Scholarly article on topic 'Application of Ground Penetrating Radar in Dam Body Detection'

Application of Ground Penetrating Radar in Dam Body Detection Academic research paper on "Civil engineering"

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{"Ground Penetrating Radar" / "reservoir dam body / hidden danger detection / spottiness feature"}

Abstract of research paper on Civil engineering, author of scientific article — Li Hui, Ma Haitao

Abstract The basic principle, the detection methods, data processing, interpretation method and the application of ground penetrating radar in the dam body detection were introduced and the problems aroused during application were primarily studied. The cause of the dam leakage was very complicated and through geophysical prospecting the reasons for the dam leakage and relevant reinforcement measures can be provided. The leakage parts will appear to be electromagnetic abnormal due to the changes of the water content, which will be an advantage in the application of geological radar method. The dam body was detected using Ground penetrating radar detection technology and the quality of the dam project were analyzed combining with geological conditions, practical situation of the project and the characteristics of the radar images. The location of the crack fracture zone in the dam was predicted and feasible technological measures were put forward.

Academic research paper on topic "Application of Ground Penetrating Radar in Dam Body Detection"

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Procedía Engineering 26 (2011) 1820 - 1826

Procedia Engineering

www.elsevier.com/loeate/procedia

First International Symposium on Mine Safety Science and Engineering

Application of Ground Penetrating Radar in Dam body detection

Li Huia,b Ma Haitaob a*

"Civil & Environment Engineering School, University of Science & Technology Beijing,Beijing 100083,China ^Institute of Mine safety Technology,China Academy of Safety Science&Technology, Beijing 100012, China

Abstract

The basic principle, the detection methods, data processing, interpretation method and the application of ground penetrating radar in the dam body detection were introduced and the problems aroused during application were primarily studied. The cause of the dam leakage was very complicated and through geophysical prospecting the reasons for the dam leakage and relevant reinforcement measures can be provided. The leakage parts will appear to be electromagnetic abnormal due to the changes of the water content, which will be an advantage in the application of geological radar method. The dam body was detected using Ground penetrating radar detection technology and the quality of the dam project were analyzed combining with geological conditions, practical situation of the project and the characteristics of the radar images. The location of the crack fracture zone in the dam was predicted and feasible technological measures were put forward.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong.

Keywords: Ground Penetrating Radar, reservoir dam body,hidden danger detection,spottiness feature

1. Introduction

After the completion of the dam, hidden problems will often occur with the increase of service years, which can easily lead to leakage or even break of the dam [1].To eliminate the hidden risk in dams, a variety of geophysical methods were used to detect them by many academicians both at home and abroad, trying to determine the factors causing the risks [2,3]. Ground penetrating radar has become a powerful tool in geophysical exploration with its high resolution and detection efficiency [4]. With the development of signal processing and electronic technology and the continuous accumulation of practical operating

* Corresponding author. Tel.: +8615210605208; E-mail address: lhui119@163.com.

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.2372

experience, ground penetrating radar technology progresses ceaselessly, and its range of applications expands. It has been applied in numerous fields such as rocky soil investigation, building Structures investigations, project quality NDT, hydro-geological surveys, ecological environment protection and so

on[5>6].

2. Detection principle

Ground penetrating radar detection is an electromagnetic technology which uses high frequency electromagnetic wave in the form of a wideband short-pulse to scan the distribution of the underground medium and non-visible objects or underground interfaces. It determines the internal structure of underground medium or the location of invisible object through analyzing the time-frequency and amplitude characteristics of reflected electromagnetic waves [7]. The working principle is as follows: high-frequency electromagnetic waves were transmitted in wideband pulses form through the antenna, reflected or transmitted by the object, and then captured by the receiving antenna.

