Light design for Grand Paris Metro station 07 Mar 2019

To ease congestion and provide a more comprehensive underground rail network between peripheral stations in the city of Paris in France, Société du Grand Paris (SGP) is adding more than 200km of new lines to the existing 400km network.

Glass-panelled station structure

Design of the new Villejuif Louis Aragon (VLA) station on Line 15 of the Grand Paris Express project, by Setec TPI with architect Philippe Gazeau, is based on ensuring an abundance of natural daylight in the underground environment and a light and spacious feel for the travelling public. Under constructed by the Vinci-Spie-Botte joint venture, Setec is also responsible for design of the underground section of Line 15 between the Villejuif Louis Aragon (VLA) and Pont de Sèvres Stations.

To increase internal visibility and allow natural light to reach the lower levels of the VLA station, which provides an interchange hub between trains, trams and buses, two skylights were designed in a single large shaft topped by a glass-panelled entrance/exit, providing the station with a 360° view.

Light permeates lower levels

The rail platforms are located 30m deep at Level -4 and the entire station is structured around the vertical connexion between the street and mezzanine Level -3, and the linking corridor route with the existing metro Line 7 on Level -2.

The overall design for construction is traditional, comprising a top-down diaphragm wall enclosure, but with some interesting features.

A number of utility networks will supply the neighbourhood with electricity, gas, drinking water or telecommunications and as these all exist together in a tight surface area the designer chose to enclose them in a compartmentalised metal framework structure across the station.

The central shaft is 10m x 60m, with a 23m wide ground slab divided in two and supported by inclined steel beams. During construction, the slab will be temporarily supported by 75cm diameter metal pre-cast in boreholes before the cover slab is poured. Once the excavation under the slab is complete, the permanent inclined beams will be placed in position.

Fig 1. Temporary metal pile lowered into the borehole

As the station is at the end of two sections of the southern metro Line 15, two TBMs will end their drives here, one from the west and the other from the east. They will be disassembled and removed by a high-capacity mobile crane (1,000 tonne approximately) via an 11m x 13 m shaft.

Challenges were faced and overcome by Setec during the building of the station foundations, where 35 people worked in two shifts to complete the work.

The first related to the drillability of the Brie limestone for the construction of the diaphragm walls. Limestone areas with silicified 1m thick blocks, often with an unconfined compressive strength (UCS) above 100 MPa and locally over 200 MPa combined with very high or extremely high abrasive and hardness values (AIN > 4.0 and DIN > 120) were encountered.

The contractor decided to remove this layer to allow for works on the diaphragm walls and three campaigns with a total of 600 boreholes were drilled 8m deep in order to remove the hard limestone layer. These boreholes were backfilled and the diaphragm wall boring machines could then easily cross this area.

Lowering diaphragm wall reinforcing cage

The second challenge related to the dissolution of the gypsum present in the Argenteuil marl and gypsum marl and blocks by the bentonite slurry used for the construction of diaphragm walls. This phenomenon led to a deterioration in the properties of the slurry due to a chemical reaction between the gypsum’s calcium ions and the clay-based minerals of the bentonite, causing instability of the borehole walls during the installation of the first panel and collapse of certain layers of soil onto the first panels. The composition of the slurry had to be significantly adjusted to overcome this problem.

The diaphragm wall works was an important part of the station construction, and resulted in a watertight enclosure of 41 panels. With widths between 3m and 7.2m and a thickness of 1.2m, a total perimeter of approximately 250m was created with an average depth of 42m and a maximum of 52m for three panels. This depth is required to stabilise this structure, below which the tunnel of the existing metro line 7 passes.

Over 13,000m³, was excavated, requiring regular loading of dump trucks to transport and dispose this soil onto the specified landfill or stockpile. Reinforcement steel cages were inserted into each panel before concreting, with a combined weight of 1,350,000 tonnes.