Jacked box underbridges using the Ropkins System

This Current Practice Sheet describes a proprietary method by which large concrete boxes are installed beneath existing railway or highway infrastructure to provide new underbridges with little or no disruption to traffic flows, unlike the traditional cut and cover methods. A tunnelling system is used to install the concrete boxes in which special measures are taken to control ground disturbance during tunnelling so as to maintain safe unimpeded operation of the traffic. Developed by John Ropkins, the system has been used successfully on many projects in the UK as well as on the 'Big Dig' in Boston USA.

John W T Ropkins (John Ropkins) and Douglas Allenby (Edmund Nuttall)

The Ropkins System

Figure 1 shows the typical stages in the construction of a concrete box underbridge beneath a highway using the Ropkins System. In Figure 1(a) the reinforced concrete box is cast on a jacking base in a jacking pit adjacent to the highway. A tunnelling shield is provided at its leading end and hydraulic jacks are provided at its rear end reacting against the jacking base. A steel wire rope anti drag system (ADS) is provided at the top and bottom of the box. In Figure 1(b) tunnelling is in progress and the box is partialIy installed. Tunnelling is carried out incrementally with 150mm of excavation being followed by 150mm of box advance. When the box has been fully installed, see Figure 1(c), any residual voids around the box are grouted up and the shield, jacking equipment, etc are removed. The underbridge is then completed by constructing the portal wing walls and roadway as shown in Figure 1(d).

Anti drag system (ADS)

Referring to Figure 1(b), 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 with shallow cover, the mass of ground on top of the box could be dragged forward causing major disturbance and possible disruption to 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 controlled by use of the ADS at the top and bottom of the box.

The ADS comprises an array of closely spaced greased wire ropes anchored to the jacking base with their free ends passed through guide slots in the shield and stored inside the box. As the box is jacked forward the ropes are drawn out through the guide slots and form a stationary inter face between the ground and the advancing box. Drag loads developed in the ropes are transmitted back to the jacking base where they oppose the main jacking force.

Shield configuration and design

An open cellular shield is used in which the dimensions of the cells and the form and structural thicknesses of the dividing walls and shelves are designed to suit the physical properties of the ground. The shield must provide adequate support to the tunnel face at all times and must be capable of incremental advance into the face without causing heave of the ground ahead.

Figure 2 illustrates a composite reinforced concrete and steel shield for a 17m wide by 6.2m high box designed for a mixed face comprising loose silt and sand in the top half and soft clay in the bottom half. The steel upper section with a sloping front is designed to be thrust into the loose silt and sand to provide face support and protection to the miners, while the concrete lower section with relatively thick walls is designed to support the soft clay.

Ground conditions and improvement

A detailed site investigation in advance of the works is essential to determine the nature and physical properties of the ground, the presence of any buried obstructions and the groundwater table. Trial holes are particularly useful in assessing the stability of the ground in the tunnel face.

The system requires the ground in the tunnel face to arch between the supporting walls and shelves during tunnelling. Accordingly, the ground must have adequate short term shear strength and hydrostatic pressure from groundwater cannot be allowed to destabilise the ground in the face.

A variety of geotechnical treatments have been used successfully to improve the shear strength of the ground and to remover groundwater pressure including dewatering, grouting and ground freezing.

Surface settlements

The system is designed specifically to control ground movements during tunnelling and to keep surface settlements to an absolute minimum. However, some settlements do occur, albeit slowly and predictably, and take the form of a gradually extending and deepening very shallow dish. This is centred over the box and extends either side of the box by a distance equal to the depth of the box floor below ground level. Based on settlement observations obtained from previous projects and using conventional tunnel settlement theory modified to suit a wide rectangular box tunnel at shallow depth, it is possible to predict with some confidence the development and magnitude of surface settlements during tunnelling.

Example projects

The following two projects illustrate the system in practice.

Underbridge beneath M] Motorway, Northamptonshire, UK

A new underbridge was required beneath the M1 motorway in Northamptonshire as part of an upgrade of the single carriageway A43 to dual carriageway. In order to minimise disruption of the 112,000 vehicles that use the M1 motorway daily, the new underbridge was tunnelled beneath the M1 using the Ropkins System, rather than by conventional cut and cover methods that would involve diversions, lane closures and contra flow systems. The underbridge box measured 45m long, 14m wide and 8.5m¬high and was installed at a minimum depth of 1.6m below the motorway surface. Figure 3 shows the box being installed beneath the M1.

Ground conditions comprised clay and pulverised fly ash fill in the motorway embankment overlying natural boulder clay. The water table was 1.5m below the box.

A three level concrete cellular shield with steel cutting edges was used to support the face. Excavation in the top cells was carried out by miners and in the middle and bottom cells by machine excavators. Ground drag was controlled using an ADS at the top and bottom of the box. The motorway surface was monitored for movement remotely prior to, during and after tunnelling.

The box was installed to within 30mm of level and 60mm of line and required a maximum jacking thrust of 4500 tonnes. The tunnelling operation took four weeks and was successfully completed in December 2002. The maximum settlement of the road surface was 28mm and motorway traffic continued uninterrupted throughout. Figure 4 shows the completed underbridge.

Underbridges beneath railway tracks, Boston, USA

Three underbridges were installed beneath the railway tracks leading into Boston's South Station as part of the new Boston Central Artery System, otherwise known as the 'Big Dig'. With more than 300 train movements each day the absolute requirement of the railway owners not to interrupt any rail services resulted in the use of the Ropkins System.

Extensive ground improvement was necessary to stabilise the weak water bearing strata. Obstructions included the remains of old wharves and building foundations. It was decided that the only way to guarantee the integrity of the railway tracks was to freeze the ground along the alignment of each underbridge. This involved the installation of 2000 vertical freeze holes to completely freeze the ground to be tunnelled through.

The underbridges were of considerable size and length, with the largest measuring 107m long, 24m wide and 10.8m high. In order to develop sufficient jacking thrust each underbridge box was split into either two or three units, which enabled intermediate jacking stations to be employed.

The ADS was used at the top and bottom of the boxes, with the ropes stored on reels inside the lead unit. With specially designed road headers excavating the frozen ground in the tunnel face, the box units were advanced sequentially in 300mm increments. Due to limited working space, the first two units of the longest underbridge were cast and the lead unit advanced into the ground before the third unit could be cast and the tunnelling operation completed.

Figure 5 shows the lead and middle units of the longest underbridge in their jacking pit. Adjacent and to the left is a second jacking pit with a three unit underbridge under construction. On the far left is a jacking pit, from which a two unit underbridge had been tunnelled into place beneath the railway tracks.

Concluding remarks

Although more expensive than traditional cut and cover methods, the Ropkins System has proved highly successful in constructing underbridges beneath valuable surface infrastructure where the cost and inconvenience of disruption to traffic and damage to the infrastructure is to be minimised. It produces a high quality and maintenance free structure. The system is endorsed by both Network Rail and the Highways Agency.

Article courtesy of Concrete Magazine (September 2007)

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk