Ground Penetrating Radar (GPR) in Geomechanical Site Investigations: A Pilot Wall Case Study

With the chosen diploma thesis topic, they primarily wanted to highlight how important it is to involve a specialist in preliminary site investigations, a geomechanics expert, before starting the preparation of project documentation for a new construction. By preparing a geomechanical report, this expert helps the designer, investor, and ultimately the contractor by providing the identified ** properties of materials in the foundation soils, thus contributing to a more complete and higher-quality picture of the area under consideration. Through the explanation of planning preliminary site investigations, presentation of laboratory tests, and interpretation and evaluation of the obtained results from field and laboratory investigations, they aimed to achieve and determine the ** depth of compact soil (grey marl), the foundation for the embedment of the pilot wall piles, by surveying the pilot wall area using the ** ground penetrating radar** method – ** GPR**.

Solution and Method

With the chosen diploma thesis topic, they primarily wanted to highlight how important it is to involve a specialist in preliminary site investigations, a geomechanics expert, before starting the preparation of project documentation for a new construction. By preparing a geomechanical report, this expert helps the designer, investor, and ultimately the contractor by providing the identified ** properties of materials in the foundation soils, thus contributing to a more complete and higher-quality picture of the area under consideration. Through the introductory chapters, they linked ** engineering geology with the fundamentals of understanding the ** structure of foundation soils** in connection with geomechanics, which studies the ** physical and mechanical properties of rocks and soils**.

Results

Through the explanation of planning preliminary site investigations, presentation of laboratory tests, and interpretation and evaluation of the obtained results from field and laboratory investigations, they aimed to achieve and determine the depth of compact soil (grey marl), the foundation for the embedment of the pilot wall piles, by surveying the pilot wall area using the ** ground penetrating radar** method – ** GPR**. The obtained results from the GPR profiles were then compared, interpreted, and analysed with the data from monitoring the excavation of piles with the drilling assembly and excavation records (pile birth certificates in the appendix).

Together with the geotechnical engineer from the construction institute ZRMK, they then analysed and interpreted the data, and based on the obtained survey, concluded that the attenuation of electromagnetic waves at depth was due to the high natural moisture content of the upper layers under consideration. The GPR method can provide useful data on newly constructed layers, such as the installation of a ballast layer in the lower structure of a railway track or, for example, the thickness of crushed stone. Likewise, the thickness of the pavement structure of trafficked surfaces can be estimated or determined quite accurately in this way.

In any case, the GPR method can be used to distinguish different material layers that are above the groundwater level or highly moist soils, but for ** material classification, the GPR survey data must be linked with laboratory tests of materials, or for accurate interpretation of all changes in the foundation soils, data on the installed materials must be known if newly installed layers are involved. The GPR method also has advantages over other geotechnical site investigations, namely the ** non-destructive nature of the survey execution itself, as the GPR only investigates or surveys the terrain and thus ** does not interfere with it**, and the execution does not cause its weakening.

Likewise, a GPR survey can be carried out on difficult-to-access terrain, and the transport of the necessary equipment to the location itself is not costly. It is also possible to repeat measurements multiple times with the same measuring equipment and thus observe any changes or deterioration over time. It is worth adding that for proper material classification, the cost of the GPR survey must include the cost of the associated laboratory tests of the collected samples, which in the evaluated example would amount to approximately €350.00.

Thus, the total cost of executing the terrain survey using the GPR method amounts to €1,026.00 + €350.00 = €1,376.00, which is almost exactly 50% of the cost of terrain investigation using a conventional drilling assembly for deep exploratory boreholes. In general, we must be aware that the GPR survey alone does not yet provide all the necessary data on the terrain under consideration, so they estimate, as stated in chapter 6.3, that the best preliminary site investigation would be a ** combination** of a terrain survey using the ** GPR method** and the execution of ** trial excavations** with a description of the geological composition and sampling of existing soils, from which soil samples are obtained for laboratory tests at the depth where the foundation of the new construction is planned, and whose laboratory analysis results are needed for the preparation of a geological-geotechnical report on the foundation conditions of structures and settlement calculations.

In this way, an accurate survey of the wider area of the new construction would be obtained earlier, and based on the identified critical locations, local trial excavations would be carried out, which would allow accurate classification of the geological composition of the foundation soils and obtain the necessary parameters of the geomechanical and geotechnical properties of soils and rocks. It should be noted that this method of ** geological-geotechnical investigations** is only possible outside the groundwater area or highly plastic and moist soils. The use of ** artificial intelligence and machine learning** methods in new technologies will significantly improve the usability of GPR in the future, precisely on the basis of parallel research.

With a sufficiently large number of comparative analyses, diagnostics and usability will be significantly improved, similar to what is happening today in the fields of geodesy, medicine, etc.