This special tool, referred to as instrumented bolt, is used to measure strain changes in rock masses; the tool consists in a steel bar with improved adhesion instrumented with vibrating wire micro-extensometers. The strain to which the steel bar is subjected is proportional to the axial load exerted on the bar.
These bolts, which act as passive point anchors, are placed inside drill holes and the holes are then filled with concrete.
Thanks to their sturdiness and ease of installation, these instruments are used in particularly harsh situations, where normal strain gauges would be damaged.
Each bolt can be equipped with one or more strain gauges according to the number of differential strain points that need to be identified. Each instrument is equipped with a thermistor to verify and correct strains due to thermal variations. The operating principle of the vibrating wire is the following: the steel wire is tensioned between two ends fixed on the support to be monitored.
Any strain in the support will affect the tension of the steel wire and the resulting resonance frequency and therefore its tension, giving the measurement of strain in the support.
Instruments bolts are used mainly for the measurement of radial strain at various depths of tunnels under construction or in operation and for testing the state of stress of the rock over time. Monitored by automatic data acquisition units with a certain frequency, they can provide interesting information on the evolution of the stress-strain condition of the rock mass in which they are inserted.
- Type of transducer: vibrating wire;
- Nominal measuring range: 3.500 µƐ;
- Resolution: 1 µƐ;
- Accuracy: 0.25% F.S;
- Non linearity: <0.5% F.S;
- Temperature range: -20°/+80°C;
- Integrated temperature sensor: NTC 3KΩ;
- Output signal: Hz;
- Typical frequency: 2800 Hz;
- Thermal expansion coefficient: 12,2 µƐ/°C
- Vibrating wire length: 51 mm
- Body diameter: 20 mm