Western Rail Link to Heathrow [WRLtH] GRIP3
Constructability Support to Grip 3 Design
The project is a westward facing link from the Great Western Main Line near Langley Station to Heathrow Terminal 5. When complete, it will permit some of the trains from Reading to divert off the Great Western Main Line and to travel using the Link to Heathrow and use the Heathrow Express route to regain the GWML at the existing Airport Junction.
It connects to the relief lines that are to the north of the main lines at Langley and then passes beneath the main lines to head south to Heathrow. In order to provide a grade separated junction between the Heathrow Link and the relief lines, the up relief line is diverted northwards so that the Heathrow Link lines can rise up between the up relief line and the down relief line and connect to both on the flat. The Heathrow Link will be constructed in a troughed ramp prior to passing beneath the down relief line and main lines at a bridge known as the intersection bridge. To the south of the GWML the Heathrow link is initially in open and cut and cover boxes which also form TBM launch boxes. Twin bored tunnels are driven under open land, buildings, some significant buried infrastructure and the M4, M25 and T5 sliproads. There are two intermediate shafts and a crossover box near Heathrow. The connection to the T5 stub tunnels from the crossover box uses SCL methods. This enables the TBMs to be removed from a location outside the airport.
There was also some additional work to support previous decisions on option selection.
The LBA role was to provide constructability support across all disciplines but principally the tunnelling and Civil Engineering Works. Support to the design of the tunnelling works covered many of the connection details between shafts, ventilation and escape adits and the running tunnels. Extensive work was also carried out on the SCL connecting tunnels to the existing Heathrow T5 stub tunnels. This included the methodology for partial face construction to provide a secure method for tunnelling beneath the motorways but also to take into account the tight spatial constraints and the variable ground expected to be found where the T5 works had been constructed in open excavations and backfilled.
At the Intersection bridge, the initial LBA input commenced with the methodology for driving two tunnels beneath the GWML to provide slide paths for the bridge to be slid in as a precast box during a blockade. As the full extent of the works necessary to complete the works as a bridge slide became apparent, LBA began to consider other options including building the bridge in two halves whilst maintaining the GWML in operation by staging the works and slewing the track between stages. An extensive staging methodology was developed as well as a construction methodology for the bridge.
The LBA contribution to the tunnelling design was significant but was far outweighed by the contribution to the intersection bridge. A highlight was an extensive workshop which led to the revised concept for the intersection bridge and which removed the need for a 10 day blockade of the GWML. A blockade would have been a major programme risk as well as political risk to the project. Other highlights were the development of a methodology for handling tunnel spoil and its removal from site by rail. Further highlights were the development of a staging scheme which facilitated all the works including maintaining all operational aspects of Langley Station during the cutting back of the platform to facilitate the up relief line re-alignment.
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