Problem
Machine foundation KA3 in Kiruna concentrator showed suspected damage in the form of extensive cracking and possible loss of rigidity of the foundation. The problem was discovered in connection with vibrations in the pellet mill. LKAB wanted to find out the extent of the problem as the mill risked being damaged if the vibrations did not stop. Invator then carried out a condition assessment using non-destructive testing and dynamic vibration analysis using mounted accelerometers to determine the status of the foundation. Of particular importance was to investigate whether the cracks affected the dynamic response and whether there was a risk of resonance phenomena or continued damage development during operation.
Results
Measurements showed that the foundation generally maintained its load-bearing capacity, but that cracks were present in parts of the structure. Accelerometers and ultrasonic measurements indicated local stiffness losses, but no signs of severe global instability. Power Spectral Density (PSD) analysis identified the natural frequencies of the foundation and showed that these were outside the dominant excitation frequencies of the machine, reducing the risk of resonance. In conclusion, the foundation could be assessed as still safe to use in the short term. However, the investigation also revealed a dimensioning error where the anchoring struts were too short, which indirectly caused the foundation to crack. It was therefore decided that the foundation would be replaced at the next production stop. Until then, monitoring by measurement continued.
The solution
The survey was conducted in several stages:
- Review of reinforcement drawings
- Visual inspection of cracks and concrete surfaces.
- Installation of accelerometers for vibration measurement during operation.
- Non-destructive testing with ultrasound and GPR to identify internal damage and reinforcement condition.
- Vibration analysis using RMS and PSD methods to determine natural frequencies and dynamic behavior.
- Comparison with previous measurement data and Eurocode-based calculations for verification.
The methodology provided an overall picture of both the static and dynamic state and ensured a reliable assessment.
Repair
It was decided that the old foundation would be removed in connection with the production stop and a new foundation would be cast. Invator carried out dimensioning of the foundation and produced documentation for implementation.
Equipment
- Accelerometers (for vibration measurement)
- Ultrasound equipment
- Georadar (GPR)
- LVDT (strain gauge) for crack opening measurement
and acquisition system for data collection and storage in the cloud - Software for vibration and signal analysis
Standards
- EN 1990 – Eurocode: Basic design rules
- EN 1991-1-1 – Loads on structures
- SS-EN 1992-1-1 – Concrete structures
- ISO 10816 / ISO 20816 – Vibration assessment of machinery and foundations
- ISO 16311-1/2 – Condition assessment of concrete structures
