As the intensity of design and construction activity on the UK’s rail projects ramps up, so does the need for geotechnical information.
Whether the project involves constructing, upgrading or maintaining infrastructure, there is a need for key data relating to the characteristics of the ground and how they might affect the construction process and the longer term performance of the completed structure.
Traditional site investigation techniques built around drilling, probing, digging, sampling and lab testing have served us well, but they are essentially based on determining conditions at sample locations and assuming some degree of continuity in-between.
To overcome this constraint, project teams are taking advantage of advanced geophysical techniques that can ‘join the dots’ and, therefore, reduce the level of uncertainty.
A factor common in most construction projects is that the source of most risk lies below the surface of the ground.
In the shallow subsurface, the presence of poorly recorded buried services and man-made structures constitutes a risk of injury to workers and delay due to damaged infrastructure.
Deeper underground, the presence of mine workings or natural cavities can delay or even prevent construction of foundations or basements.
Deeper still, the long term stability of a site can be dependent on the uniformity of its geology, with the possibility of fault activation an important consideration in the design of tunnels and major structures such as stations or viaducts.
High speed requires stable and predictable foundations
The UK is building more new railways than at any time in living memory. Major tunnelling projects in big cities are complex due to the interaction not just with the ground but with existing buildings and infrastructure. High speed railways require a stable and predictable foundation in order to ensure long term track performance.
While the route of a new line – whether at or below the surface – may be determined by broadly political and economic factors, its design will be dictated by many related factors associated with the characteristics of the ground. These include existing geological, geotechnical and hydrogeological conditions. A desk study may help identify first order suitability, but a site investigation will be required to reduce the residual uncertainty by investigating geohazards and characterising the site.
Historically, site investigations have relied almost exclusively on intrusive methods in the field and testing of soil and rock samples in the laboratory. This approach is evolving rapidly, however, and a new era of smarter, more integrated site investigation is emerging. The use of geophysical techniques enables us to optimise the spatial distribution of exploratory holes, usually resulting in a better understanding of the ground based on fewer, more intelligently placed holes. The geophysical response will reduce uncertainty by providing a continuous string of data between holes.
A smart site investigation might typically comprise the following phases:
• desk study
• reconnaissance geophysical investigation for gross characterisation of the site
• optimised programme of holes/probing with associated sampling lab testing and wireline logging
• targeted geophysical investigation
• determination of a geotechnical model of the site.
Modern geophysical methods can determine ground properties and geometry from depths of a few metres to more than a kilometre. Some of them, including magnetic, electromagnetic, seismic and gravity methods, can identify subsurface features, including geohazards of possible significance at the planning/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 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 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.