Abstract
Rock stress is causing unwanted deformations of deep underground spaces. Large deformations increase the risk of failure. The underground excavation causes rock stress changes in the surrounding rock mass, and the resulting deformations can be measured. In this paper, we present a method to monitor stress changes in the rock mass using rebar rock bolts instrumented with strain gauges to track the stress within the bolt. Next to that, we describe in-situ testing of this method using heating-induced stress in a natural underground environment. The heating experiment aims to create stress changes using rope heaters inserted into the rock mass and located symmetrically around a single instrumented rock bolt. The heat flux induced to the rock mass leads to volume expansion. The restricted thermal expansion causes an increase in the internal rock stress conditions. These conditions create a strain that can be measured and back-calculated as the rock stress change. The instrumented rock bolt and testing setup were installed in the Underground Research Laboratory located in a granitic rock below the Aalto University campus. The single bolt experiment demonstrates how instrumented rock bolts could monitor the changes in the rock mass stress state. The system can be used as a part of a real-time rock stress monitoring system in mining and rock engineering projects. The final part of the paper describes how this monitoring system can positively affect geotechnical risk management and increase the overall safety of underground construction.
Highlights
Some of the collapses of underground spaces are caused by a sudden increase of stresses in the surrounding rock mass
We present a method to monitor stress changes in the rock mass using rebar rock bolts instrumented with strain gauges to track the stress within the bolt
The improvement to ensure rock stress monitoring method over time came from Norwegian laboratory SINTEF which has developed so-called long-term door stoppers [2][3]
Summary
Some of the collapses of underground spaces are caused by a sudden increase of stresses in the surrounding rock mass. One of the main rock stress measurement methods is the overcoring method, well-known for its precise results and direct application at the location of interest. The improvement to ensure rock stress monitoring method over time came from Norwegian laboratory SINTEF which has developed so-called long-term door stoppers [2][3]. This method uses a long-term door stopper cell equipped with a strain gauge rosette attached to the bottom of a borehole in the rock mass. The real rock stress is only measured at the location close to the door stoppers at the point of interest
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