Buildings and structures
Tunnel surveys
Fugro Aperio can provide information on the construction and condition of masonry, concrete, or steel lined tunnels and unlined rock tunnels using a range of non destructive and geophysical methods.
Applications of geophysics include:
- lining and overbreak thickness
- delamination and voids at any depth within brick linings
- voiding behind the lining
- moisture above and within the tunnel lining detection and profile of invert
Masonry tunnels
Geophysical survey methods can provide a considerable amount of information on masonry tunnel condition and construction. Ground penetrating radar is the most well-established technique because of its rapid data acquisition and versatility.
Geophysical surveys of masonry tunnels can provide a wide range of information on lining construction and condition. Enlarge
Fugro Aperio has used GPR to determine lining thickness, detect features beyond the lining such as timbers, counterforts, addits or shafts, and to characterise changes in geology. We have also used it to provide condition assessment, for example mapping voids, delamination and moisture within and behind masonry linings. Sophisticated data processing methods enable images of defects to be presented as ‘slices’ through a tunnel lining, enabling maintenance strategies to be based on a far more comprehensive three dimensional understanding of asset condition than could be derived from visual or tactile methods.
GPR
survey of a Network Rail masonry tunnel to determine lining condition and detect
hidden shafts.
Most tunnel surveys are conducted from mobile scaffold towers or road/rail vehicles enabling the survey team to sweep the measuring head over the lining in a series of longitudinal or transverse profiles. Surveys using GPR can provide an overview of a 1000m tunnel in just one short possession and can be followed up by targeted drilling or by detailed geophysics from the top once the suspect location has been identified.
Infrared thermography has been used in tunnels to detect defects such as near surface delamination and has been effective in some cases in detecting blind access shafts. The method relies on optimal conditions (typically a difference of >10 degrees Centigrade between the tunnel and ambient). The major benefit of thermography is the rapid progress rate and the feasibility of surveying without expensive scaffolding or road/rail access platforms.
Far
left: Thermography provides a quick overview as part of a tunnel inspection
Left: Thermal image of suspected hidden shaft
Fugro Aperio has undertaken innovative work using ground resistivity, or electrical resistivity tomography (ERT) to detect hidden shafts beyond tunnel linings. This method is slower than GPR, but provides much greater depth penetration and is more effective in highly conductive materials such as wet clays.
Above:
Electrical resistivity image though tunnel crown showing hidden shaft (blue).
Left: and excavation of shaft from above after its detection.
Ground conductivity can be effective in mapping moisture variations within and behind masonry linings and has been used in shallow metro railways to investigate water ingress from leaking utility pipes.
Concrete tunnels
Non destructive surveys can determine the thickness and arrangement of sprayed, insitu or pre-cast concrete. Surveys also map voids within or behind the lining and in some cases map variations along a tunnel, such as changes in moisture levels or geology.
Note that GPR - the method of choice for most tunnel surveys – will not work through steel fibre reinforced or small gauge reinforcement mesh, so alternatives are needed. The most appropriate alternative method is often low frequency ultrasonics.
Metal lined tunnels
Non destructive surveys can determine the thickness of steel and cast iron linings. Surveys can detect corrosion damage and other defects. The most useful methods employ mechanical waves and include high frequency ultrasonics and impact echo.
Unlined tunnels
Non destructive surveys can map voids, fractures and man made features such as rock anchors within rock tunnels. The most useful method is GPR but the rough surfaces of unlined tunnels generally prevent the use of conventional surface coupled antennas (i.e. where the antenna is in contact with the surface).
Most surveys of rock lined tunnels therefore use an air launched setup, where the antenna is profiled along the tunnel with a gap of up to 300mm between the antenna and the surface. Using seismic methods including MASW Fugro Aperio can provide information on ground stratigraphy and geotechnical properties.
