downstream berm were presented in the Fig. 3, 4, 5 individually. The Fig. 5 shows
the semi-three dimensional results combining the all survey lines.
The overall specific resistivity values are about 100 ohm-m and the right
abutment of the embankment showed the lower specific resistivity values than the
other areas. For the right abutment area, the specific resistivity values are about 60
ohm-m in 3 survey line at the center line of the embankment as shown in the Fig. 3.
The results of survey line at downstream berm also showed about 60 ohm-m in right
abutment as shown in the Fig. 4. Therefore, the area of the lower resistivity zone can
be considered to be connected from the upstream to downstream at the right
abutment area. This lower resistivity zone is tentatively considered as high water
content area.
In case of left abutment, relatively low specific resistivity area was locally
detected from the nearby BH-1 to Sta. 11-13, but this low specific resistivity area did
not connected to upstream and downstream area. This low specific resistivity area is
considered as a small scale localized minor defect base on the connectivity.
The specific resistivity of core material sturdied in this case presented lower
values than the average of the water supply dams in Korea. This lower specific
resistivity values is considered to be related with the high water contents of the core
zone and this is predominant at the right abutment area.
Fig. 3
Specific Resistivity Values (Dam Crest)
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Fig. 4
Specific Resistivity Values (Down stream Berm)
Fig. 5
Specific Resistivity Values (semi-three dimension)
3.2.2
Borehole test
The borehole test was introduced to support the specific resistivity test results.
The SPT values in BH-1 is 4 at 7.5m depth and 4-19 throughout the depth variation.
The lower specific resistivity area presented the lower SPT N-values and the overall
N-values also showed normal to small values. Therefore, to precisely evaluate this
area two more boreholes were scheduled and tested. The location of the BH-2 and
BH-3 was selected at Sta. 11+18 to directly compare the result with BH-1 and at
12+10 to evaluate the left abutment for the low specific resistivity area respectively.
The schemes and results are shown in the Table 4 and 5.
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Table 4
Borehole Test Status
Division
BH-1
BH-2
BH-3
Sta.
No 12+0
No. 11+18
No. 12+10
Bole hole Depth (m)
15.0
22.2
18.8
S.P.T (N)
9
14
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Table 5
S.P.T Result
Depth (m)
BH-1
BH-2
BH-3
Average
1.5
6
3
8
5.7
3.0
16
4
4
8.0
4.5
9
5
5
6.3
6.0
18
11
8
12.3
7.5
4
4
1
3.0
9.0
-
9
10
9.5
10.5
15
12
7
11.3
12.0
19
12
10
13.7
13.5
14
7
9
10.0
15.0
18
17
9
14.7
16.5
-
10
12
11.0
18.0
-
17
10
13.5
19.5
-
27
-
27.0
Average
13.2
10.6
7.8
10.5
As shown in the Table 5, the average N-values at BH-1, 2, 3 are 13.2, 10.6, 7.8
and these values are relatively smaller than the average of the water supply dams in
Korea. The average N-values at each depth 1.5m, 4.5m, 7.5m showed 5.7, 6.3, 3.0
respectively, and the depth at 7.5m showed the lowest values. The overall variation
of the N-values presented that the area above 9m depth showed smaller values than
others. This variation is considered to be related to the possible poor compaction in
the upper area of the core zone.
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Fig. 6
Comparision between Specific and Sampling
The soil material at 5m depth in BH-2 presenting relatively higher specific
resistivity (Area A in Fig. 3) and the soil material at 8m depth in BH-3 presenting
relatively lower specific resistivity (Area B in Fig. 3) showed no big differences in soil
properties except the soil color. However, the Fig. 6 shows the differences in soil
properties in density and water content when the samples are compared in different
areas.
The SPT-samples in 7.5m depth presented lower density than 4.5m depth at
BH-2, 3 and this results coincide with the specific resistivity results. The analysis
presented that the area in depth 10m at BH-2, 3 is considered as weak zone in this
particular dam
3.2.3
Borehole Image Processing System : BIPS
As indicated in borehole test and specific resistivity test, the area at depth 10m
was considered as a weak zone showing low density and high water content. This
weak zone was then directly inspected using the BIPS (Borehole Image Processing
System). The BH-2, 3 were used to inspect the condition of the borehole wall, and
the results were shown in the Fig. 7 as a 360° unfolded images.
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Fig. 7
BIPS Result
The Fig. 7 indicated that the core materials showed loose, less homeogenous
and presenting unfilled empty space in soil conditions. The upper part of the core
zone such as the area at depth 3.2~8.6m in BH-2 and 3.3~13.2m in BH-3 presented
weak zone showing the less density and more water contents coincided with the
results of the borehole test and the specific resistivity test.
3.2.4
Multichannel Analysis of Surface Waves : MASW
Fig. 8 shows the distribution of the shear wave velocity including the area of the
BH-1, 2, 3 and the higher velocity implies the higher density and the opposite means
the lower density. The shear wave velocity increased with the increase of the depth
and almost constant at the same depth. This is considered as the density increase
with the overburden loading. The lowest shear wave velocity at this survey was 100150m/sec at 34-m depth, and 150-300m/sec at the depth of 4-7. This results also
presented the same tendency showing the lower density in upper part of the core
zone such as the other test.
