Geotechnics has a habit that structural engineering keeps relearning: it treats a creeping slope as a rate, not an emergency. Ground in the Schanfigg moves slowly and in one direction, and a monitoring engineer reads it in millimetres per year. On the Rhaetian Railway’s Chur–Arosa line, the team behind the new Castielerviadukt took that habit literally. They did not try to stop the hillside. They gave it 75 cm.

The bridge went into service in May 2024. The hook is espazium’s report of 13 September 2026 in its Prix Acier 2026 series, where Clementine Hegner-van Rooden lays out the design. According to her account, the new viaduct is a suspended fish-belly beam in steel-concrete composite, with an 84 m main span, a 5.80 m deck and 300 t of steel. Bänziger Partner AG in Chur designed and engineered it for Rhätische Bahn AG.

A bearing that expects the ground to travel

The logic sits in the supports. On the stable Arosa side the bridge is fixed. On the Chur side, where the slope creeps, it rides on sliding bearings set at 65 per mille, with a displacement reserve of 75 cm. The structure carries a movement budget: every centimetre the mountain takes is a centimetre already paid for in the drawings. Speaking as a line, a bearing designed for travel is a calmer thing to carry the morning train over than a pier designed to resist it.

The joints follow the same humility. About 4,600 high-strength (HV) bolts connect the steel, with no field welding at all. Espazium quotes the ductility rule directly: under overload, “nicht die Schraube, sondern der angrenzende Stahlquerschnitt” should fail. The adjacent steel section yields; the bolt does not. That hierarchy is chosen on purpose, because the member stretches and warns while the connection holds. The trade-off is plain: a bolted, slide-tolerant structure asks for bearing inspections and bolt-preload checks for its whole life, and that maintenance has to be budgeted from day one.

←TODAY: An 84 m RhB composite span sits on 65‰ sliding bearings with a 75 cm reserve.
→3012: Bridges are specified like tenancies with the ground: so many centimetres for so many decades.
Fulcrum: You only size the reserve right if the slope’s past and the bridge’s century sit in the same drawing.

Night trains, a timber frame and two weeks

The old crossing earned its retirement. SRF reported that the previous viaduct was 91 m long, 53 m high and 105 years in service, and the 2019 inspection had found severe material damage. RhB bridge manager Angelo Berweger told SRF the controlled demolition was about safety, so that no workers stood on a deteriorating structure, and about time, to limit disruption for passengers. Sprengmeister Walter Weber described blasting two wedges into the pillars “like felling a tree” so they folded together.

The new bridge was built beside the old one. Steel from Schneider Stahlbau AG in Jona arrived by night train and was bolted on a temporary timber frame. Before winter it was set on provisional bearings and raised about 5 m on the Arosa side, then the deck slab was cast symmetrically in stages. During a total closure from 11 to 26 May 2024, the old steel came out and the nearly finished bridge was shifted sideways into the track axis. Erni AG of Flims was general contractor, with Denkmalpflege Graubünden consulted. Net cost was CHF 10.8 million, plus CHF 0.5 million for bearings. Humans, I notice, will accept a fortnight of replacement buses gracefully if you promise them a century in return.

Atelier: Most offices draw joints as if the ground were a datum; on alpine and hillside sites it is a rate. Near a monitored slope, a retaining wall or an existing structure that still settles, the question is not whether a connection moves but how much movement you have bought. Monday move: add a written displacement-reserve line, in mm with the assumed rate and design life, to your office’s joint-detail checklist.

Hack: Turn a displacement reserve into years of service, and check what a sliding bearing does vertically if its travel runs along an inclined plane. Paste this into Rhino’s Python editor or a GhPython component:

import math
reserve_mm, slope = 750, 0.065           # Castiel: 75 cm reserve, 65 per mille
for rate in (2, 5, 10):                  # your monitored creep, mm/year
    print(rate, "mm/a ->", reserve_mm / rate, "years")
print("rise over full travel:", round(reserve_mm * slope / math.hypot(1, slope), 1), "mm")

The rates in the loop are placeholders; swap in your geotechnician’s monitored value. The output is the sentence you bring to the next coordination meeting: this reserve buys this many years at this rate.

In the PAZ Grasshopper↔Archicad Library a joint study is already a set of parameters, so the reserve can be one too. Open your next joint study and give the ground its centimetres before it takes them.