There is an increasing need to be able to control the permanent connections in our tunnels. Changes in load patterns and movements of rock masses mean there is a need for monitoring.

The tunnels will now be subject to routine checks and the condition of the connections which have been installed must be documented. As part of the process of being able to document the amount of load being absorbed by connecting bolts, Ørsta Stål has developed a simple little indicator which will tell when the securing bolt has reached its working load.

The load indicator is fitted on the bolt between hemisphere and nut before the bolt is installed into the drill hole and subjected to load.

The strain on the bolt will go through the load indicator, which will be locally deformed at a given threshold value of 8 tonnes, when the opening will close.

The load indicator is made of stainless tube which has been radially weakened by making a groove in the surface.The product is best suited to end-anchored rock bolts and is now offered for bolts of M20mm diameter.

If there is a need to cover the exposed end of the bolt to more easily enable later inspection, a plastic cap can be fitted over the bolt end. This can be desirable where spray concrete is to be applied to secure the rockface, in addition to the bolts.