Whilst, Japanese researcher Sawada and Takahashi(1975) reported that the
shear wave velocity was 210m/sec at the depth of 0-5m, and 240-306m/sec at the
depth of 5-10m after studying four embankment dam cases having the height from
64.5 to 107m. The embankment dam at the current study presented relatively
smaller range of the shear wave velocity than the Sawada's study.
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The previous studies conducted by the KISTEC (Korea Infrastructure Safety
and Technology Corporation) also presented average 264m/sec at the depth of 0-5m,
and 325m/sec at the depth of 6-10m based on the 11 cases of the embankment
dams. The KISTEC's study also showed the distribution of the higher shear velocity
than the current study.
Fig. 8
Distribution of Shear Wave Velocity with Dam Crest Centerline
Fig. 9 presented the dispersion curve of the surface wave at BH-1 from the 31
dispersion curve obtained with 2m interval. In overall frequencies 12.5, 30, 50, 60Hz,
the velocity layer changed. The frequency 12.5Hz can be converted to wave length
(λ) 21.6m base on the 270m/sec velocity, and as same way the frequencies 50,
60Hz can be converted to 4.1 and 3.3m respectively.
Fig. 9
Dispersion Curve of The Surface Wave with BH-1(Sta. 12+0)
In case of the Sta. 12+0, the soil density characteristics varied at the depth of
3.3m, 4.1m, 7.0m, 21.6m and this results agreed to the changes of the SPT-N values
at the depth of 1.5m, 4.5m, and 7.5m. The analysis results of 31 surface wave
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dispersion curve presented that the upper part of the embankment is weaker than
the lower part because the stiffness characteristics varied in 7-10m and the S-wave
velocity calculated from the dispersion curve was lower than the normal values
presented in the previous researches. The tendency which SPT N-values in upper
part of the embankment is smaller coincides with the results of the BIPS.
3.2.5
Seismic Tomography
In order to survey the soil properties between the two borehole, seismic
tomography test were conducted and presented in Fig. 10. Fig. 10 shows the
tomography of the P-wave velocity between BH-2 and 3. The normal P-wave velocity
is considered as 1,000~2,500 m/sec in the clay layer(John M. Reynolds, 1997). The
upper part of the embankment in the current study presented 500-1500m/sec and
this indicates that the density of the upper part of the embankment is not enough.
Especially, the P-wave velocity shows smaller than other layer to the depth
10m which presented more empty space by BIPS. This tendencyis well explained
comparing with the specific resistivity, MASW, and SPT V-values.
Fig. 10
Distribution of P-Wave Velocity (BH-2~3)
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4.
CONCLUSION
In order to detect and evaluate the weak zone in embankment dam, the various
investigation methods were introduced such as borehole test and geo-physical
survey. The analysis results are shortly described hereafter.
1) The specific resistivity test detected that the specific resistivity of the upper
part of the embankment presented lower than 60 ohm-m, and is considered loose,
high water content. The SPT (Standard Penetration Test) results presented that the
average N-values is 3-12 up to the depth 9m, and so the upper part of the core zone
is weaker than the lower part.
2) The BIPS (Borehole Image Processing System) detected more empty
spaces in the upper part of the embankment, and this coincide with the results of the
specific resistivity test and the SPT.
3) The MASW (Multichannel Analysis of Surface Waves) test presented
relatively low range of the shear wave velocity in overall area, and the sudden
change of the shear wave velocity was detected in 4-7m depth. The measured shear
wave velocity is lower than the normal values reported by the other researchers and
is considered as weak zone.
4) The seismic tomography presented lower P-wave velocity than the normal
core material, this tendency is predominant up to 10m depth. The results coincide
with the results of MASW.
The introduction of the various geo-physical survey was presented to be
effective to characterize the weak zone in the embankment dam, and this weak zone
was investigated with more direct methods such as the borehole teat and BIPS
images. The results were reasonably well matched and this presented that the
introduction of the geo-physical survey is quite useful to characterize the weak zone
of the embankment dam which does not shows any defect in eye-watching
inspection.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
KWRA, Design Criteria of Dam, 2005
MILLER, R.D., XIA, J., PARK, C.B., and IVANOV. Multichannel analysis of
surface waves to map bedrock, The Leading Edge, 1999
JOHN M. REYNOLDS. An Introduction to Applied and Environmental
Geophysics. p 221, 1997
PARK, C.B., MILLER, R.D., and MIURA, H., Optimum field parameters of an
MASW survey, SEG-J, 2002.
PARK, C.B., MILLER, R.D., and XIA, J., Multichannel analysis of surface
waves (MASW), Geophysics, 1999.
Y. SAWADA and T. TAKAHASHI. Study on the material properties and the
earthquake behaviors of rockfill dams. Pro. of 4th Japan Earthquake
Engineering Symposium, 1975.
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