Tunnel Form

Tunnel form is becoming one of the most common methods of cellular construction in the UK as its cost effectiveness, productivity and quality benefits are being realised on a wide range of projects. Even though the tunnel form method of construction is not being implemented in South Africa, this could change as speed of construction and cost effectiveness demands continue.
Tunnel form is a fast-track method of construction that is well suited to repetitive cellular projects such as hotels, apartment blocks and student accommodation. Recognised as being a modern method of construction, tunnel form simplifies the whole construction process by enabling a smooth and fast operation that can result in frame costs being reduced by 15 per cent and provide frame programme time savings of 25 per cent.

During the tunnel form construction process, a structural tunnel is created by pouring concrete into steel formwork to make the floor and walls. Each 24 hours, the formwork is moved so that another tunnel can be formed. When a storey has been completed, the process is repeated on the next floor. A strong, monolithic structure is thus constructed that can reach 40 or more storeys in height. The use of high strength concrete ensures fast construction. For the 16 storey, 268-room, Radisson Edwardian hotel in Manchester, early striking strength of 15N/mm2 was achieved at 14 hours.

Tunnel form creates cells which are 2.4 to 6.6m wide. These can be easily subdivided to create smaller rooms. Where longer spans (up to 11m) are required, the tunnel form can be extended using a mid-span section. The walls can be designed as deep beams and supported at low level on fin columns to permit car parking.

With tunnel form, the structural engineer designs the one-way spanning slabs and walls in accordance with BS 8110. The innovation is with the formwork system. As long as the architect has chosen or is prepared to work within the constraints of regular wall alignments, tunnel form is an excellent structural solution.

The techniques used for tunnel form are already familiar to the construction industry. On average, a team of nine site operatives plus a crane driver can strike and fix some 300m2 of formwork each day, including placing 35m3 of ready-mixed concrete: typically 2.5 cells. The speed of construction is underlined by the recently completed student accommodation blocks for Queen Mary and Westfield College, University of London. Here, the superstructure for a 175- bedroom block was built in only 32 days. In addition to speed of construction, the technique provides further inherent benefits of concrete: high levels of thermal mass, sound insulation and fire resistance.

The smooth face of the formwork results in a high quality finish that can be decorated directly. This reduces the need for finishing trades thereby providing additional cost savings and speeding the entire process. A hotel chain explained that the 3mm accuracy of construction meant that carpets, which are typically replaced every 3 years, can be made on a fixed loom and fitted with no trimming.

Tunnel form provides a winning combination of the speed, quality and accuracy of factory production with the flexibility and economy of in-situ construction.