Using ground-penetrating radar to non-destructively check the stability of a natural-stone bridge
These sketches were created during the last renovation in 1967. It is not clear how accurate they are. GEOTEST AG is therefore using the ground-penetrating radar (GPR) method to analyse the size of the natural-stone blocks in the arch without having to drill into them.
The Kräiligen bridge is not conspicuous from above.
The cantonal road 12, which connects the city of Bern with Solothurn, crosses an inconspicuous bridge in Kräiligen in the municipality of Bätterkinden. Beneath it flows the Limpach stream, which has its source north of Grossaffoltern and flows into the Emme river. From above, the bridge appears unremarkable. From the Limpach, however, one can see its natural-stone substructure, dating from 1858.
The original bridge, made of natural stone, can be seen from the side.
This bridge is also a crossing point for exceptional transport of oversized loads, which exert considerable force on the bridge structure. It is therefore a limiting factor in the approval of such exceptional transport. The Canton of Bern now wants to determine the maximum force the bridge can safely sustain. This raises the question of the current load-bearing capacity of the bridge.
Photos of the last renovation and widening of the structure almost sixty years ago show irregularly sized blocks of natural stone inside the bridge. The existing plans do not specify the size of these blocks. A comparison with the photos taken during the renovation raises doubts as to whether these plans are accurate or merely rough sketches of the natural-stone arch.
The plans for the section with the natural-stone arch, which were probably only rough sketches, are now being reviewed.
This photo from 1967 shows natural stone blocks of various sizes inside the bridge.
While the stone blocks visible in the photo vary in size, they are depicted as uniform in the sketched plan. However, the size of the blocks is crucial when checking the load-bearing capacity of the bridge.
In order to determine the actual size of these stone blocks without causing any damage, GEOTEST employees scan the blocks in the bridge arch using a GPR antenna mounted on a telescopic pole. This method works on the same principle as an echo sounder, but uses electromagnetic waves instead of sound waves. This produces a continuous recording, i.e. a depth section along the measurement profile.
GEOTEST employees record a vertical measurement profile beneath the bridge.
The data obtained with the ground-penetrating radar already gives an initial impression of the size of the stone blocks at the site.
GEOTEST has already carried out similar studies for SBB (Swiss Federal Railways) in old tunnels and on natural-stone retaining walls.
Non-destructive testing on natural-stone objects is not something that happens very often. It requires a great deal of confidence in your own abilities. GEOTEST has gained a wealth of experience in this field over the past decades.
- Sebastian Kleinert, project manager
The GPR method can be used to measure more than just block size and wall thickness. It can also immediately identify and detect pipes, reinforcing bars, and all kinds of other components beneath the surface.
Within a few hours on site, GEOTEST employees gathered enough measurement profiles, taken from below and above, to analyse the bridge. The data is currently being evaluated by GEOTEST. Based on these measurements, the Canton of Bern will be able to have the load-bearing capacity of the bridge checked in the future and plan reinforcement measures if necessary.
