Posted on May 14, 2008 in Construction Technology & Machines
The 3km long tunnel being bored by Gautrain’s Tunnel Boring Machine (TBM) will be lined using precast concrete segments. The segments are installed within the protection of the tail shield of the TBM.
The inner tunnel diameter is 5.860m and the outer diameter 6.460m, giving a thickness of 300mm to the tunnel segmental lining. All elementary rings are identical and 1.50m long, measured along the axis of the tunnel, with each ring consisting of 6 segments, viz:-
- 3 standard segments (S1, S2, S3)
- 2 counter-key segments (S4, S5)
- 1 key segment (S6).
To navigate curves the segmental rings are tapered. This allows for the geometric control of the general alignment with a theoretical radius of 750m, by rotating the rings relative to each other. The segment length along tunnel axis varies between 1.490m at the key segment and 1.510m at the directly opposite standard segment, S1.
To ensure a watertight tunnel lining, a compressive gasket is installed around the whole perimeter of each segment, in a specially formed rebate. In order to temporarily stabilise the segmental ring, temporary bolts are used to connect the newly installed segments with the previously erected ring. The segments incorporate box-outs and built-in plastic sockets to facilitate bolt installation. Segments are installed using a vacuum lifting device situated in the tail of the TBM.
The gap between the extrados of the ring and the soil will be filled by mortar injected through the tail shield to form an intimate contact between the tunnel lining and the ground to limit settlement and ensure even load distribution on the tunnel ring.
The segment reinforcement design is based on maximum ring loads, allowing the use of identical rings throughout the whole bored tunnel length. The reinforcement design is based on two loading conditions, the first being the construction phase, catering for loads from the manufacture to the final placing of the segments within the TBM, including the forces exerted by the thrust-jacks when the TBM is advancing [4000t]. The second load case is the permanent phase, with allowance for all loads created by the geological ground conditions and by live loads during operation.
Since much of the tunnel is located beneath the groundwater table, a stringent quality control system is being implemented to assure the water tightness of the final tunnel lining.
The precast concrete segments are manufactured by Southern Pipeline Contractors [SPC], at a factory located approximately 25 km east of Johannesburg. Concrete is produced at the factory owned batching plant with a production capacity of 18 m3 per hour. 42No sets of segment formwork (constituting 7 rings, each of 6 segments) have been supplied by the French tunnel engineering group CBE, a world leader in the highly specialised market of steel moulds for tunnel concrete segments. Each mould is equipped with pneumatic vibrators and is required to produce concrete segments to the very tight tolerances of +/-1.0 mm. The dimensional tolerances of the moulds themselves are +/- 0.3 mm and these are checked regularly with appropriate templates.
CBE also supplied parts for the mould evacuation line, such as two vacuum lifting devices hooked to the factory’s gantry cranes to extract the segments from the moulds and transport them inside the factory; the segment rotation machine to turn the segments around from the production position to storage position; and the gasket bonding frame which applies pressure on the gasket after it is glued into the purpose formed recess in the segment. The completed segments are then lifted by locally manufactured grabs and transported to the factory’s storage area.
The production cycle commences with the preparation of the 42 moulds, including cleaning (removal of concrete, dust and other residue from previously produced segment casting), assembly of moulds (including periodical checking of tolerances) and application of release agent to the surface of the moulds. Completion of preparation requires acceptance by a Quality Controller before proceeding further.
The pre-assembled reinforcement cages (welded on specialised jigs) are placed with spacers fixed at defined positions (cover to reinforcement checked by QC). Cast-In items, such as bolt sockets, are installed before the pre-concreting final check and closure of mould.
Concrete is manufactured on site at the factory by an accredited batching plant according to an approved mix design. Slump tests are performed on every batch before casting of segments. Overhead concrete skips pour concrete into the steel shutters whilst vibrating the moulds but in order to prevent segregation, vibration is not permitted to exceed 300 seconds per mould per pour.
Fifteen minutes after the concrete has been placed and compacted, the cover of the mould is opened to allow concrete surface finishing and application of an approved curing compound to the extrados. To prevent dehydration of the fresh concrete the opened mould is covered with a tarpaulin.
