‘Walking on water’ by Dietmar Feichtinger, July 2014
Creating a new access to one of the most remarkable cultural monuments and landscapes in Europe is a rare challenge for the design of a causeway. The estuary of the Couesnon River surprises by the immensity of the bay surrounding the Mont Saint-Michel, by the beauty of light and the colours of the natural elements. The changing sea level by tides rising up to 14 m, creates a repeated event exposing the forces of nature and offers a variety of landscapes alternating between land reaching far out into the sea on low tide and the water filled bay with the Mont transformed into an island as only landmark on high tide. This radical change contributes to the magic of the site.The minimal slope of the estuary and one of the world’s highest differences between tides accentuate the phenomenon: the sea is approaching the coast like galloping horses according to Victor Hugo.
Built on a rock in the bay, the medieval town of Mont Saint Michel topped by its abbey and monastery represents a major cultural landmark on one of the most visited sites in France. The Mont-Saint-Michel and its bay are part of the Unesco list of World Heritage Sites since 1979. In 1897 a dam exceeding the highest level of high tide was built to connect the land to the Mont Saint Michel Island to assure permanent safe access for visitors. By obstructing the Couesnon River partially sand was accumulating approaching the continent to the island. In order to preserve the island a new causeway and a 756 meters long jetty replace the existing road allowing water to circulate and to restore the insularity of the Mont-Saint-Michel.
The new causeway and jetty, a delicate answer
In this sensitive context human intervention is delicate. The new causeway and jetty with a total length of two kilometres search for continuity and perfect integration into the site. The jetty’s design blends into the landscape by achieving a maximum of transparency. Perfectly horizontal, the deck merges into the horizon contrasting the verticality of the abbey. Its slenderness is achieved by a sequence of 134 pillars with only 24,4 cm of diameter and a distance of 12metres between each pair. Obstruction to water is minimized. The pillars are fixed on the bottom into the foundation –concrete pillars with a diameter of 120cm founded on rock about 30 meters under sea level – and on top to the deck avoiding any diagonal structural element to achieve its pure appearance.
From the mainland to the Mont, the general geometry forms a continuous curve. Slightly offset to the East before turning back, it offers a variety of views. The jetty becomes part of the bay following the smooth lines designed by water.The new causeway and jetty assure a safe walkway for visitors as well as a central roadway for shuttle services.Bi-directional shuttle busses assure the connection of the new parking on the continent to the Island using the central part with a width of 6m50. It grows constantly up to a width of 8m50 at the terminal which is situated in a distance of 300 meters to the entrance in the defense wall of the Mont.
Walkways for pedestrians run along the road on either side with varying widths according to the expected crowd of pedestrians: a main connection 4m50 wide on the west side – 5m50 on the terminal, a secondary walkway 1m50 wide on the east – 2m50 on the terminal. The central road carries the walkways on cantilevers, large balconies offering fantastic
perspectives. The columns are situated under the central part. The shadow of the balcony enforces their discrete appearance. During high tide sea level is just under the level of the deck giving the impression of walking on water.
The last two hundred meters of the jetty are descending by 1% leading to a concrete platform facing the Mont. The platform is submerged several days a year on high tide and provides total insularity of the Mont Saint-Michel.
A transversal steel structure carries the concrete deck in the center. On each side of the roadway, cantilevers – T-formed steel consoles – support a wooden deck. Few materials are applied harmonizing with the colours of the bay. Whenever possible they are applied untreated. The modification of the aspect with time and weather conditions corresponds to the roughness of the medieval constructions. The main structure in steel is coated with a metallic light grey finishing on the anti-corrosion complex. All connections of the main steel structure are welded allowing a clear legibility of forces.
The central part supports a concrete slab accessible for vehicles up to 38 tons, with asphalt coating hydro treated and polished. The deck of the walkways on the sides is in oak wood without treatment. The wood cladding is factory pre mounted forming panels with stainless steel fixation underneath. The curved handrail is also in oak wood composed by three elements and mounted on steel posts. Horizontal pre stressed stainless steel cables provide the filling of the railing. Separation elements are installed between the road and the main walkway on the west made of untreated fibre concrete mounted on steel posts. Like a continuous bench they offer seating to rest and contemplate the site.
Light is discrete. It guides the visitors and underlines the walk path. The splendour of the lit up monument contrasts with the spare light on the access paths. LED light bars are fixed underneath the separation element and underline the horizontality with indirect light. Guiding lights are integrated in the wooden panels on the small east part and on the causeway. All supplies – water, electricity, telecommunication – for the village situated on the island are integrated in the access road. They are situated under the concrete slab. Exposed to walkers in the bay the tubes are part of the design and carefully integrated. The formal simplicity allows perfect integration. It hides the complexity of a sophisticated technical approach. The coherence of the architectural and technical approach reflects the ambition to magnify this beautiful site.
