After completing the diaphragm walls and northern section of the cover slabs for the future Orly Airport Station (L14/L18) with our joint-venture partner Demathieu-Bard, the consortium’s teams began the earthworks, tie beam, and strutting phases back in December 2019.
The tie beam works involved 143 type-P2 units and used Repetitive & Selective Injection to stabilise the diaphragm walls as the soil excavation advanced, until the raft foundations and strutting slabs were completed.
The tie beams were set up in very dense, compact ground including greensand, and both Pantin and Argenteuil marlstone. This required pre-drilling then inch-prefect drilling using a range of drilling tools to extract all the cuttings to ensure the sleeved pipes are set up properly.
To avoid disrupting the passage of the L18 TBM, a specific, fully-destructible system featuring no steel parts was required. To this end, Sefi-Intrafor used twelve 27lm glass-fibre tie beams in partnership with Freyssinet for the first time.
The glass-fibre tie beam method involves:
- Core boring into the diaphragm wall and setting up distribution plates;
- Pre-drilling, drilling then setting up glass-fibre tie beams with built-in PVC sleeved pipes;
- RSI (Repetitive & Selective Injections);
- Tensioning (including acceptance-testing);
- Relaxing and removal of metal elements (cover, plate, head, etc.) before the passage of the L18 TBM.
The glass-fibre tie beam features 4 separate parts:
- 4 detachable 3lm steel strands,
- Four 11.8lm CGA GF strands connected with 4 steel "A" couplers (steel/GF)
- Four 11.8lm CGA GF strands connected with 4 steel "B" couplers (GF/GF)
- Smooth, sleeved PVC pipe along the entire tie beam for SRI
For this innovative glass-fibre-reinforced process - which is unusual and non-standard in France - to be approved, the teams had to submit a full set of documentation (detailed technical data sheet, risk data sheet, glass-fibre tensile strength tests, Freyssinet accreditations and references) then successfully complete a control test on the first tie beam in the series. This was required to confirm the quality and strength of this new material.
To guarantee constant monitoring over the next 2 years, half of these innovative tie beams were equipped with dynamometers.
To make the logistics easier, the tie beams were assembled directly on site. As a result, assembly benches had to be set up and a 30m x 5m space cleared in the centre of the groundwork during the earthworks in collaboration with Demathieu-Bard. These glass-fibre tie beams were assembled on site by Sefi-Intrafor and supervised by Freyssinet based on very precise specifications.
Only 11.8m glass-fibre bars were available so had to be connected using steel couplers. As they were made from steel, Sefi-Intrafor kept them well out of the way of the L18 TBM. The drilling angles were adjusted accordingly.
Given the length of the tie beams (26.6lm) and the rigidity of the material, the full worksite team (10 people) was required to set them up in the boreholes.
Client: Aéroports de Paris
Prime contractor: Systra and Alpes Contrôles
Consortium members: Demathieu-Bard, Sefi-Intrafor
GF supplier: Freyssinet