Vermont Day: VTrans Uses GPR to Pinpoint Geohazards
Envisioning a multimodal transportation network that supports vulnerable groups and is safe, dependable, and cost-effective, the Vermont Agency of Transportation (VTrans) has implemented the use of intelligent compacting with enhanced methods based on recent research.
Ground Penetrating Radar
The Vermont Agency of Transportation (VTrans) recently used a ground penetrating radar (GPR) to determine the size and scope of evident holes resulting from a failing metal pipe culvert that was located beneath a local road and heavily used interstate, I-89.
Introduced in the March 2022 edition of the Federal Highway Administration’s EDC News Biweekly Newsletter, VTrans was highlighted for their use of GPR to identify geotechnical hazards. Geohazards, such as landslides and similar ground movements, present safety risks to workers and can affect the efficiency in which an overall project can be completed.
Although GPR is not one of the techniques highlighted by Advanced Geotechnical Methods in Exploration (A-GaME), it is still deemed a useful and adaptable subsurface exploration tool.
There are two different kinds of GPR systems, ground- and air-coupled, both provide different benefits that assist with gathering accurate data. Antennas in air-coupled apparatuses are placed above the surface of the structure and mounted on vehicles for rapid scanning, while the antennas on ground-coupled devices must stay in constant contact with the ground.
During the use of GPR, VTrans collected subsurface data to determine the size of the holes and areas of disturbed soil between the pavement and culvert structure along I-89. VTrans staff found that numerous holes had formed in the median causing the road to crack and sink-in. However, they were able to certify that the passing lane could continue to be used by vehicles.
Using the data collected, VTrans has concluded that GPR is a useful supplemental technology that can effectively determine the depth and presence of bridge rebars below the asphalt surface.
The performance of pavements and other earth structures depend on the compaction efficacy of embankments, subgrades, and base materials. High quality compact materials are required to ensure the long-term performance of pavements and to reduce costly maintenance expenses.
In September 2020, the VTrans Research Selection and State Transportation Innovation Council hosted their Research and Innovation Symposium. During the event, former University of Vermont Graduate Researcher, Dr. Maziar Foroutan briefly discussed his experience investigating strategies to improve the performance of intelligent compaction (IC) in pavement construction projects.
Through research and the integration of measurement systems that use GPS accelerometers, enhanced IC addresses issues associated with conventional compaction. The GPS system records the coordinates while the accelerometers provide feedback for both hot mix asphalt (HMA) and warm mix asphalt (WMA). Accelerometers are also able to detect the frequency and amplitude of the vibratory rollers. The data is then converted into stiffness (ICMVs) to calculate specific coordinates.
Due to inconsistency and non-uniformity in traditional compaction methods, the shift from the current density-based acceptance process to a stiffness-based inspection technique has been extremely desirable.
Dr. Foroutan developed recommendations on the implementation of IC by evaluating the consistency of the operations, assessing feasibility of using IC as a quality control (QC) tool, analyzing the data already available from a prior IC project the agency carried out, and conducting field tests to validate and establish correlations with the data obtained from the IC test sections.
Focusing on the reliability of IC, Foroutan and his team conducted a myriad of studies including a geostatistical analysis. The analysis resulted in a plethora of data. However, the researchers expressed a need for a higher level of certainty around the interpretation of the results before it is used in manuals and other resources for contractors.
The results of this study are available in a variety of places so that local contractors can incorporate the data in their IC work plans.
Sources: FHWA, VTrans