A major hillside excavation and complex foundation works have been necessary to create a new tower block in Monaco. Adrian Greeman reports.
It is not often that a few ground anchors could cause an international incident, but for the 160m high Tour Odeon in Monaco that could have been the case - if they had been too long.
“They would have crossed under the border with France,” explains Soletanche Bachy’s site manager Lucas Allevena, who is currently overseeing the foundations and an underground carpark for the luxury apartment building.
The boundary lies just at the back of the site on the steep chalk slopes that make up the Mediterranean coastline here.
Since the tiny principality of Monaco itself is only a narrow strip of land, boundary problems are not uncommon for schemes in Monte Carlo.
And logistical and site access in general is always difficult among the ranks of expensive apartment buildings and hotels that jostle for space in the famous casino city.
For the Tour Odeon that is even more the case.
The building will be one of the largest in the city when finished, standing out from the rest at almost twice the height of its surroundings on the steep hillside.
Local daughter company Soletanche Sam is part of the six-firm joint venture which is making a space for the building and the 10-storey deep car park beneath.
The JV will then complete the main tower structure.
“initially there was no room at all for the big machines we need”
The other partners are JV lead Vinci Construction France and its local Monaco manifestation SGTM, Triverio Constructions, GTM Sud, and GTM TP CA.
The foundations include a diaphragm wall to enclose the deep basement, plus an assortment of deep barrettes for the foundations to support the double tower, which will rise to 44 floors on one side and 48 the other, with a swimming pool at the top.
There is also a seven-storey podium at the tower base, above the car park.
The work is not straightforward since the initial development site comprised nothing more than a tightly enclosed section of hillside, sloping steeply towards the sea.
There was almost no room for machines and limited street access.
The project called first for the hillside to be excavated away to create a flat platform big enough for the tower to rise up and the car park to drop down 10 stories below the main access point at street level.
Diaphragm walls 52m deep form the perimeter and enclose the excavation for the car park. Barrettes will be up to 55m deep.
“But the excavation to make the site platform could be done only in three phases because initially there was no room at all for the big machines we need,” says Allevena.
First, a series of steps was excavated at the top of the slope to create progressively larger working platforms from which excavators could dig out the slope.
The smallest of the platform spaces was 6m wide, enough for two Soilmec SM18 mini-pile rigs to work.
These drilled a row of 52 piles, each 300mm in diameter and reinforced with a 220mm steel tube.
In front of these, 12.5m deep piles form a retaining wall at the back of the site.
The slope was then removed, with six rows of anchors going in to support the wall as excavation proceeded.
The 30m long anchors were carefully measured to stay within Monaco’s boundary to avoid the developer having to seek further permissions from the French authorities.
Completion of the pile wall allowed another, larger, platform to be made with enough space for a much bigger Mait 240 bored pile rig to come in, says Allevena.
With this, around 40 large 1m diameter piles were installed, this time to a depth of 20m.
The spaced piles were the basis of a “Berlin wall” of piles as kingpins with intermediate shotcrete lagging, finished with a concrete facing to support the chalk marl hillside.
Eight rows of the 30m long anchors were installed for this wall to support it as the hillside was excavated, between July last year and March.
The deep space now available here after excavation of this next section of slope has created a much larger site platform, currently in two levels, one 8m above the other, from where two of Soletanche’s hrydrofraise machines are carving out the deep 1m wide panels needed for the perimeter and the barrettes.
Rotary head cutters, rather than panel grabs, are needed to grind through the relatively hard marl which underlies a top weathered layer which is up to 20m deep.
Panels are made to average depths of 47m and in places 50m, the last 10m or so keying into an even harder marly limestone.
The big barettes are going in even deeper at up to 55m.
Using the hydrofraises creates further site constraints, however, since each machine must have its own bentonite supply and separation plant for spoil.
Room has had to be found for those two sets now perched over the site on the hillside to one side.
Bentonite comes in from Bentofrance near the French town of Valence and is made up on site.
The spoil is separated with centrifuges.
A number of other machines have to operate too, including some smaller drill rigs and some very big mobile cranes with the reach and capacity to lift the tall reinforcement cages and slowly slide them into the bentonite supporting the finished wall trenches.
The largest is a Sonnebogen HD 5500 with a capacity of 180t.
A four-storey block of site offices also had to be set up.
This rests on a steel framed “bridge” over the road along the site front, so as not to take up valuable site space.
“But we were not allowed to take any possession of the road at all, except for some truck movements at restricted parts of the day, mainly after 7pm,” says Allevena.
“Logistics and access has been one of the most difficult parts of the job”
Each truck must have a separate individual Monte Carlo city council permission to enter Monaco with its load, he adds.
Site work hours are limited, though with a 6am start allowed and a 10pm close this does allow two shifts to work, albeit within tight 75dBa noise limits.
“Logistics and access has been one of the most difficult parts of the job,” says Allevena.
But that does not mean the work on site is easy either.
The panels are large and deep and reinforcement cages require complex manoeuvring, especially where the panels are up against the steep vertical pile wall at the back of the site.
An added complication is that the diaphragm wall will include a ground heat energy extraction system.
This technology is relatively new to Monaco and installation of the tubing for the system is part of the learning curve for all concerned.
The blue HDPE piping runs inside the reinforcement cage and is installed at the same time as the cages are made up.
“Reinforcement cages are prepared outside the site and delivered by trucks,” explains Allevena.
Sections reach a maximum height of 14m and are added section by section.
“The wall can be up to 53m deep so we can need six cages in height and three in width which is 18 in all” says Allevena.
Where three cages are needed they are first each assembled vertically, the growing cage lowered into the bentonite-filled trench where it is supported at the top while a new piece is lifted in and fixed to the top.
Two or three of these full length cages are made in sequence side by side.
They are then lifted out of the trench again and held side by side by the cranes, while intermediate horizontal connections are made between the three vertical sections, primarily for additional resistance in accordance with local seismic standards.
For the biggest, with 18 pieces, he says, two cranes are needed to hold and then lower the whole assembly, “though mostly one crane is enough”.
Last month, work on the walls and the barrettes was about half done, on track for completion in late autumn.
The joint venture will then begin the top down excavation work while the superstructure above will be able to start up at the same time.
The tall tower itself will also get under way with completion scheduled for 2014.