Going Deeper

GPRGPR can increase safety and reduce project liability.   

By Daniel Bigman

Ground penetrating radar (GPR) is a non-invasive mapping and subsurface imaging tool that is growing in popularity in the construction industry. When applied properly, this technique can increase worksite safety and reduce project liability by creating 2-D and 3-D renderings of buried infrastructure and subsurface geology. 

A GPR consists of a control unit and an antenna. The antenna is pushed along the ground or concrete surface and produces an electromagnetic wave at regular intervals. The wave travels through the subsurface and some of the wave’s energy will reflect off any discontinuity in the subsurface and travel back towards the ground. The control unit will record the two-way travel time and the amplitude of the reflection. 

When the GPR is pushed along the surface, usually called a transect, it produces a 2-D image of what is below the ground which is a representation of a trench profile excavated in the same location. The major difference is that GPR is non-invasive and avoids ground disturbance.

The depth of a potential target can be calculated by recording the two-way travel time of the wave and estimating its speed. This allows the construction professional to locate the depths of small targets such as reinforcements or pipes, and large geological structures such as bedrock or the water table. The amplitude of the reflected wave will depend on how different the physical properties are between the two materials. 

A larger difference will produce a higher amplitude reflection. For example, concrete is more like dry sand than wet clay, so the GPR signature will be subtler when detecting concrete in dry sand and more dramatic when detecting it in wet clay. 

Construction Benefits of GPR 

Like other geophysical methods, GPR is a non-invasive technique that doesn’t break ground. It maintains the integrity of the site and is cost effective. However, there are several additional benefits of GPR compared to other common imaging techniques. First, data collection with GPR is quick compared to most other geophysical techniques used in construction. 

Seismic surveys and earth resistance can be time consuming as each geophone or probe needs to be set and these techniques can require teams of people. GPR can usually be carried out with a minimal sized team and collects data rapidly when tied into a GPS and pushed by a cart or attached to a vehicle. 

Second, GPR has a very high data resolution. While other techniques also can have high resolution, the tighter the survey resolution the shallower most techniques will image. GPR, on the other hand, can “see” deep while still maintaining a sampling resolution on the order of centimeters. This allows the GPR practitioner to create high-resolution models of the subsurface at multiple depths and creates a unique situation where the same data sets can be used to extract multiple lines of information.

Finally, GPR is a dynamic instrument that can be applied to solve numerous problems for construction projects. Other techniques can be one-dimensional, such as EM locators for finding and marking pipes and utilities. EM locators are fantastic tools for quickly locating buried services, but these tools are limited to this use. While not a replacement for EM locators, GPR can be used to enhance locate projects, but also can be used in other ways, some of which are outlined in the next section.

Applications of GPR 

The dynamic use of GPR is one of its major strengths as a non-invasive imaging technique. Here is a brief list of some of the ways GPR can benefit construction projects:

• Estimate the unknown buried concrete pile length to help establish load-bearing capacity.

• Measure the pavement layer thickness since deficiencies can reduce the use life of pavement. 

• Estimate depth to bedrock to determine rippability and plan for proper excavation equipment.  

• Locate rebar, post-tension cables and conduits through concrete scanning to avoid damage before cutting, drilling and sawing.

• Locate buried pipes and utilities to prevent damage before boring and excavating or to find non-metallic pipes that are unlocatable with traditional EM locating methods.

• Evaluate bridge and road conditions to identify cracks, corrosion and overall integrity.

• Locate voids and sinkholes to determine construction feasibility and reduce liability.

Limitations

While GPR has many benefits, and can be used to solve a variety of problems in construction, there are some important limitations of this technology that construction professionals need to be aware of. The quality of GPR data is directly related to site conditions. Unlike the seismic survey technique which is uninhibited in its depth of prospection when used in clay (because it works on mechanical waves), GPR struggles to see buried targets or deep geological structure in clay because it produces EM waves. 

The signal strength can dissipate quickly on sites with these conditions. The technique can work in clay, but often takes an expert to generate useful information from the data. For example, if your signal dissipates before it reaches a buried pipe, it may be possible to identify the filled in excavation trench at the ground surface. These alternative perspectives usually come with proper training and experience.

GPR has wide-ranging applicability for the construction industry. The technique uses electromagnetic waves to image the subsurface and records the reflections and two-way travel times of waves to identify targets of interest and map geological stratigraphy. The benefits include rapid data collection, high resolution data sets and 3-D imaging, and can provide useful solutions to many problems. 

However, GPR is not a magic bullet and suffers from some limitations, the most significant being its dependence on environmental conditions. With experience and proper training, many of these limitations can be minimized or overcome, and in situations where these are unavoidable, the practitioner can manage client expectations and extend accurate probabilities of project success. 

Daniel Bigman, PhD is the founder of LearnGPR.com, an online training and certification platform for GPR practitioners in the construction and engineering industries. He may be contacted with any questions about GPR at [email protected].

 

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