Big rocks are thrown from a bridge over the Sill river – what is behind all this?
> VIDEO of the 3 drops
The motorway bridge of A13 over the Sill river in direction of Innsbruck, shall be removed and replaced with a new structure. The scenario to be checked is that the deck slab and the pillars are conventionally removed and the remaining hollow arch is blasted.
The vibrations through the detonation itself have already been determined by in an experiment and are not subject of the present study. What has to be analysed is the following collapse of the arch, its impact on the valley bottom as well as on the slopes and the resulting vibrations of the neighbouring bridge objects, the A14 bridge in direction of Brenner as well as the Sonnenburg railway bridge of the ÖBB.
An experimental approach was chosen in which three defined falling weights (boulders of approx. 5t) were thrown from the A13 bridge in direction of Brenner in such a way that they hit the valley bottom and produce a defined vibration with their known kinetic energy. The aim was to perform an impact both on the bare rock and on the gravel bank (orthographically the left, northern bank).
The punctiform vibrations at the two concerned bridges and the pylon were documented by measurements and the local dissemination characteristic along the orthographically right river bank was determined in the course of the experiment.
On the basis of these measurement data, dissemination laws, energy relations and transfer functions the measured vibration signals of the punctiform impact of the falling weight were assembled to a linear excitation with the expected impacts according to the blasting concept (concrete parts of max. 8m³, approx. 9.2t) with their energy (total mass of 1470t, falling height of max. 65m) and the expected timing (in accordance with the arch and valley geometry) and their effect on the bridges to be inspected was determined. In order to be able to assess the spectrum, several impact conditions (rock, gravel, inclination) as well as the assumptions were varied.
Service. Concept, test planning and execution, forecasting, survey