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Portsmouth Harbour tunnel

Britain’s first immersed tube tunnel for a light rail line

The South Hampshire Rapid Transit Project is currently the subject of a Public Inquiry as part of the process by which powers are being sought to build and operate the system. One of the key aspects of the scheme is the proposed tunnel underneath Portsmouth Harbour. Piet Meeuwissen, Managing Director of the specialist Dutch engineering consultancy Delta Marine Consultants explains what is proposed.

South East Hampshire in England includes the coastal area between Hayling Island and the River Hamble and its hinterland. Over 500,000 people live in the local area of Portsmouth, Gosport, Fareham and Havant.

The current transport problems of the area are broadly similar to those of most other major urban areas. However, accessibility is influenced by the particular geographic characteristics of the area and the growing number of car journeys entering and leaving the Fareham-Gosport corridor and Portsea Island have been concentrated on a limited number of traffic routes. It is also a fact that Gosport is the largest settlement in the United Kingdom without a rail station.

To conquer the very congested traffic routes Hampshire County Council and Portsmouth County Council are proposing a rapid transit system between Fareham - Gosport and Portsmouth [see map of area]

One of the key parts of this system is a tunnel under the harbour of Portsmouth.

There are in this case three options for constructing this tunnel in the harbour:

Considering all the aspects of the different construction methods, the Project Office chose the Immersed Tube Tunnel option. If these plans come to fruition, this tunnel will be the first immersed tube tunnel in the United Kingdom for either heavy or light rail. There are already two immersed tube tunnels for vehicular traffic, one under the river Medway and the other under the estuary of the river Conwy in North Wales.

The Portsmouth Harbour tunnel will be 1 km long in total, although the immersed tube portion itself will be only approximately 670m. There will be six tunnel elements or section, three on the Gosport side of 107m length each, and three on the Portsmouth side of 116m each. The crown of the immersed tube tunnel will need to be only 2m below the over dredging limit for the harbour bed (by contrast, a bored tunnel would need 9m of cover above the tunnel crown). The likely minimum cross section of the tunnel elements is 11.5m wide by 7.5m high. These dimensions will allow sufficient space for two tunnels within each section (one in each direction) each with sufficient space for the kinematic envelope of light rail vehicles (LRVs) up to 2.65m wide and for the tunnel to act as a conduit for various sorts of statutory undertakers’ apparatus, which will provide revenue streams to assist in the financing of the approximately £40m cost of the tunnel. Jet fans will be provided for smoke control in an emergency and evacuation of passengers through fire rated partitions with escape doors at regular intervals will be provided

The horizontal alignment is determined by the constraints at the Gosport and the Portsmouth side and the minimum acceptable horizontal curves of the LRT system.

The vertical alignment is defined by the minimum depth of the tunnel below the harbour bed, the width of the existing shipping channel and the maximum allowable slopes in relation to the LRT system.

The cross section of the tunnel is first of all determined by the kinematic envelope of the proposed LRV and the minimum clearances above, below and to the sides of the vehicles for ventilation and walkways. In addition to this requirements there is a need to provide sufficient space for internal ballast. This ballast [concrete] is required to replace the temporary water ballast which provides the tunnel sections with sufficient negative buoyancy.

After construction of the tunnel sections and the dredging of the trench the immersion of the tunnel sections can start. First of all the sections, which already float by means of two bulkheads and are trimmed by means of water ballast tanks inside the section, will be fitted with the necessary immersion equipment.

An access and control tower installed on the roof of the tunnel is necessary to get access to the tunnel section after immersion together with the survey tower positioned at the opposite end of the tunnel section. The towers are used for positioning the tunnel section when it is below the water surface.

On the tunnel sections bollards and lifting points installed for the control over the horizontal and vertical position of the section. Temporary supports are installed inside the sections. These supports are jacks through the tunnel bottom slab and with these jacks it is possible to adjust the vertical position of the rear end of the tunnel section. On the other side of the section a ‘nose’ is constructed to make it possible for the section to find, with his ‘nose’, a support on the adjoining previously immersed section.

The actual immersion starts with a correct positioning of the tunnel section above its final position. Each section will be immersed by taking in water in the ballast tanks. The location of the section and its vertical stability [in this situation, the section has a negative buoyancy] are controlled by two immersion pontoons installed above the section being lowered and by means of winches from the pontoons to the lifting points of the roof.

The horizontal location is controlled by means of anchor lines connected to the immersion pontoon and the tunnel section.

To achieve a watertight and flexible joint, each tunnel section has a rubber profile [the, so called, “Gina”] installed along the cross-section.

After the nose hits the previous immersed tunnel section the section being lowered into place will be surveyed again and adjusted if necessary. Then the tunnel element will land with jacks on pre-installed concrete slabs on the bottom of the trench.

After the element is positioned on its final location the first compression of the Gina-profile will be reached by means of horizontal jacks connected to bollards on the two sections.

When the Gina is compressed over its full length the water between the bulkheads of the two elements can be pumped out. The water pressure on the other bulkheads of the immersed section will fully compress the Gina-profile.

It is very important that the Gina seal will be compressed during the total life of the tunnel because this compressed Gina is crucial for the water tightness of all the joints between the elements. From the inside a second rubber sealing (the so called “Omega” profile) will be installed to provide a second seal against leakage of water.

The next step of the immersion process is the installation of a sand foundation for the section. The sand material will be pumped as a mixture of sand and water by means of pipes through the bottom slab and will completely fill the gap between the element and the bottom of the dredged trench. The sand material can be pumped from the shore by a vessel on the surface. After completion of the sand foundation the jacks of the temporary supports can be drawn in and sealed. Due to the flexibility of the joints (they can rotate) the total tunnel is flexible in his longitudinal direction and is able to absorb some settlement of its foundation.

The last task in completing the tunnel structure is filling the gap between the last section immersed and the face of the penultimately lowered section.

The fill is constructed by placing a purpose made steel shuttering around the gap. To maintain the required compression force in the element in view of the Gina-profile struts have to be placed, the water can be pumped out in the gap and the joint can be constructed from the inside.

During the construction of the tunnel in Portsmouth’s harbour it is very important to reduce the impact on the harbour users as much as possible. During the dredging operations the present shipping channel will be diverted and during the immersion operation itself there will be a short period that the harbour is completely closed.

In the present stage of the design six tunnel elements are foreseen which means also six immersion operations. Therefore good co-operation is necessary during the construction between the Harbour Masters, the port users and the contractor. Much detailed consultation has been undertaken with the harbour authorities and users to ensure minimum disruption to shipping.

The overall construction time for the tunnel will be approximately three years, but the immersion of the individual elements will be only approximately one day each. Thus the significant economic and environmental benefits that the new rapid transit system will bring to Portsmouth and the Fareham-Gosport peninsula will not be at the expense of any significant disruption to the economic or leisure activity of the harbour or, indeed, to the densely pack hinterland.

Portsmouth Harbour tunnel: top