A new wildlife crossing in Switzerland using an innovative post-tensioned timber design has been completed over the motorway between Zurich and Bern to reconnect habitats and improve road safety.

Commissioned by the Federal Roads Office (Astra) and delivered with input from VSL International, a subsidiary of Bouygues Construction, the bridge spans the motorway and the Koppigen cantonal highway. The structure is covered with about one metre of soil and vegetation to form a continuous ecological corridor, allowing animals to cross without encountering traffic.

The project addresses a long-standing barrier between two natural areas that had previously restricted animal movement and contributed to vehicle–wildlife collisions. By restoring ecological connectivity, authorities expect reductions in habitat fragmentation and in incidents that pose risks to both biodiversity and motorists.

Technically, the bridge is notable for its use of post-tensioning in a timber superstructure, a method more commonly associated with concrete. The span comprises two timber elements, including a 37m main span, which support a concrete slab, backfill and the vegetated covering. The considerable loads from the backfill required the application of post-tensioning to control deformation and ensure long-term performance.

VSL International adapted post-tensioning systems originally developed for concrete for use with timber. The solution uses a large steel plate with numerous small anchorages to distribute high tension forces across the timber without causing damage, according to the company. The work incorporated several years of local research and testing in collaboration with the Higher Technical School of Wood in Biel and ETH Zurich, which examined the structural behaviour and maintenance implications of post-tensioned wood.

Experts say the project is among the first worldwide to apply post-tensioning to timber at this scale for infrastructure, demonstrating that bio-based materials can meet the demands of heavily loaded structures. Proponents highlight the lower embodied carbon of timber compared with traditional construction materials as a key advantage in efforts to reduce the environmental footprint of infrastructure.

Bouygues Construction, which has participated in high-profile projects such as the Hong Kong–Zhuhai–Macau Bridge and the Simone Veil Bridge in Bordeaux, described the Koppigen crossing as a step towards low-carbon construction solutions that combine innovation with structural performance.

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