Cum laude Master's graduate Francois Swanepoel (left) with the sensor and Dr James Jena lead researcher and CBC postdoctoral research fellow
The collaboration between the Centre for Broadband Communication (CBC) in Physics and the Department of Geoscience Pormed Lab expands on previous optical fibre sensor development at the University.
Ongoing research has resulted in devices able to detect changes in vibrational states and related instability before the actual collapse of sinkholes and other geo-hazards, says Dr Jena, lead researcher and CBC postdoctoral research fellow, whose field of expertise includes physics, optic fibre communication and sensing.
Geo-hazards are geological processes posing a threat to the environment, human health and safety, such as earthquakes, landslides and tsunamis, he explains.
Apart from threats to life, they cause financial loss through damaged buildings and homes, disruption of business and livelihoods, infrastructure repair, reconstruction or remediation, evacuation and relocation costs.
The team’s research paper, ‘Simultaneous vibration and soil moisture sensing using a single mode fiber for structural health monitoring applications’ was published in Optica Publishing Group in January.
Water, water everywhere
Dr Jena was assisted by recent master’s graduate Francois Swanepoel, who specialises in structural health infrastructure monitoring for the civil and mining industries, using fibre optics, together with Dr David Waswa and Geoscience senior lecturer Dr Gaathier Mahed.
Swanepoel was awarded his MSc in Geology, with cum laude, in April, and will soon be commencing his PhD studies at the University of Cape Town.
The dual-purpose single mode optical fibre sensor was developed for simultaneous soil moisture and structural health monitoring.
In practice, it is used to detect environmental changes due to ground movement and moisture content variations.
Why are optical fibre sensors so important in terms of public safety and building integrity?
“With the expansion of fibre networks by the telecommunications industry, fibre optics could become increasingly valuable for infrastructure monitoring,” says Dr Jena, who holds a PhD in physics.
“Its high resolution and sensitivity offer engineers and environmentalists more efficient and accessible methods for monitoring and maintaining this infrastructure.”
The team’s latest fibre sensor acts as a moisture detector, explains Swanepoel. “Perturbation (a small change in a physical system) from the addition of water changes the physical properties of the fibre, causing it to detect the amount of water in the soil.”
The sensor detects moisture levels in the soil and subsequent movement of soil in the subsurface, while lower frequency monitoring keeps an eye on the structural integrity of buildings.
“This means that any movement below the surface can be detected and understood in terms of its possible impact on the surrounding environment and eventually human life.”
The device serves a dual purpose, making it superior to single-purposes devices with similar properties, explains Dr Jena.
“Dual-purpose systems eliminate the need for extra equipment and sensing techniques, streamlining monitoring process to be easier, faster and more cost effective.
Early warning signs
For the average Joe Public, how can geoscience monitoring be applied to everyday situations? Would a building owner, or layperson, be able to determine if a structure is compromised?
Yes, says Dr Jena: look for the obvious, such as cracks in the side of a building and walls subsiding or lowering into the soil.
A call to the building contractor who erected the structure is a wise first step in these cases, he says. Box ends
The high cost of sinkholes
Sinkholes are a constant threat in South Africa, with 2500 sinkholes and related subsidence events recorded by 2011. Nearly 100% of these occur in the Gauteng province, says Dr Jena.
The National Geographic Society defines sinkholes as a depression or hole in the ground that forms when the surface layer collapses, often due to the dissolution of underlying soluble rocks, such as limestone by water. They can vary in size and shape, occurring either suddenly or gradually.
In 2011, statistics revealed the estimated cost of sinkhole damage to be more than R1.6-billion, with further projected costs of R11-billion to safely relocate around 30 000 households from a danger zone west of the city of Johannesburg.
There are three categories of sinkholes, explains Dr Jena: cover collapse, subsidence and solution. Of these, collapse sinkholes are the biggest threat, as they are impossible to identify by any known means, due to lack of available instruments and because they do not exhibit any ‘on-surface’ signs.
They form when the cave roof of the top covering material suddenly fails, collapsing into the cave. “These are more dangerous because they have no visible signs of imminent surface collapse.”
Solution and subsidence sinkholes, on the other hand, can be detected with the naked eye. Red flags include gradual subsistence of ground surfaces, cracking walls, bending door posts and tilting of trees and poles.