High Spatial Resolution Measurement of Tendon Reinforcement in Underground Construction Works

Abstract

Distributed optical fiber strain sensing is investigated as a technology to enhance the assessment of tendon reinforcement elements that are commonly used in mines and tunnelling projects. Central to this research effort is the application of a particular high spatial resolution Rayleigh optical frequency domain reflectometer that is able to distinguish strain along a standard optical fiber at spatial increments as low as 0.65 millimeters. In applying this technology to reinforcement elements, the objective is to measure a nearly continuous strain distribution, such that complex and non-uniform in-situ reinforcement responses that are induced by ground deformations can be characterized and quantified. This has been approached from an intrinsic sensing perspective, whereby several instrumenting procedures have been developed to directly couple fiber optic strain sensors with various reinforcement elements. Notably, a delta-shaped sensor arrangement and accompanying analysis have been developed in order to calculate the coaxial strain scalar and bending moment vector mobilized along a reinforcement element. Significantly, this allows the maximum strain distribution along a reinforcement element to be measured without prior knowledge of the location(s) or orientation(s) of strain inducing features. Through a series of laboratory experiments and in-situ studies it was demonstrated that load experienced by a reinforcement element can be measured as a function of distance from the load inducing source. This critically improves common pull test assessment of reinforcement because bond stress and load development length can be quantified and related to bond strength models. Furthermore, several in-situ reinforcement mechanisms mobilized by excavation advancement were measured. These included stress redistributions facilitated by umbrella arch elements in the form of distributed bending moments as well as localized dowel reinforcement across shearing discontinuities. The latter was found to concentrate strain within several centimeters of an active discontinuity, necessitating the high spatial resolution measurements. In view of use of this technology by practitioners, it has been established that the sensor arrangement can be installed in conformance with standard reinforcement handling and mechanized installation procedures. The end benefit is the developed sensing technique can be used by ground control engineers to more confidently justify alterations and optimizations to support design.