Technical description by Michael Zimmermann, July 2014
Constraints of the project
The structural design of the bridge was not only determined by the loads but also by important constraints regarding the site, maritime conditions and the ambitious operation restoring the Mont Saint Michel’s maritime character. Thin steel columns have been used to ensure a minimal obstruction rate of the water flow from falling and rising tide. In addition, these columns have to work considering two different situations concerning the sand level in the bay: the current high sand level embedding the columns and causing an important lateral bedding stiffness for the integral frame structure and the future 6m lower sand level, where the thin columns have to provide buckling resistance according to their important column lengths.
The bridge has a total length of 756m and is divided into 7 units (2x78m at the end and the beginning and 5 x 120m in the middle part). All sections are separated from each other in
longitudinal direction by expansion joints. The width of the deck changes from 4,00m to 8,50m.On each side of the primary deck structure cantilevers form a pedestrian balcony with a width reaching from 1,50m to 2,50m in the East and from 4,50m to 5,50m in the West. A pair of columns supports the deck structure every 12m in longitudinal direction, following the curvature of the bridge. Double columns are arranged at the transition points of the bridge units to reduce the length of the cantilever deck to a minimum.
The bridge deck consists of a steel-concrete composite structure with two longitudinal welded steel box girders with a height of 450mm (east) and 550mm (west) and a constant width of 750mm. All steel elements are in S355 grade. Together with the transversal T beams, which are welded every 3m between the longitudinal girders, a steel grid is formed and equipped with connection bolts on the upper flange. The side balcony structure is supported by T beams with a varying section welded in the axes of the transversal girders.The vertical structure is performed by slender steel tubes with a diameter of 244mm and various thicknesses between 40 to 60mm. The columns are rigidly connected on both ends:at the bottom into the concrete foundation piles and on top by welding onto the longitudinal box girders.
In a next step the steel grid structure is covered by prefabricated 25cm thick concrete slabs with anchorage reinforcement bars sticking out at the borderline. To get the two structures work as one composite element, the joints between the concrete slabs are cast in-situ. To limit the shrinkage effect, slabs have been prefabricated in advance. Despite the repetitive aspect of the structure, the variation of the width of the deck imposed specific dimensions for each slab. A numerating system has been applied and adaptable formwork tables have been built.
To keep the same speed of form working for all the parts of the bridge (variable or constant width), the prefabricating factory ordered a second form working table. Once poured, the slabs were stored in the factory before being transported and directly set up on the bridge. For the pedestrian walkway on both sides of the bridge, T profiles are fixed on the cantilever beams to provide the support for wooden blank elements on top.
The structure corresponds to a semi-integral system, which means that all elements in each unit are rigidly connected. The ends of the deck are not fixed and thus allow longitudinal movements due to temperature elongation. This structural behavior fits perfectly to the slender rigid connected steel columns. It allows the whole unit (especially the outer columns) to deflect against the rather small lateral column stiffness. The big advantage of built-in connections in all the foundations, abutments and top of the steel columns is to limit the maintenance operations since there is no need of bearing points.
Realization of piling and column fixation from existing sand level: Every column is anchored in a drilled in-situ pile with a diameter of 120cm, transferring the loads and bending moments to good bearing soil. Therefore 134 vertical elements had to be realized.The connection of the steel column in the concrete pile is situated 7m below the current sand level. It was necessary to find a method for realizing the fixation working from the ground level to avoid any big excavation work in the bay (with an important impact on the planning). The height of the bridge is presently about 2m above the ground level. The number of drilling procedures and column connections also required a safe and fast process, ensuring both security for the workers and quality for the result. A centering device of the steel element, fixed on the tube above the pile, has been conceived to set up the column in the right position in all three dimensions. The workers could operate from the current sand level, without having to go 7m deep, where the piles start. The length of the columns to put into the concrete of the pile is 1,50m.
To validate the design, a test pile has been realized with measurement of the horizontal stresses in the concrete. A force was applied at the top of the test column by a hydraulic jack and strain gauges were placed in the concrete to measure the strain-deformations undergone in the zone of the built-in connection between the test column and the test pile.The results validated the calculations, so the columns-piles of the jetty could start to be set
up. Welded connections and the ease of accessibility of the main structure assure efficiency in maintenance.
Architect: Dietmar Feichtinger Architectes
Engineer: Schlaich Bergermann und Partner