Ensuring Dam Safety through Ground-Penetrating Radar

As improving critical infrastructure is a global issue, many are looking for non-destructive testing (NDT) methods that can accurately determine what can be repaired and what needs to be replaced.

Where dam safety is concerned, NDT methods are seen as an approach that can limit the actual work that has to be done, while ensuring condition data is most accurate. Ground penetrating radar (GPR) is one of the primary non-destructive testing methods used to guarantee the structural integrity of a dam.

The Benefits of Non-Destructive Methods for Identifying Dam Repair Needs

Engineers, facility managers and dam operators require accurate information on the structural health of their facility to take active measures to prevent catastrophic incidents. Earthquake monitoring systems can support decision making by offering predictive data before an earthquake and assess the structural integrity of the dam or levee prior to and immediately after an earthquake.1

GPR is growing in status among the available NDT methods for dam evaluation applications. Both public and privately funded dam owners are increasingly looking to GPR technology to evaluate the dam infrastructure and surrounding areas.

In a cost-effective manner, GPR can be integrated with other methods of ground surveys to compile a visual understanding of the overall subsurface of the site. The technology can be quickly rolled-out, offer an effective means to assess subsurface information and grant continuous monitoring and condition assessment throughout the lifetime of a structure.

As a response to the requirement for the best available NDT methods, numerous industry experts established the Dam Safety Group, which supplies industry with the most extensive range of geophysical and seismic techniques and technologies to tackle non-invasive investigation, maintenance and monitoring of dams, embankments and levees.

The founding members of the Dam Safety Group include Geometrics Inc., Kinemetrics, Inc., Robertson Geo, IRIS Instruments and GSSI.2

Ground-Penetrating Radar Basics

GPR utilizes radar (radio detection and ranging) for the transmission and reception of high frequency electromagnetic waves to capture images of the subsurface. A small pulse of radio-frequency energy is relayed into a material using an antenna.

This wave reaches a target and is reflected back to the receiver antenna as a high frequency, electromagnetic reflection. The entire process is very quick - at about 100,000 pulses per second.

GPR has very low power emission rates – less than 1 percent of the radio waves a cell phone uses – so there is no need for safety training or additional safety equipment to use these antennas.

The wave is sent into the concrete, soil or subsurface and reflects off various materials with which it interacts. The equipment reads “time of flight,” or the time taken for the wave to pass through the material, bouncing off an object or boundary and travelling back to the receiver.

Subsurface variations generate reflections that are detected by the system and stored on digital media. A built-in computer records the strength and time analogous to the return of any signals reflected.

These reflections are generated by a range of different of materials, such as geological structure differences and manmade objects like foundations and pipes. GPR can be deployed in rock, soil, pavements, ice, fresh water and concrete structures. The signals that are reflected are used to detect objects, variations in material and voids and cracks.

GPR is regarded as the most precise, highest resolution geophysical technology. Generally, GPR functions best in dry, sandy soils with nominal salt content; dense clay-based may be hard to penetrate with GPR.

In some scenarios, penetration depth may be restricted to just a few feet or even less within clays, whereas targets buried in sandy soils can be identified at depths of 30 feet or more. GPR equipment utilizes a wide range of frequencies (measured in cycles per second), depending upon the required depth and the potential targets being investigated.

One cycle per second is 1 Hertz (Hz). GPR is in the megahertz (MHz) or gigahertz (GHz) range. Utilizing a lower frequency will deliver a signal that permeates deeper in the ground but will generate lower resolutions. The opposite is true for high frequency antennas – improved resolution, but diminished depth penetration.

Figure 1 displays an image captured using a 2.7 GHz frequency antenna demonstrating a series of hyperbolas. These hyperbolas relate to segments of rebar situated at various depths ranging from 1 to 5 inches.

Ensuring Dam Safety through Ground-Penetrating Radar

Figure 1. Image Credit: Geophysical Survey Systems Inc. 

GPR Used to Detect Deterioration in Concrete Dam Slabs

Concrete inspection is crucial when establishing a set of priorities for identifying which areas of a dam need to be repaired and which areas need to be replaced. Among the tools available for concrete inspection, GPR has several advantages over others and is especially well-suited to establishing budgetary priorities.3

GPR is generally used to establish the type and location of concrete reinforcement and identifying deterioration in concrete slabs. Figure 2 exhibits a GSSI StructureScan Mini XT GPR system used for a concrete dam condition evaluation.

