At more than one kilometre long, the Riddes viaduct is the longest motorway interchange in Switzerland. It crosses the Simplon line of the SBB, the N9 motorway and the Rhône river, and is subject to heavy traffic on a daily basis. GEOTEST Ltd has carried out a detailed investigation of the condition of the structure.
In the course of a regular routine inspection by the canton of Valais, various damages and defects were visually detected on the viaduct near Riddes. The 1250-m-long prestressed concrete bridge is an important traffic junction and is crossed daily by around 10,000 vehicles.
As a consequence, GEOTEST Lausanne was commissioned to carry out a comprehensive assessment of the condition of the structure. Using non-destructive or semi-destructive investigation methods, the structural concrete, the existing tension cable and the grouting mortar were examined between June 2019 and January 2020.
The primary focus of the condition investigations was on the tension cables and the degree of filling of the injected ducts. The degree of corrosion of the prestressed cables was determined for a total of 150 tension-cable tests and samples in order to better estimate the extent of the damage. Furthermore, samples were taken from the viaduct in order to analyse, experimentally or microscopically, the concrete quality with respect to the penetration of chlorides and the degree of the alkali aggregate reaction (AAR). Finally, a mineralogical analysis of the grouting mortar was carried out to evaluate its preservation status and its influence on possible corrosion of the prestressed cables.