Geotechnical
buildings. This calls for information on the structure, condition and loading capacity of walls and floors and is where the high resolution capabilities of engineering geophysics can help. Techniques such as ground
penetrating radar, ultrasonics and sophisticated remote visual inspection cameras can determine the thickness of masonry or concrete and the arrangement of structural steelwork or reinforcement. They can also help to evaluate the condition and integrity of structures, and to map subsurface voids, corrosion and moisture.
Integrated site investigation In a recent project for Irish Rail, Fugro surveyed more than 1,000 km of railway trackbed using a train-mounted ground penetrating radar system integrated with the client’s survey train to simplify logistics. Fugro Aperio’s transport team
resolved challenges of equipment set-up and simultaneous collection of multiple data-streams to complete the survey in just seven days without disruption to scheduled services. With antennae mounted on board
the Irish Rail track recording vehicle, the team collected six continuous data- streams at a normal operational speed of 65 km/hr. Coverage included the mainline passenger routes between Cork, Galway, Sligo and Westport. Irish Rail plans to use the data to help
design stage. These include:
• cavities – natural (e.g. karst related) or man-made (e.g. mining features)
• bedrock structure – stratigraphic layering, faults, shear zones, joints, fractures
• obstacles – boulders, existing foundations or buried structures such as services, culverts.
Upgrades bring unique challenges Upgrades to the network include everything from replacement of a level crossing to enhancing the track speed or structure gauge of an entire route. The biggest upgrades on the drawing board currently are route electrification projects and these will bring their own set of unique geotechnical challenges. Electrification projects require
thousands of gantries to secure overhead line equipment. The placing of each gantry base in the ground brings its own set of questions. Where are the buried services or pre-existing buried structures? What are the soil conditions? What type and depth of foundation is optimal? Careful observation by an experienced geotechnical engineer together with trial holes and probing will provide a
valuable insight into ground conditions. But is this type of conventional mini-site investigation at every location affordable or achievable in terms of access constraints? An alternative approach could be
based on a combination of non-intrusive scanning and targeted cone penetration testing. Scanning with cable tracing and ground penetrating radar can mitigate (but not eliminate) the risk of hitting unrecorded services or structures. Cone penetration testing is usually
carried out from heavy truck-mounted rigs, but there are alternatives with a lighter footprint. Engineers at Fugro use a system based on a modified excavator arm. This allows rapidly mobilised testing at the proposed location of gantry bases using nothing more than the hydraulic system of the vehicle to provide the reaction force necessary to push the instrumented test cone into the ground. Alternatively, highly mobile systems
are mounted on mini-crawlers offering the potential to access the ground trackside without occupying the track. Although most overhead line
equipment will be mounted to gantries anchored in the ground, a proportion will be fixed to tunnel linings or station
determine the condition and thickness of track ballast, as well as ballast formation, sub-formation and presence of water. This will assist engineers in assessing, prioritising and designing track rehabilitation work and associated drainage improvements. Highlighting the benefits of an
integrated site investigation, the rapid scanning geophysical technology was used to target trial holes with results from the holes helping in turn to calibrate thickness measurements of the geophysical data and refine their interpretation.
Working smarter in geotechnology We can be certain that there will always be a degree of uncertainty regarding the possible effects of ground conditions on the construction and operation of a rail network. With increasing pressure to improve
performance in terms of safety, environmental impact, programme and outturn cost, investment in the joined up picture of smarter geotechnical surveys can pay dividends by reducing the risks posed by the unknown.
Simon Brightwell is a director and technical head of structures at Cambridge-based Fugro Aperio, specialists in non-destructive investigation for transport, structures and ground engineering.
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