High frequency electromagnetic waves in the form of a wideband short-pulse were transmitted through ground penetrating radar. On the ground they were sent to the underground through antenna T, reflected by the underground formation or object, and then received by the other antenna R. Pulse waves travel time can be shown as that equation below:

t = ^-J- (I)

In the equation: t is used to represent electromagnetic wave two-way travel time; z is used to represent the depth of the reflector; x is the distance between transmitting and receiving antenna; v is the velocity of electromagnetic wave in the underground medium; propagation velocity v can be analyzed and determinated according to the available data, or be calculated by the equation below:

In the equation: ^ represents permeability; e is dielectric constant; c is conductivity; ra is angular frequency of electromagnetic wave. Rock and soil, which generally is nonmagnetic and nonconductive medium, satisfy c/rae<< 1, so formula (2) can be wrote as the equation below:

v = (3)

In the equation: c is propagation velocity of electromagnetic wave in vacuum, c=0.3m/ns; er is relative dielectric constant.

Radar images generally were recorded in the form of echo impulse waveform. The positive and negative peaks of waveform were represented by monochrome respectively, or by gray scale or by color. Thus, underground reflecting surface or object can be vividly represented by events, equal ash lines, or isochromatic lines. Waveforms of every measuring point were composed by radar profile image, which recorded through the measuring line perpendicular direction in waveform map. Underground puzzling obstacles can be estimated according to radar images.

3. Methods and techniques

There are generally three GPR testing methods, which are profile, wide-angle and transmission method [8]. Profile method is the most common detection method (Fig.1). The measuring results can be showed by time profile image, in which the horizontal axis is the position of the antenna on the surface and the vertical axis is the reflected wave two-way trip. It can also be converted into the depth of the geological slope. Wide-angle method applies to the situation in which underground medium is relatively homogeneous and reflection interface is close to horizontal (Fig.2). Transmission method is mainly used for state detection of ancient buildings, bridges and so on and close quarter inter-porous observation.

Fig. 1. Schematic diagram of GPR principle Fig.2. Schematic diagram of the wide-angle method to determine the velocity

For the radar system itself, the higher the frequency used by the antenna, the smaller the depth of the probe, and vice versa. In general, the radar waves have a better penetrability in dry sand or other soil with poor electrical conductivity, and in clay, water or other high-conductivity medium have a poor penetrability [9]. According to the electrical detection of regional differences in underground medium, the depth of radar ranges from tens of centimeters to tens of meters with different antenna frequencies. Take 100MHz frequency antenna as an example, under the smooth interface conditions, when the subsurface attenuation is at 0.1dB / m, the GSSI radar maximum depth will be 112m; while in the rough conditions of the reflecting interfaces the depth will be 178m. But in the high consumption medium (10 ~ 100dB / m), the influence of the radar system parameters on the maximum depth is not obvious, an it mainly depends on the electrical properties of underground medium, which is shown in Table 1.

Table. 1. Effect of different dielectric constant and propagation velocity of radar

Material Dielectric constant Velocity(mm / ns)

Air 1 300

Gravel(dry) 3-6 120-170

Gravel(wet) 25-30 55-60

Silty sand(wet) 10 95

Clay(wet) 8-15 86-110

Clay(dry) 3 173

Granite 5-8 106-120

Limestone 7-9 100-113

Mudstone (wet) 7 113

Sandstone(wet) 6 112

Quartz 4.3 145

Asphaltum 3-5 134-173

PVC 3 173

Concrete 5-8 55-120

4. Project Analysis and results

The Sanqingting Dam sits in the downstream of Tingjiang tributary streams Jingmei, the actual control of the drainage area above the dam site is 2.06km2 and the total reservoir volume of 2.08 million m3. Flood control standard is of 30 years and case checking length is every 200 years. 150 # concrete dam (secondary grading) assemble a single center rubble arch dam, crest elevation of 244.20m, the lowest dam elevation 192.00m, the maximum height of 52.20m. After the basic foundation excavation to weak weathering zone, the backfill concrete plug groove processing were carried out, combined with hydraulic structures processing. As the Sanqingting Dam construction methods use rockfill dam, ground penetrating radar detection method can be used to detect the dam defects. In order to conduct a comprehensive inspection of the dam, four survey lines were arranged in this test.

4.1. Measuring line layout

There are four sections of detected layouts, which are shown in Figure 3 - 4, and the measured line layout parameters are shown in Table 2.

