Northern Ticket Hall hub shaft at King's Cross Station

The redevelopment of the King's Cross/St Paneras underground station involves upgrading and expanding the existing main ticket hall and constructing two new ticket halls. The hub shaft alone required 150Om^sup 3^ of concrete.

Morgan Bemo, as main contractor for a 16 12 19m-deep, piled and braced shaft required wall and soffit support that would allow the construction of an 8m single-sided, tieless wall varying in thickness from 450mm to 1500mm, supporting a 2.4m-thick diaphragm slab which, in turn, would support a 4.2m single-sided, tieless wall. From this, a steel grid would support the precast planked floor with an in-situ edge stitch to the perimeter piles. The final 3.6m single-sided, tieless wall would spring from this level.

Doka UK was asked to design and supply the formwork using standard equipment for all aspects of the shaft. For the first 8m single-sided wall, Morgan Bemo requested that Doka split the pour height into two (4.5m and 3.55m) and adhere to a pre-arranged hit-miss pour pattern of approximately 6m.

Formwork design

Doka provided Framax panel formwork and single-sided supporting frames with two anchors per frame, per pour, to be cast into the base slab. The frames were constructed in two or three frame units to suit the full 8m with 4.5m-high formwork attached at base level. A total supply of 16 frames allowed the construction of six units giving a three-pour sequence. Once the first pour was constructed the units were withdrawn, cleaned and additional panels added to construct the upper pour.

After construction of the first 8m of wall the top panels and top portion of frames were disassembled to retain 4.5m-high units to construct the upper 4.2m wall above the diaphragm slab. For the Sm-high support to the diaphragm slab, Doka made the decision to use 1.5 1m Staxo support towers to give high-capacity leg loading with WSlO primary beams supporting 200mm H20 secondary beams with a ply overlay.

The towers were quickly assembled in the horizontal position at the top of the shaft and craned into position. Scaffold tube lacing and bracing was then applied.

Lightweight Frami panels, raked from primary slab formwork, were used to cater for internal void box-outs. Once the slab was constructed the 4.5m single-sided wall units were craned back into the shaft, repositioned at the new anchors, and the walls poured. After erection of the steel grillage and the positioning of precast planks, Dokaflex 1-2-4 soffits, comprising Eurex props and 200mm H20 beams, were used to construct an in-situ stitch between the slab and the piles. The wall process was then repeated for the upper wall with back propping at the frame positions to take the loading back to the main diaphragm slab.

Concluding remarks

Over a nine-month period the tunnelling contractor, Morgan Bemo, excavated and lined the walls and slab of the hub shaft. It was from this shaft that the tunnelling works required for the construction of the Northern Ticket Hall were undertaken.