The jacking of large structures, such as road underpasses, pedestrian subways and culverts uses trenchless jacking methods.

I developed and pioneered these techniques in the 1960s and 1970s when working with a major international contractor. The initial projects were for pedestrian subways using large diameter pipes to provide the structure. The next development was to jack box sections for culverts and pedestrian subways. With this experience we increased the dimensions to accommodate larger structures. Figure 1 illustrates a typical situation where a pedestrian subway is driven under a rail embankment to allow separation of traffic and pedestrians.

The most basic approach, which is still widely used, is box jacking where the structure is constructed on a launch pad adjacent to where it is to be installed and is jacked into the ground with excavation taking place in the shield.

Dealing with a face excavation which can be typically 15 m wide and 8 m high you need to compartmentalise the shield, which is fixed to the front of the structure – very much like Brunel’s shield for the very first tunnel under the Thames back in 1823. You also need to provide thousands of tonnes of jacking capability to push such large sections, which in turn needs the provision of suitable reaction. The frictional loads on a rectangular structure are much greater than experienced when installing circular sections so there is a need to introduce anti-drag systems to reduce this.

Many installations have been undertaken under roads, rail tracks and runways. Figure 2 shows one of the first projects undertaken in the early 1970s where two underpasses were driven below a major road in North London to provide access to Brent Cross shopping centre. Perhaps the most high profile installation was the three large underpasses jacked under rail tracks in Boston, US as part of the Big Dig project. These boxes were approximately 24 m wide, by 11 m high and the longest drive was 109 m. Figure 3 gives some idea of their size and the large number of hydraulic rams that were used. Because of the ground conditions – which were mainly soft marine deposits with old piles and buried quay walls – the ground was stabilised by freezing.

Box jacking, where the track is temporarily supported and structures are installed directly below the rail tracks, is widely used in Europe.

There are some inherent limitations in box jacking and in recognition alternatives approaches have been developed.

If provision is made with the rail authority, it is possible to obtain brief possession to undertake work. With this in mind I developed the modular approach.

Using a series of precast jacked boxes, abutments and central piers can be constructed without disrupting rail operations. As can be seen, a whole series of operations can be undertaken from the boxes such as cross strutting and finally stressing the boxes together and infilling with concrete. During a short track possession the bridge deck can be slid in or bridge beams placed to complete the structure. A large number of such jobs have been undertaken in the UK. A typical installation is the Wandsworth under-bridge shown at figure 4. This concept minimises the exposed face at any time and requires much less launch area as only relatively small jacking pits are needed.

More recently a new patented development is the jacked deck which can be combined with the modular system to provide a complete structure without disrupting rail services. The modular system is used but the top box is modified internally to allow a slide track to be installed. These top boxes are also designed with a removable section which exposes the track as the deck is jacked into place. The jacked deck, which can be formed from a complete cast on site structure or formed from precast beams, is fitted with a compartmentalised shield where excavation takes place. This same concept can also be used with a jacked circular tube or box providing an access gallery which allows piling to be undertaken to provide abutment walls and the installation of the slide track.

This approach is currently being used to install a 20 m wide by 126 m long traffic underpass under rail tracks in UK.

Another patented approach is to install initially jacked foundation boxes with removable lids and install in these boxes a slide track. In this case, precast arch sections are jacked in along the track with lids being removed within the shield as excavation takes place.

The Jacked Arch has a number of advantages such as the reduction of jacking loads and face stability is improved. The use of precast arch sections also minimises the launch area. This approach enables larger spans and longer length to be installed. Metro station structures are quite possible and full support to the excavation as work advances would be provided thereby eliminating the risk of collapse.

There are other techniques used such as the provision of advanced support structures where a canopy is installed as an initial support for the final structure. Typical approaches are shown in the graphic at figure 5. The steel tubes that form the canopy can be installed by pipe ramming, microtunnelling or auger boring.

Although the majority of Jacked Structure installations have been in Europe, projects have been undertaken in US, India, Canada, South Africa, Indonesia, Taiwan, Korea, Japan and Australia.

The relentless growth of cities is creating a demand to install structures below existing facilities and buildings. The potential applications for creating large underground spaces by jacked methods without disruption are unlimited. The separation of road and pedestrian traffic from rail traffic is essential to avoid the all too frequent accidents that occur at rail crossings. Valuable and scarce development land cut off by rail tracks and highways can be accessed. New underground space such as parking garages and storage can be created.

One interesting possibility that was put to me in Australia was the possibility of installing wine storage caverns. I am hoping this will go ahead and I might get the opportunity to check out the quality of wine stored.

Biography

James Thomson has been involved in Trenchless Technology over many years and in developing and designing the installation of large structures since the late 1960s. He was the owner and principal of Jason Consultants Group until 2003, a company specialising in Trenchless Technology including a number of jacked structure project. His experience in the field has led him to develop some concepts that have been patented. Along with experienced colleagues who have worked with him over the years he recently formed Jacked Structures SA. A consultancy specialising in developing concepts, detailed engineering, installation design and contract management for these projects. Work is undertaken for owners, consultants and contractors.