Distributed fibre optic sensing during different anchor pullout tests

Although ground anchors are widely used in fields of civil engineering, as in soil and rock stabilization or anchoring structures, such as excavation pits, retaining walls or tunnel constructions, the load transfer behaviour has not been entirely investigated yet. Ground anchors are usually monitored by load cells at the anchor head or by strain gauges at selected points along the anchor, which both do not deliver reliable information along the entire anchor. Distributed strain sensing provides an opportunity to get the strain information along the entire length of the anchor, on the tendons as well as in the grout, as we have already shown in a preliminary anchor test. However, there were still lots of issues, which did not allow a general conclusion for all anchor types and ground conditions. Thus we investigated further ground anchors in different soil conditions (clay, rocklike material and gravel), within a research project. This paper gives an overview of the sensor systems used, the obtained results of the anchor pullout tests, gathered experiences and finally gives a brief concept of a monitoring anchor for long term monitoring.