Ensuring Dam Safety through Ground-Penetrating Radar

Figure 2. Image Credit: Geophysical Survey Systems Inc. 

Evaluating the condition of concrete can be can be conducted using ground-coupled antennas or a GPR system. A ground-coupled antenna offers clear horizontal resolution, which is adequate enough to facilitate image capture of individual rebars in the top mat. This is not generally possible with other types of GPR antennas.

Ground-coupled antennas are employed to acquire measurements of densely spaced areas along lines that are oriented so they intersect with the top rebar in the upper mat at right angles (or close to a right angle if the rebars are skewed).

The radar wave amplitude reflected from each rebar is recorded against its location on the bridge. Relative changes in the rebar reflection amplitudes generally indicate the condition of the rebar and/or concrete above it.

Software is utilized to amass, process and interpret GPR data in the context of a dam condition assessment. The procedure draws a map of rebar reflection amplitudes. The areas with the least rebar reflection amplitudes (yellows and reds) correlate with the section of the concrete that contains the greatest amount of concrete deterioration and/or rebar corrosion.

The GPR data is then modified by visual inspection and other accessory condition information, such as previous maintenance records. Figure 3 illustrates data collection using GPR.


Figure 3. Image Credit: Geophysical Survey Systems Inc. 

GPR in Action

GSSI customers have been employing GPR systems for decades to perform non-destructive dam investigations that supply sufficient information on the build up of sediment, risk of leakage or void detection.

One example of interest was a survey conducted for the CuaDat Irrigation Dam, one of the largest irrigation dams in Vietnam. The Department of Geophysical Application Research – Institute of Ecology and Works Protection conducted the entire concrete surface inspection survey.4

Vucico Vietnam, a GSSI representative, assisted in the concrete dam’s survey, which is part of the CuaDat Reservoir, the largest hydropower and irrigation structure in Vietnam, with a total capacity of 1.5 billion cubic meters.

This reservoir, built between 2004 and 2009, is geographically situated upstream of the Chu River, Southwest of Hanoi and helps manage flood control as well as sustaining a consistent water supply for more than 86,000 hectares of agricultural land, as well as supplying water for industrial and domestic use.

The maximum height of the dam is 119 meters with a total length of 1,023 meters long. The survey scanned the entire surface of the dam over a period spanning several years. Some of the work could only be completed when the water reservoir level dropped enough so that access to the surface of the dam was provided – an area usually covered by water.

The team employed the GSSI SIR® 30 system and 900 MHz antenna for the first phase of the survey for locating potential voids in specific areas. The voids identified were marked for repaired which is done by injecting concrete through a drill hole.

The next challenge is avoiding damage or destruction to the existing rebar in the structure. Later, the team used a StructureScan™ Mini XT to clarify the survey area that needed repairing by assessing the rebar locations and marking an area where it was safe to drill. See Figure 4.

GSSI using the Structure Scan™ Mini XT to refine the dam survey area.

Figure 4. GSSI using the Structure Scan™ Mini XT to refine the dam survey area. Image Credit: Geophysical Survey Systems Inc. 

Inspecting the Future

Improved GPR technology has significantly improved dam inspection results. GPR is one of the primary non-destructive technologies employed to clearly evaluate the structural integrity and physical condition of dams.

Understanding the dam’s true physical condition facilitates proper maintenance and repairs which ultimately ensures the physical safety of the surrounding community and environment.

References

  1. Ground-Penetrating Radar for Dam Investigative Applications, Dam Safety Group, https://damsafetygroup.com/wp-content/uploads/2021/02/GSSI-CS03-Ground-PenetratingRadar.pdf.
  2. GSSI is a Founding Member of Dam Safety Group, https://www.geophysical.com/gssi-is-afounding-member-of-dam-safety-group.
  3. Using GPR for dam safety projects, Dam Safety Group, https://damsafetygroup.com/wpcontent/uploads/2021/02/GSSI-CS02-Using-GPR-for-dam-safety-projects.pdf.
  4. Survey of CuaDat Irrigation Dam, Vietnam, Dam Safety Group, https://damsafetygroup.com/survey-of-cuadat-irrigation-dam-vietnam/.


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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