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Diaphragm walls: Paris’ grand vision

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By 2030 Paris will have added 200km to its metro network through the Grand Paris Express project. GE visited one of the largest shafts on the scheme.

Crossrail once claimed to be Europe’s largest infrastructure project, but it has now lost that title to the €30bn (£26bn) Grand Paris Express (GPE) scheme. With plans to add 200km of network across four new lines by 2030 to better serve the regions, the Paris project makes Crossrail look small.

grans paris map for web

grans paris map for web

Construction of GPE is being phased and the first new routes to open will be the circular Line 15 and extension of Line 14, which will provide better connections to Orly Airport. The intersection of these lines at Villejuif Institut Gustave Roussy station to the south of the city is firmly on the critical path, but shaft construction is on track to be completed in May.

Project promoter Société du Grand Paris has let Lines 14 and 15 under several different contracts, fortunately those that intersect at Villejuif Institut Gustave Roussy are being undertaken by the same joint venture partners.

The CAP joint venture delivering the €926M (£782M) TC3 contract Fort d’Issy-Vanves-Clamart and Villejuif Louis Aragon on Line 15 and the €400M (£355M) GC02 contract between Orly Airport and Olympiades on Line 14 is formed by Vinci Construction Grands Projets, Spie Batignolles Fondation, Botte Fondations and Dodin Campenon Bernard.

“The station at Villejuif Institut Gustave-Roussy is the biggest station and also has the largest shaft at 60m in diameter and 51m in depth,” says Spie Batignolles Fondation diaphragm wall manager Stephane Argenson.

“All the other stations are being constructed using boxes but the intersection at the Villejuif Institut Gustave-Roussy station made the shaft design a better option.”

The shaft is formed by 29, 600mm thick panels. The thin panels were introduced to save costs, but have created a number of interesting challenges for Spie.

Before work on the shaft could get underway a retaining wall formed by 35 diaphragm wall panels had to be built to depths of up to 17m around the shaft site to allow 114,000m3 of landfill material to be excavated to depths of up to 11m. The site was once a sand quarry but had been backfilled with refuse.

The retaining wall has two levels of anchors that vary in length from 14m to 34m with capacities varying from 150kN to 950kN.

The retaining wall was started in July last year and Spie moved off site while excavation took place. It returned in December to start work on the shaft.

villejuif gustave roussy 2 1

villejuif gustave roussy 2 1

Part of Line 15, which includes the section through Villejuif Institut Gustave-Roussy, and extension of Line 14 to Orly are set to open in 2024

So far 13 out of the 29 panels have been constructed and the team is completing two panels per week but are hoping to achieve three soon.

“The ground conditions are complicated so cutters and grabs are being used and tools need to be changed several times in order to complete each bite, which slows down the productivity,” says Argenson.

The made ground at the site is underlain by a deposit that is specific to the Paris region – Calcaire de Brie. While the mention of brie might make you think it is soft as the cheese of the same name, the truth is far from it.

At the Villejuif Institut Gustave-Roussy, Argenson says that the Calcaire de Brie is more blocky which is easier than in some areas where the material forms a solid 2m to 3m band.

Testing has found parts of the Calcaire de Brie to have strengths of up to 200MPa.

Where the Calcaire de Brie is solid, Spie has used a continuous flight augur (CFA) rig to core through it.

At the Villejuif Institut Gustave-Roussy site this layer has resulted in the use of a heavy duty 16t grab for the diaphragm wall. Once through this layer there are clays – green then brown – and Spie is using grabs through these sediments too.

But the diaphragm wall extends into white marl at 36m depth, which Argenson says is more like mudstone, and gypsum limestone at depth, so the cutter is brought back to form the last 14m of the bite through this.

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dscf0190

The diaphragm wall work is expected to be completed in May

While the gypsum limestone requires the use of the cutter, the site has been subject to mining activity in the past and Spie has had to undertake grouting work around a number of its shaft sites to minimise overbreak and bentonite loss as a result of these voids.

The contract calls for a verticality of 1:200 and care is needed at the start of each bite as the strength and blocky nature of the Calcaire de Brie can cause a twist in verticality.

Argenson says that the skill of the operator is critical in achieving the verticality in the ground conditions here.

Originally the shaft was designed with 800mm thick panels but this was changed to 600mm thick panels with an internal bracing beam to reduce costs. The detailed design for the shaft was undertaken by Spie, based on an initial design by the client.

According to Argenson, the 600mm thickness means that the verticality is even more challenging.

The change to thinner panels also meant that standard stop ends could not be used on the scheme. It also limited the choice of grabs for the work.

The stop ends have been supplied by Botte Fondation and go the full depths with a full depth water bar. Argenson says that the thinner panel meant the team was unable to use Spie’s own hydraulically releasing stop ends on the scheme.

In France it is preferred to use a water bar to create the connection rather than overbite which is more common in the UK.

The stop end is being used to help maintain the verticality and clean the panel before concreting. Once the cutter has excavated the bite to full depths, the grab is being used with – the stop end as a guide to maintain verticality – to clean the excavation.

“It is time consuming to swap tools but it would be too time consuming to excavate through the limestone with the grab,” explains Argenson.

There is storage for 1,000m3 of bentonite on site and each panel has a volume of 250m3, which is where Argenson says the benefit of the 600mm panel comes in. 

“Although it is not faster to excavate, the amount of concrete required is less and it takes less time to cleans the bentonite and concrete,” he explains.

The ground conditions have also led to the use of a bespoke bentonite mix, which uses polymers and soda ash to avoid a reaction with the gypsum limestone.

Argenson expects the shaft to be completed in May, clearing the way for excavation to start.

The shaft will be excavated to 47m and Line 14 will pass through at 32m depth, while Line 15 will be at 42m.

The TBMs driving Line 15 will be supported on a deck at the base of the shaft, while a temporary frame bridge will be built to aid the transit of the Line 14 TBMs.

The open shaft will allow the breakthrough to be done by traditional methods and avoid the use of glass fibre reinforcement for soft eyes, which would have been challenging given the depth of the shaft and the breakthrough at two levels.

Part of Line 15, which includes the section through Villejuif Institut Gustave-Roussy, and extension of Line 14 to Orly are set to open in 2024, but the full circular route of Line 15 is not set to open until 2030.

Shaft solitaire

While Villejuif Institut Gustave-Roussy may be the largest shaft on the scheme, just a short distance away is the smallest at just 11.5m diameter. It is probably being constructed on the smallest site too at just 42m by 27m and bounded on all sides by houses, roads and a cemetery.

pic for box

pic for box

The 45m shaft – known as P01 – will be used for services and as an emergency access shaft with a horizontal connection to the new metro tunnel at depth.

The diameter of the shaft is so small that it will be formed with just five panels and use the triangular void formers that Spie developed to deliver its work on a shaft at Beckton in London last year.

The ground conditions at P01 are the same as Villejuif Institut Gustave-Roussy so Spie’s team is again alternating the use of grabs and cutter, which is a significant challenge on such a small site. There is only room for one machine so each panel will have to be excavated and concreted before moving onto the next.

Strict planning will also guide the delivery of reinforcement cages and concrete deliveries.

The size of the site means that the team only expects to complete one panel per week and the shaft is scheduled to be completed in early May.

 

 

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