The application of distributed optical sensing for monitoring support in underground excavations

Abstract

An observational approach to excavation design and construction is commonly employed in order to assess excavation driven displacements which, in turn, affects the design of support systems used to maintain the stability within underground projects. A correct and accurate evaluation of the support system performance will therefore, be critical to both the safety and economics of the construction process. This necessitates a monitoring program capable of capturing the support system behaviour in order to verify assumptions made at the design stage and as the excavation advances. Conventionally, this has been approached from an external perspective where the behaviour of individual support elements has been inferred from the measurements of the excavation periphery (e.g. geodetic monitoring) and displacements surrounding the support (e.g. multi-point-borehole-extensometers). This is possibly due to the difficulties of operationally instrumenting ground support. However, this has ultimately lead to a gap in knowledge in terms of the distinct performance of each support element in isolation and as part of a multi-component support system. Within this context, a novel distributed optical strain sensing (DOS) technology has been implemented with rock bolt and forepole support elements. Unlike conventional, discrete strain measurement methods, the optical technology captures a distributed strain profile along the length of a standard, low-cost, single mode optical fiber (i.e. 1.25 millimeter spatial resolution). In this regard, this research has been devoted to developing a technique whereby the optical sensor is capable of being installed with rock bolt and forepole elements to be used as primary support in situ. The development of such an optical technique is a non-trivial undertaking. Furthermore, the DOS technology has never before been implemented within the geomechanics community and was therefore subjected to a comprehensive laboratory testing program, which considered: bending, axial loading, and shearing of optically instrumented support elements. Results of the testing program demonstrated the capability of the optical technique to capture expected loading mechanisms of the support elements throughout their serviceability life, agreeing well with numerical and theoretical predictions. Additionally, the optical technique was found to capture complexities of support behaviour at an unprecedented level, overcoming limitations of conventional monitoring.