Kingsway Tunnel

[THE BOSS]

The two parallel tubes of the Kingsway Tunnel under the River Mersey, between Liverpool and Birkenhead, opened in 1971 and 1973, currently carry approximately 15m vehicles per annum. Each tube is some 1.4 miles long with an internal diameter of 9.65m. These circular sections were segmentally constructed after excavation by a full-face tunnel boring machine. Almost all excavation passed through fissured waterbearing Bunter Sandstone, which also forms the riverbed here. For most of the length of both tubes, the tunnels were lined with bolted precast concrete segments cast on site. These precast concrete segments are in excellent condition, although there is some leakage through joints.

The original tunnel design, taking into account emergency exits, involved escape stairwells leading to the two ventilation stations at approximately quarter points of the tunnel. Recent consideration of tunnel safety in emergency situations, particularly fire, has led to a change of emphasis. Where there are two parallel tubes, the use of cross passages for self-rescue and emergency access has become the preferred solution.

Although the existing two means of escape provide cross passages, they are 0.8 miles apart and form an escape route significantly longer than the optimum. The two tubes are about 27m apart from centre to centre, providing a barrier of just over 17m. Another hurdle was that the work had to be undertaken without affecting this strategic part of the road network.

Mott MacDonald was commissioned to carry out a feasibility study into the construction of new cross passages. This study focussed on the geology of the site, using the records of the original projects to identify areas with fewer geotechnical problems. This data was matched to the preferred spacing of the new cross passages and it was concluded that three passages could be introduced, giving an average spacing of 320m.

It was recognised that significant work would have to be carried out to control water ingress and that the excavation would probably be carried out using a small mobile excavator/breaker. The design required the cross passages to have a segmental lining with an internal diameter of approximately 3.3m. Sections of two original rings were removed to provide a 2.4m wide access. Care had to be taken to replicate the strength of these removed panels both during and after construction.

Consideration was given to the method of procurement, a partnering form being adopted to gain the early involvement of a contractor before any detailed design. Following a competitive tender, based upon quality and price criteria, AMCO Donelon was awarded the contract.

The positions of the cross passages were confirmed by trial horizontal holes. These indicated the extent of fissures and groundwater. After service diversion works, the next step was to deploy grout holes in two rings around the boundary of the cross passage excavation for the full length of the passages, with the lower ones inclined. Grout holes were constructed in 6m stages to achieve improved grout distribution.

The resulting pattern of 18 grout holes at each cross passage was filled with a Portland cement grout with a water/cement ratio of 2. The mix was reviewed to reflect actual conditions on each grout hole. Shear dowels, supplied by Dywidag Systems, were installed above each opening location to replicate the strength of the panels to be removed. In the final works, this strength will be provided by a structural steel support frame within the plane of the original tunnel and an in-situ concrete collar. Spile bars, also supplied by Dywidag, were horizontally installed above each cross passage to stabilise the crown of the excavated passage during lining installation.

Excavation is to begin soon. The sandstone is expected to be quite sound while the segmental lining is installed, working from one tube. The precast concrete segments that will form a 3.35m internal diameter tube will be a standard bolted segmental ring cast offsite from C50 concrete by CX Buchan. This will be internally lined with an in-situ secondary concrete lining 150mm thick. The external annulus will be grouted with a proprietary GM4 pre-blended grout from CMS Pozament. It is expected that construction will be completed by August 2002 with internal work following immediately afterwards. The total cost will be around L 2.4m.

As the cross passages are a safety feature for emergency use only, the hope is that they will never be used.