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As a result of stresses caused by water load, water saturation and erosion, engineers are looking for reliable methods that will enable them to probe the structural integrity of dams.
Dams are critical structures that are built across waterways to control the movement of water. From small embankments made of earth to large concrete structures, these constructions are essential for power generation, water supply and irrigation.1
Some of the most significant problems that can affect dams are fractures and voids. However, traditional investigative methods are invasive and destructive and only cover small areas.
Discontinuities and anomalies are also easily missed when a visual inspection is carried out, and there are other defects that would not be discovered without using non-destructive methods.
Non-destructive investigation methods that can be employed without disrupting the usual operation of a dam are crucial to maintaining safety.2
To that effect, ground penetrating radar (GPR) is a non-destructive testing technique used for the investigation of voids, fissures, fractures, and other deformations in earthen and concrete dams.3 GPR enables the investigation of large areas of a structure while providing superior spatial resolution.
RADAR (Radio Detection and Ranging) is a familiar technology that uses radio waves to determine the distance and speed of distant objects such as aircraft and ships by bouncing radio waves off them.
In GPR systems, an antenna transmits high-frequency radio waves that are reflected by different materials and return to a receiver. GPR systems measure the time it takes for these radio waves to travel through materials and return to the receiver.
The dielectric permittivity of the encountered material determines the velocity at which radio waves from a GPR system travel.
During a GPR survey, an antenna travels across a survey line and records a series of data points which are then plotted to form a profile of the area.4
GPR is best in dry, sandy soils, ice, rock, and freshwater. In clay soils, however, penetration depth may only be a few feet compared to that of sandy soils, which are typically 30 feet or more.
Low frequency antenna signals can penetrate deeper than high frequency antennas, but the images produced are coarser in comparison to the high-resolution images produced by high frequency antennas.
Inspecting a dam in Lapalisse, France. Image Credit: ARKOGEOS
In comparison to alternative non-destructive testing methods, such as infrared thermography, acoustic emission testing, microwave and ultrasonic-based techniques, ground penetrating radar offers greater depths of survey in less time.5,6
GPR works within a range of frequencies in the megahertz or gigahertz ranges. High frequency antennas are useful for investigating small scale deteriorations, reinforcement bars (rebars), voids, fissures, and delamination.
On the other hand, low frequency antennas are ideal for the investigation of deep structures, large leaks, voids, and fractures. These low frequency antennas are also ideal for obtaining large datasets, which would be much harder to collect using more invasive techniques.
Problem areas are identified after the survey has been completed; special software is used to post-process and interpret data from GPR systems. This data gives dam operators the option to perform a crucial assessment of dam conditions such as rebar corrosion and concrete deterioration and take preventative action to avoid catastrophic failure.
This GPR data may also be correlated with visual inspections and maintenance records to provide a complete picture of dam conditions.
Used in combination with alternative ground survey techniques, GPR can provide complete assurance of structural integrity throughout a dam’s operational life.
For decades, GPR has been used to perform non-destructive environmental, geological, and dam evaluations. It is regarded as a valuable tool for producing accurate data on structural profiles and repairs while reducing labor, time, and costs.
Concrete inspection is essential when it comes to finding and prioritizing structural defects which must be repaired.
GPR is used to locate deterioration in concrete slabs as well as determine the appropriate types of reinforcements. GPR is a highly valuable non-destructive tool in the investigation of dam integrity, as it enables the effective identification of structural defects for quick restorative action at minimal risk and cost.
Inspecting the CuaDat Irrigation Dam with a GPR system. Image Credit: Vucico Vietnam
Vucico Vietnam took part in the investigation of the CuaDat Irrigation Dam in Vietnam over several years. Forming part of the CuaDat Reservoir built on the Chu River, southwest of Hanoi, between 2004 and 2009, the dam is 1,023 meters long and 119 meters high.
The reservoir supplies water for industrial and domestic use as well as 86,000 hectares of agricultural land. This survey was conducted over the course of several years, scanning the entire surface of the dam.
Throughout this dam inspection, Vucico used two different GPR antennas; 2700 MHz and 900 MHz. The team used the SIR 30 with a 900 MHz antenna to identify possible voids, with the 2700 MHz antenna as part of the StructureScan Mini XT being used to locate existing rebar for better resolution.
Artificial levees are key components in flood defenses. Their effectiveness can be severely compromised by structural defects. In Hungary, where a 4,200 km long levee system protects one-third of the country, GSSI’s SIR® 3000 GPR unit with 270 MHz and 200 MHz antennas were employed to investigate these levees.
Performing a fracture assessment to detect leakage zones in the concrete. Image Credit: Szeged University
GPR is strongly suggested here to survey parts of the levee as it gives a more realistic image of the subsurface settings of the study area. The GPR survey was conducted using SIR 3000 control unit (Geophysical Survey Systems Inc.) attached to a 200 and 270 MHz center frequency antennas in the survey wheel mode. Each GPR profile had a length of 100 m. Profiles were successive, meaning that the endpoint of one profile is the starting point of another profile.
After processing, the GPR profiles were analyzed in detail to identify interfaces and to record different types of anomalies. Anomalies were then categorized and evaluated in terms of flood risk. The spatial distribution of the different categories was analyzed in relation to other parameters referring to levee health. Besides mapping structural differences and defects, changes in levee composition were also assessed, mostly by investigating the attenuation of signals.
Diaa Elsayed, Ph.D. Student, Szeged University, Hungary
A full profile of the levees was formed from individual GPR profiles, 100 meters in length. Six different types of anomalies were identified from this and subsequently categorized according to the flood risk they posed. Their spatial distribution was mapped, with alterations in levee composition also being assessed.
A correlation was found between the spatial distributions and frequency of anomalies and flood occurrences. This demonstrated the capabilities of GPR in anticipating flood risks.
Representative profiles for the identified GPR anomaly types: A) contraction cracks, B) remarkable changes in dielectric permittivity, and C) Photo showing the contraction cracks on the levee crown. Image Credit: Szeged University
In large earth dams, there is a dam core, filter, support embankment and slope lining. These structures vary on a case-by-case basis. Water always seeps through the dam, under the dam and around it. Problems arise when internal erosion begins, i.e., the flow of water is so great that it takes away ground material and forms corrosion in the dam structure. Noticeable deviations in GPR profile are a consequence of these kinds of problems. These are the undesirable findings we try to locate. Therefore, dam owners can plan for the right type of repair. In our opinion, GPR is a cost-effective tool for risk assessment.
Terho Makinen, Suomen Maatutkapalvelu
Inspecting an earthen dam. Image Credit: Suomen Maatutkapalvelu
The Dam Safety Group (DSG) provides the widest range of seismic and geophysical techniques and technologies to address the non-invasive investigation, maintenance and monitoring of embankments, levees, and dams.
DSG brings together ground penetrating radar, seismic tomography, resistivity imaging, seismic monitoring and wireline borehole logging. Find more information about the Dam Safety Group and a range of dam inspection technologies here:
Ground penetrating radar is a non-destructive testing method that aids in the inspection of the structural integrity of dams in a cost-effective and time-effective way. GPR can be reliably used together with other survey techniques to provide a comprehensive picture of dam health.
This information has been sourced, reviewed and adapted from materials provided by Geophysical Survey Systems Inc.
For more information on this source, please visit Geophysical Survey Systems Inc.
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