Smart Monitoring of Sinkhole Formation Using Optic Fibre Technology

Sinkholes are common geohazards, frequently responsible for sudden catastrophic ground collapse. Development on these areas is affected by historic and ongoing geomorphologic processes that may result in surface movements. Centurion and Pretoria in the province of Gauteng, South Africa are large urban environments underlain to a significant extent by dolomitic bedrock and have been affected by sinkhole hazards in the past decades. Sinkholes occur with little or no warning and directly or indirectly affect communities and/or major and minor infrastructure. Thus, effective monitoring would allow for further understanding of the mechanism of occurrence of sinkholes and lead to the development of a potential early-warning system to provide an alarm or a warning of incipient collapse. In the current study, fibre Bragg gratings (FBGs) were used to instrument a reduced-scale model, simulating a sinkhole event. The tests were conducted in a laboratory environment by embedding optic fibre sensors in the soil and inducing failure until critical conditions were reached. FBG sensors were manufactured in various optic fibre cables at predetermined locations. Strain and temperature data were obtained using the FBGs with an optical interrogator. The measurements were recorded simultaneously and in various positions, giving reliable and comparable results. We believe that this is a promising technology that, once expanded to distributed strain measurement on long lengths of optic fibre, would assist local authorities, municipalities and civil protection authorities towards a strategic sinkhole risk management.