References
Brest - dry dock no1
FRANCE, 2006-2008
Widening and modernisation of Dry Dock N°1 at the Brest Port de Commerce.
The Brest dry dock N°1 came into service in 1905. With the arrival of ever larger vessels, this dry dock was only capable of taking a small number of vessels, others being sent to larger dry dock facilities.
The modernisation project consisted of widening the dry dock, creating a new flat base slab by demolishing the old steps and installing a new system for collecting water and separating soiled bilge water from clean water. Additionally, beams were built on both sides of the dry dock and equipped with a hauling rail anchoring system. Furthermore, a crane track was rebuilt on the platform behind the wall to take the new handling crane subsequently installed by the Brest C.CI. de Brest which operates the dry dock.
Thanks to this project that has been financed partly by the Brittany Region, by the European Community and the Government, the Brest Chamber of Commerce hopes that the dry dock usage will increase from 145 to 245 days of the year.
Description of the structure
The dry dock is being widened by demolishing one of its side walls. Before this can take place, a diaphragm wall was built behind the side wall and secured using passive ground anchors to form the new wall. Doubts about the stability of the side wall left in front of the diaphragm wall have resulted in the working platform level being lowered by 2 metres. In order to achieve this, the first step consisted of constructing a watertight curtain by installing a slurry wall around the ground anchor drilling area. Metal sections were embedded in this wall to form the attachment points for the diaphragm wall passive ground anchors. The ground anchors were installed after excavating 25,800m3 of spoil.
The height of the diaphragm wall then has to be raised by casting a concrete wall which is then topped off by the capping beam and haulage track. Once the peripheral steps in the bottom of the dry dock have been demolished, a reinforced concrete base slab will be constructed over the existing base slab and anchored into the substratum rock using micro piles. This base slab will incorporate a new drainage system.
Phasing the works and specific points
Phase 1: from the top platform behind the side wall, construction of a reinforced slurry wall, in two shifts, using a KS 2 rig.
Phase 2: checking the low level of the water table (dewatering could have been used) and earth moving in order to lower the working platform level by 2m.
Phase 3: demolition of the side wall upper offset using 3 heavy duty 3 BRH (Boom mounted demolition hammers).
On the floor of the dry dock:
Construction of the first 550 base slab anchor points. In view of the considerable amount of water circulating in the bottom of the dry dock and of artesian phenomena, a silicate cement slurry was injected to tie in the ground anchors.
Phase 4: a KS 2 and a drilling rig were used to construct the diaphragm wall.
Phase 5: earth moving and construction of the working platform for installing the ground anchors which consisted of a bed of sand between 2 layers of geotextile.
Phase 6: the ground anchors were installed and the excavation backfilled up to the level of the diaphragm wall. In the bottom of the dry dock: installation of base slab reinforcement bars and then concreting at 7 locations.
Phase 7: construction of the extension wall using tamped concrete.
Phase 8: construction of the North and South haulage beams.
Phase 9: demolition of the side wall which is 6.50m wide at its foot. Demolition took 4 months to complete.
Phase 10: construction of the rear crane track.
In the bottom of the dry dock: ground anchors were installed in the area of the demolished base of the side wall, installing reinforcement bars and concreting the slab.
Techniques
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.
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.
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.
A pile is a structural element driven into the soil for transferring loads and prevent deformation. Its slenderness ratio is not limited.
Pile shafts can be uniform and rectilinear, telescopic and belled out.
Piles can be installed either separately or in groups. They can also form a retaining wall, a mixed curtain wall, contiguous piles, secant piles and composite curtain walls, such as Berlin walls and similar. Piles are also used as precast beams to be placed in the structure of the building they support.
Ouvrages
Voyage au coeur de l'ingénierie de pointe
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