ETD

• GPR
• tunnels

The main aims of this thesis are processing and interpretation techniques of Ground-Penetrating Radar data collected from three tunnels located on the A6 Highway near Altare city in the Northwestern part of Italy. Their names are “Altare”, “Bocca d’Orso” and “Fonte di Altare”.
The main targets of interest during processing and interpretation are reinforced concrete tunnel linings that are vulnerable to ground water infiltrations, developing of cracks or voids, corrosion of metal rebars and drainage systems. It is necessary to collect this type of data in order to maintain safety and structural integrity inside tunnels.
Overall, a group of specialists collected about 150 GPR profiles from sidewalls and roofs of tunnels using STREAM T multi array GPR system with 200MHz and 900MHz antennas mounted on a moving vehicle with a remote control platform for better access to highest points of interest. The GPR profiles were obtained along total lengths of tunnels (varying from 43 m to 335 m) in order to get long profiles for more convenient and productive analysis. The combination of high and low frequencies antennas made it possible to obtain profiles with up to 4 meters depth resolution and recognize the most important radar features.
For proper processing and following interpretation steps, I generated and analyzed 2-dimensional B-Scans in GRED HD software developed by IDS GEORADAR and then generated more robust 3-dimensional C-Scans in GPR-Slice software.
Each of B-Scans required proper filtering sequences and sets of configurations. During my processing sequences in GRED HD I applied wobble corrections, time-zero corrections, gain functions, bandpass filtering, Hilbert transform. I also had to take into consideration a unique type of acquisition in tunnels and some particular challenges like the absence of GPS coordinates for proper positioning of antennas and distances between profile lines, vehicle and equipment vibrations, deviations from straight acquisition line, presence of unwanted noise and signal reflections.
After processing, I obtained about 50 filtered radargrams from every tunnel and started interpretation of the data using my own experience and literature examples for detection and description of radar features and anomalies. I recognized engineering and technical features like reinforcing rebar meshes with strong hyperbolic type of reflections, sequences of drainage systems for ground water control and metal anchors. Moreover, I recognized thicknesses of concrete linings and borders with host rocks, water filled cracks, voids and corrosion zones.
For more robust processing and interpretation of concrete linings, I generated 3-d C-Scans in GPR- Slice software. I did not have detailed information about antennas positioning and locations of acquisition lines, but the software allowed me to create a set of coordinates and artificial markers in order to make a proper survey grid. I obtained information about the distances between antennas channels (about 30 cm) and distances between profile lines (about 2 m). After filtering sequences, I generated sets of radar time slices. After compilation of time slices I obtained volumetric radar datasets. It allowed me to visualize and analyze dimensions of radar features and construction elements more carefully. Data analysis allowed me to identify metal rebars spacing, water drainage systems locations and their dimensions, inner defects inside linings and host rock borderline contact dimensions with water infiltration zones.
The information and results from my research can be applied for very detailed interpretation, visualization of inner concrete linings features in tunnels, and planning of repair works in the future because the tunnels of interest were constructed in the 1960’s in the region with specific, partially unstable geological and climatic conditions. Mentioned factors require periodic geophysical assessment and monitoring acquisitions in order to detect structural and surrounding host rocks defects and anomalies, which could affect safety of people and local infrastructure.
I have also included some other geophysical investigation and monitoring approaches for modern tunnel construction industry, and a few practical and conceptual strategies for GPR applications in tunnels for comparison and discussion.