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Paris underground

The tradition of using limestone for Paris’s buildings has left the city standing on a network of quarry voids. Alexandra Wynne treads carefully on a site where work is under way to stabilise the ground.

The famously grand buildings of Paris have taken their toll on the ground below over the centuries. Quarrying of gypsum – the main ingredient in plaster – and limestone has been going on since the Middle Ages and over 300km of subterranean galleries serve as a reminder of the city’s building heritage.

Some of the 835ha of quarries have been kept and preserved as utility tunnels. However, the network of underground voids – predominantly quarried for limestone along the Left Bank and south part of the city and gypsum to the north – has become a cause for concern for developers.

Soletanche Bachy won a !2.3M contract to stabilising one such site for its client, developer Sadev 94. Semofi is the developer’s grouting consultant engineer.

The 15,000m2 site in Vitry-sur-Seine in south east Paris is being subjected to grouting work in preparation for its transformation into a residential and commercial development. Information supplied by the l’inspection générale des carrières, – an office set up in the late 1700s to monitor the French capital’s underground quarries – suggested the site was underlain by a system of limestone quarries, the tops of which lay 22m below ground level.

Five rigs are working on site to drill the holes needed before the grouting work can be done – three Soletanche-produced Domine rigs and two Commachios. A 150mm diameter rotary drill with water flush is being used to do these at between 2.5m and 7m spacings across the site.

The inspection office has responsibility for deciding how close together the injection holes needed to be. The site will eventually be a mixed-use development, so the holes need to be closer together where there will be buildings above yet can be more widely spaced apart where there are only gardens.

The holes are also closer together around the perimeter of the site to contain the grout beneath and stop it seeping beyond the site boundary. Soletanche Bachy deputy communications manager Stéphane Monleau says because the perimeter injection holes are closer together “they create a sort of dam” that helps to keep the grout from stretching outside the site and causing rig operators to continue grouting indefinitely.

This dam is formed before the interior grouting is done and uses a mix with more gravel. This means it is a less fluid grout than the mix used across the remainder of the site.

Site manager Charles-Eric Macé says the company has experience in this kind of work – it came up with a similar solution on a large gypsum cavern under Paris’s Gare du Nord in 1975.

The team considered making this perimeter dam with building blocks, which can be done where a quarry is safe enough for contractors to work below ground. However, Macé says the 22m deep shaft on this site was too unsafe so the grout curtain proved to be the better option.
Following completion of this grout curtain site workers inject grout comprising 250kg/m3 cement under gravity to bulk infill the voids at between 21m and 27m into the ground.

The strata is typically up to 1.2m of made ground overlying about 2m of sands, 2.6m of alluvial deposits, 13m of a limestone and marls mix and then limestone. This technique accounts for the majority of the grouting work on site and 20,000m3 of gravity mortar is being used for this part of the project.

A liquid sand and cement mortar – made with 1t of sand and 150kg/m3 of cement – is pumped under pressure to ensure grout flow into smaller voids.

The five rigs are drilling up to 1200 linear metres per day. Every day the site receives deliveries of 500t of sand for the grout mix.

Macé says the speed of drilling, the pressure on the bit, rotating pressure on the tool and drilling fluid pressure are all monitored during the work. When the speeds increase or pressure decreases the team can tell the tool is aligned with a void: “The method gives us information and is a site investigation in itself,” he says.

In places, the soil in the boreholes has a tendency to collapse while work is carried out. To combat the problem, rig operators include 30kg to 40kg of bentonite in the water flush, which supports the hole.

After reaching the required depth, site workers retract the auger and insert a 60mm diameter PVC tube with 50mm holes along the lower part of the tube for the grout to be pumped through to fill the voids.

Where there are two levels of quarries, two separate tubes are used in the same borehole to ensure each level gets the right treatment.
In weaker soil areas a tube à manchette is used. This involves using PVC tubes fitted with rubber sleeves surrounding the grout holes that act as a one-way valve – allowing grout to flow into the soil without letting it back into the tube once pumping stops. This method accounts for 1500m3 of grouting on the project.

When the four-and-a-half-month contract is complete – scheduled to happen by the beginning of next year – over 1100 holes will have been drilled. At that point the developer is likely to sell the property (with its outline planning permission completed) ready for a new developer to build on.SOLETANCHE STAYS BUSY IN PARIS

The company is busy working down the road from the Vitry site on the north bank of the River Seine on a separate !12M contract for a 53m deep concrete shaft as part of an upgrade to the city’s stormwater system.

It involves building a diaphragm wall box that will redirect flow from two existing tunnels down into a new deeper tunnel (being bored under the River Seine for a separate contract).

The work has been made challenging by the need to keep the tunnels open as much as possible throughout the excavation and building work.

A Soletanche Bachy-built cutting head (called a Hydrofraise) is mounted on a Soilmec SM129 for the work. It has two rotating cutting heads that take three bites to carve out the 1m wide, 7m long and 400m3 panels. Once 12 of these are complete they will form the walls of the 4.9m diameter, roughly circular shaft.

The shaft receives the flow from the two water tunnels and provides an area for the water to be re-channelled down to the new tunnel below.

Work started on the project in January near Charenton-le-Pont in the south east of the city for client Paris wastewater treatment authority SIAAP. It is due to be finished by March 2009 when the new larger tunnel will have a greater capacity to carry more water away from the city and out to a treatment plant.

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