In addition to visual inspections, geophysical techniques can assist engineers with infrastructure maintenance and monitoring by providing non-destructive in-situ subsurface information. This subsurface information can help reduce the risk for an engineer when conducting condition assessments of civil infrastructure e.g. inspecting tunnel lining integrity, bridge deck integrity, bridge abutment integrity, subway wall integrity. Geophysical techniques can also assist with interpreting the thickness of tunnel lining, locating unknown positions of steel reinforcement and monitor subsurface changes over time of an infrastructure over 3D-space; this is referred as 4D monitoring.
- Ground Penetrating Radar (GPR) is a quick and effective tool for providing assessments on subsurface damage zones and regions of deformation within civil infrastructure. GPR is a technique that images the radar reflections that occur between materials/regions when a strong electrical contrast exists between material interfaces. In the case of damage zones, regions of cracking and water seepage, these regions will generally produce a strong electrical contrast in comparison to undisturbed surrounding material either-side of the damage zone. This males GPR particularly useful in highlighting where there are regions of fouling in railway ballast, regions of sinkholes/damage on roads and bridge decks and also where there is cracking and seepage found in tunnel lining or subway walls. When a base image has been created of the subsurface using GPR, it is also possible to rescan these regions and monitor for changes in the GPR dataset over time. GPR is also commonly used to locate reinforcement in concrete, provide indications of the thickness of road-base and basement slabs, and provide indications of the thickness of tunnel lining and grout lining.
Geophysical technologies can provide quick and accurate non-destructive solutions for understanding the depth to pile footing and the unknown spatial location of buried piles. Common effective techniques include GPR, ERI, downhole seismic and magnetic methods.
- Ground Penetrating Radar (GPR) is a quick and effective technique for interpreting the depth to buried objects such as the tops of buried piles. Generally, the targets chosen for further investigation with GPR are often located prior using EM and Magnetic methods. These objects often have a large electrical contrast in comparison to the host material, which provide good targets to produce a strong GPR reflection. In the case of buried piles, the top of the pile should produce a large radar reflection due to the large electrical contrast between the top of the pile and the host geological material.
- Electrical Resistivity Imaging (ERI) maps the differences in the electrical properties of the subsurface if an electrical resistivity contrast exists between materials. The ERI technique is highly dependent on the porosity and water content of the earth material e.g. the greater the water content the larger the electrical conductivity. But, the ERI method is also highly influenced by the presence of buried metal or services located beneath an ERI profile line, as these buried objects are often highly electrically conductive. It is possible to locate a region of buried steel reinforced piles along a profile line using the ERI method, as this region will be highly electrically conductive in comparison to the host geological material.
- Magnetic methods (downhole) is a useful tool to utilise when the foundation pile contains ferromagnetic material, such as reinforcement bars. The reinforcement generates an observable, interpretable induced magnetic field relative to the background earth’s magnetic field. If the surrounding soils are generally non-ferromagnetic, then a relatively strong induced magnetic field may be observed in data as a result of the steel rebar in the concrete. The base of a foundation pile containing the steel reinforcement, should be observed as a strong magnetic response in the measured data. As a result, the base of the foundation pile can be estimated.
- Seismic methods (downhole), in particular parallel seismic, measure the change in seismic velocity of the propagated seismic wave generated from impacting the top of the pile in a downhole tool, located at depth parallel to the pile. The seismic velocity of the pile is generally faster than the host geology the pile is located within. This can be due to the host geology being less dense, or less consolidated, so the seismic wave travels slower through his less consolidated material. At the point when the seismic velocity is measured to be slower than the constant speed of the pile, the depth to that velocity change is estimated to be the foot of the pile.
- Ground Penetrating Radar (GPR) is a quick and effective technique for interpreting the depth to buried objects such as underground services, underground storage tanks (UST) and other large buried man-made objects. Generally, the targets chosen for further investigation with GPR are often located prior using EM and Magnetic methods. These objects often have a large electrical contrast in comparison to the host material, which provide good targets to produce a strong GPR reflection.
- Electromagnetic (EM) methods are excellent pre-screening tools for locating buried electrical services and steel pipes. Buried metal objects will generally have a large electrical contrast in comparison to the host geological material. EM methods can map the lateral extent of the buried object across the site, and the depth to the object can often be determined using the complimentary GPR technique.
- Magnetic methods target metallic or ferrous objects related to buried service. The magnetic method can detect magnetic objects such as steel pipes, conduits or tanks that may otherwise be too deep for GPR or EM methods to detect. EM methods may not necessarily pick up highly magnetic objects, so the magnetic technique acts as a great pre-screening and complementary tool to EM methods; especially if the investigation site is known to contain possible buried magnetic objects. Depths to interpreted objects can often be determined using the GPR technique.
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