Twenty-four hours after casting, and, provided that an early-age strength of 15MPa has been reached, demoulding of segments takes place. After the sides of the moulds have been removed the segment is vertically removed out of the mould with a vacuum lifting device, suspended from a gantry crane, and transported to a temporary storage area for quality checking, which includes inspection of the gasket recess, minimum cover of reinforcement, extrados surface finishing, and finishing of arises. At this stage, necessary, permissible concrete repairs are carried out, according to approved segment repair methods and each segment receives an individual identification mark which relates to its production history.
For storage, handling and transport reasons the segments are rotated 180º by a segment rotation machine. Once the segments have been rotated so the intrados surface is face up, the segment is lowered down onto the transfer trolley and moved to the next production station where gasket fixing takes place. Before the gasket and guiding rods are placed, the purpose formed rebates are dusted and glue is applied using an airless spray gun. The guiding rods are fixed to each segment so they will mate with the corresponding receiving recesses of the adjacent segment in the ring, thus assisting the accurate placement of segments inside the tunnel. After placing the neoprene gasket, the gasket press is lowered and pressure applied to achieve proper bonding.
The next station in the production line applies curing compound onto the exposed concrete surface of the segment before the segment is transported to the temporary storage area and a further quality control check is undertaken.
Although the factory is equipped with 42 moulds, such that 42 segments can be cast simultaneously, the factory possesses one evacuation line which is the ‘bottle neck’ of the production cycle. The production cycle is designed for a production output of one segment every 10 minutes at the bottle neck thereby achieving 42 segments per working day. The total period of production planned for the 12,000 segments is approximately 15 months.
In order to achieve the production of one segment per mould per 24 hour cycle the segments require an early-age strength of at least 15 MPa after 20 hours. The segments require to have a compressive cube strength (150mmx150mm) of at least 50 MPa at 28 days. Before erection in the tunnel, the concrete segment strength requirement is 55 MPa. Therefore, three extra test cubes are cast for each batch of segments and tested at 56 days to check that the compressive strength has reached 55 MPa.
The concrete mix consists of Cement CEM I 42.5 N (350kg/m3), Blastfurnace Slag (100kg/m3), Fly Ash (100kg/m3), Aggregates (19.0mm max size), water and plasticiser. Workability calls for 60mm slump concrete and the maximum water/cement ratio is set at 0.45.
The segments include 82.5kg of steel rebar per m3 with the reinforcement being accurately assembled with the help of specialised jigs. The minimum concrete cover to the reinforcement is 30mm.
The 3 km long tunnel will be lined with 12,000 segments, consisting of 17,420m3 of concrete and 1,437t of reinforcement steel. The reinforced concrete volume of one ring is 8.71m3, one standard or counter key segment comprising 1.585m3 of concrete weighing 4.1t (concrete density 2.55t/m3), and one key element comprising 0.787m3 of concrete weighing 2.0t.
Storage / Transport / Installation
4,000 segments can be stored at SPC’s factory at any one time and 50 rings (300 segments) can be stored on site next to the tunnel portal. The segments are transported from the precast factory to the tunnel site on normal road trailers (one ring of six segments per trailer). From the site storage area the segments are lowered into the shaft by a tower crane and transported to the TBM, stacked in pairs on rail wagons, from where they are forwarded to the point of installation on a segment feeder and installed by the TBM’s vacuum segment erector.
The excavation production is planned on an average rate of 10m advance per day, i.e. 7 rings.
Challenges / Solutions
A huge advantage in the production of the TBM segments has been the Civil Contractor’s prudent selection of an established precast factory, run by an experienced management with significant TBM segment experience. The greatest challenges have related to developing the necessary standards of workmanship and Quality Assurance to match the Specification requirements. This has been successfully achieved by breaking down the production cycle into manageable elements under the control of designated supervisors and implementing strict quality control procedures at each stage in the process. The resultant quality of the product is self evident.
Written by Ian Thoms and Oliver Gertsmann, Bombela Turnkey Contractor.