ROAD INVESTIGATION

 

 

Non-destructive road investigation

There are more than 64 million kilometres of roads in the world, all of different age and condition, some paved and some unpaved. The road infrastructure is important for innumerable reasons and needs to be maintained to a certain level to ensure safe access, both for private and public transportation 

Today, most inspections and maintenance plans are based on invasive investigations with drilling or sampling of the roadbed at discrete locations; this gives precise information but only at those points. With a growing road network requiring maintenance due to age and use, the need for information on its condition is increasing. Geophysical methods can then be a suitable solution to create a more comprehensive picture of the internal structure of our roads.  

Using GPR to detect asphalt and reinforcement layers in roadways

GPR (Ground Penetrating Radar) is a non-destructive investigation method that uses electromagnetic pulses to produce an image of the subsurface, without digging or drilling. Greater data coverage can be achieved compared to traditional, point-by-point, geotechnical investigations such as drilling or digging, which makes it a very cost-efficient method.

GPR can be compared to a fish-finder (or echolocation device) on a boat. A transmitter antenna sends a pulse into the ground, and that pulse reflects whenever there is a contrast in the dielectric properties of the medium it’s being sent into. Such a contrast can be due to layers (such as asphalt, soil layers, bedrock), objects in the ground (e.g. utilities), or voids. The reflected pulses are then collected by a receiver antenna, which will measure the time (which can be converted to depth) from when the pulse was transmitted to when a reflected signal was received. The receiver antenna will also record the amplitude of the reflected signals.

The transmitter and receiver are often put in one antenna box for easier handling in the field but can also be housed in separate units. Different horizons, such as asphalt and reinforcement layers in the road construction, will typically give linear features in the resulting radargram.

 

 

 

 

 

Choosing the right GPR antenna

The depth and resolution that can be achieved depends on the frequency of the transmitted electromagnetic pulse. A higher frequency results in higher data resolution, whilst a lower frequency results in better depth penetration. What frequency to use depends on the application, and what resolution and depth is required. For road embankment mapping (with total thickness and internal layers) frequencies around 450 MHz to 2.3 GHz are quite convenient. If the geology underneath the road are of interest as well, frequencies of 100 to 250 MHz can be used.  

Most often it is efficient to use two antenna frequencies or more at the same time, providing high resolution for thin asphalt layers with a high frequency antenna and a good overview of the total road embankment with a lower frequency antenna. As seen in the table below resolution changes with antenna frequency but this is also true for an increasing depth, meaning that objects need to be larger, at depth, to be identified.  

Table 1. GPR antenna frequency and estimated resolution and depth.  

 

Frequency is not the only factor that determines the achievable depth and resolution; the ground through which the signal is sent, also determines the quality of the collected data. Resistive soils such as sand and gravel, as well as rocks, are usually suitable for GPR surveys, whilst less appropriate soils usually contain conductive materials, for example, clay. Soils saturated with water can also be problematic for GPR surveys. 

GPR measurements for road applications are most often done by pulling the antennas in a cart along the ground, which can be efficient for smaller areas. For longer investigation lines, Guideline Geo also provides a convenient cart, the RoadCart, for holding 1-3 antennas behind a vehicle. The MALÅ MIRA HDR can be an efficient tool if wider and very precise investigations are needed.

During measurements, the position of the GPR antenna is tracked by an external or internal GPS for precise positioning of your investigation results.  

GPR for different road applications

Suitable applications to solve with GPR within the area of roads are:

  • Mapping asphalt thickness
  • Recording internal road layers
  • Mapping geology underneath the road (such as bedrock, peat, soil layers)
  • Locating utilities within the road and/or embankment
  • Mapping piled decks underneath the road embankment
  • Detecting voids within the road structure
  • Mapping boulders/stones within the road embankment
  • Mapping ground frost, moisture

GPR investigation results with 2D measurements

 

The results from a GPR investigation on roads is rather straightforward. Data can be analysed in different types of software (for example ReflexW and GPR-Slice) where the data is processed and interpreted according to application. Layer data is most often presented as X, Y and Zwhere X and Y gives the positions and Z the depth to the interpreted layer. This data can be viewed superimposed on maps with easy color scheme representing the layer thickness interpretation. The example image shows a road investigation of asphalt. As seen in the mid parts of the profile, asphalt has been added on several occasions. 

 

GPR investigation results using multichannel array measurements

Road investigations can sometime benefit from multichannel array measurements. This will give a 3D visualisation of the road. Instead of making several runs with a single radar antenna you can measure several parallel profiles simultaneously (the technique is explained in Application Area Utilities). Guideline Geo provides several different options of multichannel arrays, for the best possible solution to your application.

 

In the image below a multichannel array investigation is carried out with MIRA HDR. 4 parallel utility lines crossing the road together with one utility along the road construction is clearly seen. The multichannel data also reveals different structure (due to stones, boulders) in the road embankment. The data is processed in rSlicer.

Products

Guideline Geo has several solutions for investigation of roads, both for road embankments and its internal layering and the conditions beneath the road structure  

Most often road investigation comprises investigation of thin layers of asphalt to deep road embankment constructions. With Guideline Geo´s ProEx GPR system several antenna frequencies (from low to high, up to 8 antennas) can be measured simultaneously, saving time and costs. The system is very robust and has a number of accessories suitable for road measurements including the convenient Road Cart upon which 3 different shielded antennas can be mounted. Interpretation is efficiently done with packages such as ReflexW from Sandmeier Geophysical Research or GPR-Slice. 

 

If asphalt is the main only application, Guideline Geo´s CX system is the easiest and most convenient system to usededicated to high frequency antennas (in the range of 1.2 to 2.3 GHz). The system is fitted in a rugged case and with an easy interface, measurements can be carried out very efficiently. Single lines can be collected as well as aided grids for a 3D view of the investigated area. The system is also very suitable for concrete and rebar investigation or inspection of other types of constructionsuch as brick and wood. Interpretation is easily done directly in the GX Controller or with Reflexw GPR-Slice.  

If you are going to work on larger areas, instead of single lines, and are interested in utilities, rebars in concrete roads, voids and other discrete targets within the road embankment or on runways, Guideline Geo´s multichannel MIRA, MIRA HF or MIRA HDR is a suitable solution giving you multiple, close-spaced parallel lines (total number varies by MIRA model) in one single swath, perfectly positioned and results easily processed for 3D views in rSlicer and GPR-Slice.

If your main application is estimation of thicker layers, the total road embankment and the geology underneath the road structure, Guideline Geo´s GX system with antennas from 80 to 750 MHz can be suitable. This is the easiest to use system with its WiFi option, making all cables unnecessary. Interpretation is efficiently done with Reflexw or GPR-Slice.  

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