Fig.3. Measuring lines 1-3 diagram Fig.4. Schematic diagram of measuring line 4

Table.2. Parameters of ground penetrating radar scanning measuring line

NO. Position Trend Length Measurement method

1 Top of the dam North(dam trend) 87 Free run

2 Dam body North(perpendicular to the dam body) 45 Point mode

3 Dam body North(perpendicular to the dam body) 50 Point mode

4 Dam body North(perpendicular to the dam body) 50 Point mode

4.2. Results and recommendations

According to the characteristics of Sanqingting dam, the 100M shielded antenna detection was selected on the top of the dam and the radar receiver range 800ns, so the maximum depth was about 40m. The 400M shielded antenna detection was selected on the Dam body and the range of radar receiver 50ns, the maximum depth about 7m.

Measuring line 1:

Measuring line 1 was on the Top of the dam, which length is 87m and sounding was 40 m, using free run mode from south to north uniform scan, a profile image was obtained, as shown in Figure 5. The radar scanning displays that layers of rockfill dam body were uniform and dam structure was in good

condition, in 15m Detection distance no significant aquifers were found, infiltration dam body line was not exposed through the radar reflection images found three anomalies. Outlier 1 is 40m-43m from the initial point at a depth of 3m-20m, the anomaly may be caused by small-scale cracks of masonry within the dam body; outlier 2 is 45m-48m from the initial point, at a depth of 3m-20m, the abnormally may be caused by small-scale cracks of masonry within the dam body; outlier 3 is 53m-55m from the initial point at a depth of 4m-20m, may be caused by small-scale cracks in local concrete.

Fig.5. GPR scanning profile 1

Measuring line 2:

Line 2 is located in the west middle part of dam body perpendicular to the dam body horizontally, with the length of 45m, sounding of 7m. Point mode was used to scan the dam body from south to north by point to point scanning and profile image 2 was obtained which is shown in Fig. 6. The radar scanning displays that the middle layers of rockfill dam body were uniform and dam structure was in good condition and in 7m detection distance no significant aquifers were found.

Fig.6. GPR scanning profile 2

Measuring line 3:

Measuring line 3 was at the bottom of the dam perpendicular to the dam horizontally, with the length 50m, sounding of 7m. Point mode was used to scan the dam body from north to south by point to point scanning and profile image 3 was obtained which is shown in Fig. 7. The radar scanning displays that the lower layers were more uniform rockfill dam body, dam structure was well and in 15m detection distance no significant aquifers were found. By radar reflection images one exception was found. Outlier 1 is 25m-30m from the initial point at a depth of 2m-4m, which may be caused by small-scale cracks in internal rockfill dam body.

Fig.7. GPR scanning profile 3

Measuring line 4:

Measuring line 4 was at the binding sites of water and dam in the east of the dam body, perpendicular to the dam horizontally, with the length of 50m, sounding of 7m. Point mode was used to scan the dam body from south to north by point to point scanning and profile image 4 was obtained which is shown in Fig. 8. The radar scanning displays that the middle layers of rockfill dam body were uniform and dam structure was in good condition, dam was well-structured, and strong radar reflections were received in the right section suggesting that the cause was behind the parts of the outer rock. By radar reflection images one exception was found which may be partially due to loose rockfill. Outlier 1 in the range profile is 5m-8m to the initial point of the section plane with the depth of 2m-4m.

Fig.8. GPR scanning profile 4

Fig.9. Schematic diagram of outliers

Dam body may be inferred from Fig. 9 in the position shown in Fig. 9 there is a top-down cracks and on-site water seepage also confirmed that. The blue circles represent outliers.

5. Conclusion

The results of ground penetrating radar detection show that: the overall dam structure is good and no greater risk found; local existence of cracks within dam body, loose with other defects. In order to avoid the gradual expansion of defects in construction, the positioned weak, broken areas should be reinforced with the filling, compaction, grouting and other strengthening measures.

The ground penetrating radar is one of the most effective methods in detecting leakage of the dam and project quality. Compared with other methods, it is of high resolution, high efficiency, non-destructive nature, intuitively strong, not easily affected by site constraints and so on. Analysis and interpretation of radar images and the actual situation need to be combined, giving full consideration to dam body structure and geological conditions, to eliminate interference factors on the impact of profile, avoiding misinterpretation.

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