Considering stricter infrastructure standards and regulations – as well as digitalisation across the public sector and industry – geophysical techniques are set to play an even greater role in infrastructure planning, design and management.
A key contribution will be to address the current challenge of data sparsity in drilling, test pits and geotechnical data, according to Wesley Harrison, a principal geophysicist at SRK Consulting (South Africa). Harrison highlighted how the recent Engineering Geophysics Symposium, hosted by the South African Geophysical Association (SAGA) last month, showcased the immense potential that geophysical techniques could play in improving risk modelling.
“Through non-invasive subsurface imaging, we can efficiently and cost-effectively fill the gaps in traditional data sets,” he said. “This helps us gain a holistic understanding of subsurface conditions and properties for project planning, infrastructure safety, and disaster mitigation preparedness.”
He added that by combining geophysical techniques with geotechnical methods for site investigations, engineering firms were able to add further value by reducing the number of expensive boreholes and laboratory tests, resulting in cost and time savings.
Bruce Engelsman, a partner and principal geotechnical engineer at SRK Consulting (South Africa), said that the value that geophysics techniques provided had largely gone unnoticed due to the expertise needed to correctly interpret geophysical data for conclusive outcomes.
“However, our geophysical investigations have demonstrated the value of these techniques by providing a cost-effective and fast way of enhancing geotechnical studies,” said Engelsman.
Harrison said that the growing interest in the potential of geophysical techniques for de-risking infrastructure projects was discussed at the symposium.
“The event was well-attended not only by geophysicists, but by civil, geotechnical and mining engineers from both the private and public sectors, as well as members of academia,” he said.
Piecing together data assets
At the symposium, he presented how SRK had deployed ground-penetrating radar (GPR) alongside geotechnical investigations to gain a holistic and detailed understanding of the health of rail foundations.
“GPR was deployed to investigate subsurface conditions between the 2 m-deep test pits that were excavated every 5 km to 10 km along the railway line, enabling us to measure the foundations down to 2 m at a 20 mm horizontal interval,” he explained. “The subsurface images that we obtained enabled us to accurately piece together the 1D datasets that were derived from the geotechnical investigations.”
The survey was undertaken with a “rail ground-penetrating radar” which the company helped to develop with technology partners.
“High-frequency electromagnetic waves were emitted by a trailer-mounted ground-penetrating radar towed behind a vehicle,” he said. He added that the technology could be easily adapted for various surveying applications by adjusting the programming and antennas.
“We foresee this technology playing an important role in transport infrastructure condition monitoring and maintenance planning moving forward,” he said.
Understanding dolomitic ground
Harrison said that there was also increased interest in electrical resistivity tomography/imaging (ERI) to provide a better understanding of dolomitic terrain and improve cavity monitoring.
“ERI provides more reliable spatial constraints than gravity surveys for imaging subsurface features at depths typically below 10 m to 30 m, depending on site conditions and survey design,” he said. “This is due to ERI's high sensitivity to significant electrical resistivity contrasts between air- or water-filled cavities and surrounding soil or rock, coupled with robust 2D and 3D inverse models for accurate depth calculations.” ERI also offers improved detection and characterisation of subsurface features within this depth range, as well as more reliable detection and characterisation of subsurface features.
“This is a significant step forward in terms of de-risking infrastructure projects sited in unpredictable site conditions which require specialised engineering approaches,” he said.
Modelling seismic waves
Harrison added that geophysics technology, such as Multichannel Analysis of Surface Waves (MASW) helped to design resilient surface infrastructure by modelling how seismic waves propagated through the rock mass.
“In this way, mining engineers gain a sound understanding of subsurface characteristics related to engineering designs, such has shear wave velocities and stiffness, in a 3D space,” he explained.
He added that work was currently underway to understand how the dynamic data generated by geophysics techniques could be used to determine geotechnical properties like Young’s modulus, Poisson’s ratio, shear modulus and uniaxial compressive strength, which are usually measured in the laboratory.
The loading conditions of dynamic tests are rapid and transient and are the rock response to high frequency low amplitude stress (seismic) waves propagating through the rock mass.
“In contrast, laboratory tests are considered static since they show the response to slowly applied, high-amplitude stresses, mimicking long-term, quasi-static loading conditions. Dynamic and static tests produce elastic properties that can differ widely,” he explained.
Towards tailings dam performance monitoring
Engelsman presented on how SRK Consulting (SA) was using geophysical techniques to integrate real-time monitoring with accurate deformation modelling, to improve performance monitoring of tailings dams.
“The performance monitoring of tailings dams is increasingly critical in the light of the Global Industry Standard on Tailings Management,” he said. “Geophysical techniques enable us to reliably zone tailings by calibrating them against drilling data and then by dramatically improving stability assessment modelling so that they resemble what tailings are actually doing.”
Engelsman said that geophysical data related to pore water pressure (CPTu) was used to characterise the tailings material that lay between current data points derived from drilling and cone-penetration tests; this was useful for monitoring tailings material that was inaccessible with such methods, particularly towards the centre of tailings dams.
“At present, it is impossible to delineate 3D zones in tailings dams, even though it is known that tailings zones with different characteristics exist – by virtue of the way in which tailings are deposited,” he said. “Considering that engineers cannot prove this with confidence, the tendency is to overestimate the risk and to be more conservative in the assessment – in the interests of safety. With more information, the stability assessment can more closely reflect the real situation.”
Detecting triggers of liquefaction
Geophysical techniques can also potentially detect whether liquefaction has been triggered by earthquakes, a sudden rise in water level, construction activity or blasting.
“Historical data has shown that liquefaction occurs locally in tailings bodies and can trigger this occurrence in adjacent areas, ultimately resulting in tailings dam failure,” he explained.
He added that liquefaction could occur over timeframes ranging from minutes to days, with current monitoring methods providing very little time in which to detect it and raise alarms.
Engelsman noted that it is currently difficult to assess whether tailings housed within dams have brittle tendencies and are, therefore, prone to liquefaction.
“This is due to the challenges associated with collecting and transporting representative tailing samples to laboratories – as any disturbance reduces their integrity for accurate lab testing,” he explained.
One of the key outcomes of the symposium, Harris said, was the need for greater collaboration between geophysicists and the fraternity of mining, geotechnical and civil engineers, to further innovate geophysical techniques.
“We need to break down silos so we can share different perspectives,” he concluded. “This will inform the development of new geophysical solutions and techniques, which will advance the disciplines.”
