Augmenting the in-situ rock bolt pull test with distributed optical fiber strain sensing

Abstract

A high spatial resolution distributed optical fiber strain sensing technology is demonstrated to advance the assessment of support element behaviour during in-situ pull tests. A technique to instrument typical tendon support elements with a fiber optic sensor is discussed and was trialed at an underground salt mine through a series of pull tests on instrumented rebar elements. It is shown that a continuous strain profile can be measured along the length of a support element under pull test load, which, in turn, allows the interfacial shear stress distribution and deformation of the support element to be resolved. In comparison to load and displacement measurements solely at the support element head, which is what is traditionally reported for in-situ pull tests, the optical fiber strain sensor is found to provide significantly more insight into the mechanistic behaviour of the support element by readily measuring highly variable strain distributions and by quantifying the load development length of the support element. The sensor arrangement allowed the conventional handling and installation procedures to be followed without any implication to the integrity of the sensor. Accordingly, an instrumented reinforcement element can be installed with minimal interruption to on-going mining/construction procedures at the given project.