Distributed fiber optic shape sensing along shotcrete tunnel linings: Methodology, field applications, and monitoring results

Abstract

Deformation monitoring and structural reliability assessment are key components in modern conventional tunneling. The state-of-the-art monitoring design is usually based on displacement measurements of geodetic targets using total stations paired with pointwise geotechnical sensors inside the tunnel lining. In recent years, distributed fiber optic sensing (DFOS) has become more popular in tunneling applications. DFOS measurements basically deliver internal strain and temperature distributions, but no direct relation to the tunnel shape’s behavior. This paper introduces a novel sensing and evaluation concept, which combines DFOS strain measurements and geodetic displacement readings for distributed shape assessment along curved structures, such as tunnel cross-sections. The designed system was implemented into shotcrete tunnel cross-sections as well as shaft linings and enables the determination of displacement profiles with high spatial resolution in the range of centimeters. Evaluations of continuous monitoring campaigns over several weeks as well as epoch-wise measurements performed by different DFOS sensing units in combination with stochastic analysis demonstrate the high potential of the developed approach and its capability to extend traditional monitoring methods in tunneling.

Long-Term Performance of Distributed Optical Fiber Sensors Embedded in Reinforced Concrete Beams under Sustained Deflection and Cyclic Loading

“…The study and application of DOFS has not been limited to laboratory experiments and several cases exist where this sensing technology has been successfully deployed and applied on-site for the monitoring of real structures. Some examples of real-world applications are the Sarajevo bridge [ 28 ], the precast tunnel lining of the L9 subway line in Barcelona [ 29 ], the shotcrete lining in the Sammering Base tunnel [ 30 ] or the monitoring of driven ductile piles [ 31 ], to mention just a few. Despite the successful implementation of DOFS systems in real structures, the existing applications are often either limited to the measurement of strains or unable to be validated against complementary systems or carried out in a discontinuous way and/or for a short period of time.…”

Distributed Fiber Optic Shape Sensing of Concrete Structures

“…This can be especially advantageous if more than two supporting points can be utilized to provide redundancy and handling of potential erroneous data in practical applications on-site. Using DFOS in combination with pointwise displacement readings, e.g., from total station measurements, can provide an integrated approach, which allows fully distributed shape assessment along the civil engineering structures [ 52 ].…”

“…For closed ring system, it can be assumed that the displacement value and its gradient at the starting point is equivalent to the last integration position by extending the functional model with constraints instead of pointwise displacement readings. More detailed information on the functional model and the updating process is given in [52]. It is obvious that displacements in the plane of the x-axis are minimal for measurements at the test rig due to the loading orientation (cf.…”

Distributed Fibre Optic Sensing (DFOS) for Deformation Assessment of Composite Collectors and Pipelines

“…By means of the distributed fibre optic technique, it is possible to detect local damages [22], such as leaks or unsealing [23], that are usually related to changes in temperature. Another aspect is the mechanical loading caused by the ground movements.…”