References

Culture and Exhibition Centre

MONACO, 1996

A turnkey 17 000 m2 excavation.


The project for theCultural and Exhibition Centre (CCE) enclosure, a 17,000 m2 excavation des-cending to a depth of 25 metres, i.e. 19 metres below the level of the Mediterranean sea, is located in a seismic site with uneven geology.

The area available for construction in the Principality of Monaco is becoming scarce and new buildings are being constructed on land reclaimed from the sea. The CCE development falls into this category. It is located by the sea, on an earth platform, bordered by a dyke composed of concrete elements laid on a rock fill spoil bank.
Preliminary earthworks brought the level of this earth platform from an elevation of + 5,50 to + 2,50, the level of the work platform.
The substratum on the land side consists of hard Jurassic limestone, fractured and permeable. On the sea side, the land consists of compact lime marl which is not very permeable. The Larvotto fault divides these two areas in the longitudinal direction of the fault.
Half of the excavation hits this substratum, which then plunges nearly thirty meters on the sea side, where it is covered with very fine quaternary sand and backfill which was used there to reclaim this land from the sea.
The CCE development is almost entirely buried, for architectural and aesthetic reasons, to avoid having a wall that acts as a screen along the sea front. The length of the project is 220 m for a width of 55 to 85 m. The chamber is a self-stabilising and watertight box.
Water drainage is through a drainage slab and a drainage tunnel, as far as the collection pit (-25 NGF) where a permanent pump mechanism is installed.
To carry out the excavations, a self-stabilising ground support was required on three sides. This consisted of a diaphragm wall held in place by anchors and a wall- buttress-slab assembly during the service stage. There are two types of foundations for the structure: shallow foundations where the rock is flush and deep foundations (barrettes, piles and micro-piles).

The initial support consisted of a diaphragm wall, 1.02 m thick, which went down to the level of the lime marl, with a maximum depth of 34 m. This is maintained throughout the works by 220 prestressed anchors 5 to 7 t, anchored in the quaternary sand and the lime marl. A 15 m grouted wall extends the diaphragm wall to ensure the relative watertightness of the box. During the service phase, the self-stability of the ground support is ensured by the use of reinforced concrete buttresses, which are supported on the slab and deep foundations.
The project has changed considerably during the course of the work. The southern part of the site was deepened by 3 m in the central part and diaphragm barrettes with micro-piles were used to transfer the thrust loads.
A second layer of 7.15 t anchors was placed under the ground water table in the central area to ensure a tension of 0.6 TG approximately is maintai-ned in the first layer of anchors (duration in excess of 18 months). The northern area of the site was deepened by 9.00 m.
A second ground support was built inside the first ground support, using diaphragm wall T panels, anchored in the rock. The two ground supports were connected using a 2 m thick slab on which reinforced concrete buttresses were erected. Prior to all of these support works, new watertight bases were grouted in the Jurassic limestone wedged at altitude in relation to the new excavation bases to ensure under-pressure stability. The drainage mechanisms include a drainage blanket made of 4/60 materials and a layer of anti-contamination "bidim"* (see note) and 2.50 x 2.50 surface drains pointing towards the drainage tunnel, as far as the collection pit, the lower point of the development at -25 NGF. The tunnel enables the collection of all the water, the inspection and cleaning of drains, as well as any eventual additional works that may be required over time.

* Translator’s note: "bidim" is a brand name used for an under-layer placed between the watertight covering and the earth.

The second ground support:
T panel wall
Taking into account the nature of the land, 40% sand and 60% rock (resistance to compression exceeds 800 bars), "Hydrofraise" rig was used for the drilling.
First of all, however, we had to demolish previously existing foundations.
This work consisted of 70 mm drilling, spaced out at 0.8 to 0.9 m followed by blasting using charges in the region of 350g/m3 of concrete. For the demolition we used chisels and buckets for clearing.
The excavations were carried out perpendicular to the axis of the foundations to be demolished, they were then filled with lean-mixed concrete.
We encountered several types of difficulties during the drilling of the diaphragm walls:
- the hardness of the land, leading to the consumption of a large number of picks
- the presence of 109/127 micro-piles from the initial development and HA 50 steels
- the characteristics of the mud with high cake and filtrate (intake of sea water)
We used three types of bentonite, "Cforage", C2 and CM20 (mud mixed with polymer).
The 2,80 x 5,50 T cages were constructed on site in one single unit of 24 m (35 t each). A particular feature of the cages is the inclusion of HA56 steels and a high density of 210 KGs/m3 steel. Two spreaders, one T spreader and two cranes (one 200 t and 50 t) were used to handle the cages. The concrete used was specially designed for the purposes of this site. It consists of low grade aggregates (0/14) to ensure improved mixing and a better steel/concrete traction coefficient for the reinforcing cages.

Other works
Blasting was used for the excavations in the rock (more than 30,000 m3) according to a mesh size of 1.50 x 1.50 approximately and loads of 250 to 350 kgs/m3 of land.
Close to the diaphragm wall constructions and the grouted wall along a 3-m strip, we used a hydraulic rock breaker.
A tower crane was installed in the deep section to the north of the excavation to facilitate the digging work, the drainage blanket and slab works.
The site constraints were essentially of an environmental nature, that is to say the discharge into the sea of the water from the pump, noise and air pollution.

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Techniques

Civil engineering

Regardless of site size, and providing it involves a significant proportion of works in the ground, Soletanche Bachy will handle the complete project, and all aspects of the works, including site supervision, excavations, foundations, civil engineering and all construction operations.

Diaphragm wall

A diaphragm wall is a reinforced concrete wall that is made in situ. The trench is prevented from collapsing during excavation, reinforcing and casting by the use of supporting bentonite slurry. The slurry forms a thick deposit (the cake) on the walls of the trench which balances the inward hydraulic forces and prevents water flow into the trench. A slurry made of polymers can also be used.

Ground anchors

A ground anchor is a load transfer system designed to transfer the forces applied to it to a competent stratum. An anchor is said to be temporary if it has a lifespan of under two years and permanent if the lifespan is over two years.

Grouting

Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).

Ouvrages

Building

Suitable foundations are required for all building and civil engineering structures to ensure that they perform within the settlement criteria established in the design of the structure. Special foundations are used where shallow footings do not provide adequate support for a structure.

Urban excavations

Major urban excavations are one of the specialist activities of Soletanche Bachy. Such excavations are required for basements to buildings that are usually part of the foundations of the structure and also used for under-street car parks, cut and cover tunnels for roads, rail, metro and storm water tank...


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