The method in outline

Jacked box tunnelling is a non-intrusive method for constructing a new under-bridge, culvert or subway beneath existing surface infrastructure, for example railways and highways.

The method enables traffic flows to be maintained throughout the construction period, and maintained with only minor restrictions during the brief period of tunnelling. The inconvenience and costs of disruption to infrastructure and traffic flows experienced with traditional construction methods can be avoided.

An example of the method is illustrated simply in Fig. 1. An open ended reinforced concrete box is cast on a jacking base adjacent to a railway embankment, see Fig. 1a. A purpose designed tunnelling shield is provided at its leading end, and thrust jacks are provided at its rear end reacting against a jacking slab. The box is then jacked slowly through the ground under the railway in a carefully controlled tunnelling operation, see Fig. 1b. Excavation and jacking take place alternately in small increments of advance. Measures are taken to ensure stability of the tunnel face and to prevent the ground from being dragged forward by the advancing box. When the box has reached its final position, Fig. 1c, the shield and jacking equipment are removed, and bridge construction is completed with the addition of wingwalls and road pavement.

Principal Benefits

The principal benefits of jacked box tunnelling are:

• A non-intrusive construction method
• Minimal disturbance to surface infrastructure
• Traffic flows maintained throughout the construction period
• Traffic flows maintained with only minor restrictions during box installation
• An efficient structural form incorporating a low bearing pressure foundation
• A high quality maintenance free structure

Historical development

Jacked box tunnelling in the UK developed from the pipe jacking of the mid-1960s. Initially, small precast concrete boxes were jacked to form pedestrian subways and portal bridge foundations. Later small boxes were jacked one on top of another and filled with concrete to form bridge abutments

In recent years, the development of high capacity jacking equipment and sophisticated techniques for controlling ground disturbance has led to the jacking of very large boxes each capable of accommodating a highway, railway or flood defence channel.

Several large box structures have now been installed in a wide variety of ground conditions.

Anti-drag systems

Referring to Fig. 1b, it can be seen that as the box is jacked forward it will tend to drag the ground along with it. In the case of a wide box at shallow cover the mass of ground on top of the box could be dragged forward, causing major disturbance and possible disruption to the infrastructure above. Similarly, the underside of the box will tend to drag and shear the ground, resulting in remoulding accompanied by a loss in volume that will cause the box to dive.

These effects are minimised by the use of a proprietary anti-drag system (ADS) at the top and bottom of the box, as illustrated in Fig. 2.