Tunnel Form System

Overview

A tunnel form casts the two walls and the floor slab of a room in a single concrete pour, which is why it is the standard method for repetitive cellular buildings such as hotels, student blocks, prisons, and apartment towers where the same room shape repeats hundreds of times. The form is shaped like an upside-down U. Two L-shaped Half-Shell units are set facing each other and bolted together along the ceiling line at the Crown Joint, so each half supplies one wall and half the soffit. Reinforcement and any services are fixed, the concrete is poured in one go, and the next morning the form is struck, rolled out, and flown up to repeat the same cell one floor higher.

The appeal is speed and finish. Casting walls and slab together gives a monolithic structure with no cold joint between them, and the steel face leaves an off-form surface smooth enough to paint with no plaster. A site running enough forms can turn a floor every few days, which is far faster than building walls and slab as separate operations. The cost is heavy plant: each form weighs several tonnes, needs a tower crane to move it, and only pays off when the same cell repeats often enough to spread that cost.

The half-shells

Each Half-Shell is an L. The upright leg is a Wall Panel and the foot is a Soffit Panel, joined by a Corner Gusset that holds the angle square under load. The face that touches the concrete is a Sheet Metal Panel skin, 4 to 6 mm steel plate welded up flat. Behind the wall skin run vertical Face Stiffener channels and horizontal Waling sections that carry the full liquid pressure of the fresh pour without the face bowing. Where two facing walls must be held to each other, a Tie Socket socket takes a through-tie. The soffit skin is carried on deep Soffit Bearer beams, and a set of Camber Shim pieces pre-cambers the soffit so the slab casts flat once it deflects under its own wet weight. Stiffness without dead weight comes from the Rib Truss units behind the panels, each a light lattice of Truss Chord and Truss Diagonal members bolted at Node Plate joints.

Closing and tying the form

The two half-shells meet at the cell ceiling. The Crown Flange along each soffit edge is pulled together with Crown Bolt fasteners, a Alignment Cone guides the halves into exact register, and a Crown Seal strip stops grout leaking down the ceiling line. The open ends of the tunnel are closed by the Stop Ends assembly: an End Panel at each end held by End Clamp clamps, with a Door Box and a Window Box set into the wall to leave the door and window openings. An Edge Seal strip closes the joint against the previous day's pour so the new wall keys cleanly to the one below.

Holding it plumb and level

Before the pour the form must sit dead level and dead plumb, because every wall it casts inherits its alignment. The Prop System carries the soffit load to the floor through telescopic Prop Jack units, each built on a Ball Screw running in a Prop Tube, driven by a Jack Nut and held at height by a Jack Collar. The props are triangulated with Brace Tube diagonals and stand on spread Prop Foot plates, and a Plumb Screw tilts each wall a fraction to bring it true. At the base the Levelling System system sets the form down: a Levelling Jack under each corner, again a Ball Screw and Level Nut on a Base Shoe, raises the soffit to the set level, while Alignment Bolt push bolts and a Laser Target read against a site laser to set the line.

Striking and moving

The form has to release a cell it has just cast without damaging the green concrete, so it strikes inward. Short Kicker Jack units at the foot of each wall, each a Ball Screw driving a Kicker Shoe against the slab with Coil Spring return, push the panels a few millimetres off the cured wall to break the bond. Then the Striking Gear takes over: a Servo Motor turns the Striking Screw pair through the Striking Drive drive, lowering the soffit in a controlled drop, while the Drop Linkage and its Retract Arm arms draw the wall feet inward so the whole form folds clear. Once free, the form rides out on the Rolling Chassis. A Chassis Jack lifts it onto its Wheel Set units, each a steel Wheel on a Axle turning on Ball Bearing supports, and a Track Guide keeps it running straight to the slab edge.

Flying and pouring

At the slab edge the Lifting Frame takes the load. A Strongback spreads the lift across the form so it does not distort, a Balance Beam keeps the Sling Set slings vertical, and rated Lifting Eye points let the crane fly the form up to the next floor in one piece. Through the pour and stripping the crew works off the Working Platform, a cantilever Platform Deck on Cantilever Arm brackets with Handrail guards, Toe Board kickplates, and an Access Ladder. The whole loop, set, level, pour, cure, strike, roll, and fly, runs on a 24-hour cycle, so one form casts one cell a day and a crew with several forms turns a full floor in a working week.