Liverpool is the sixth largest city in the UK with a population approaching 500,000. Situated in the north west of England, it has excellent road and rail connections, along with rapid trans-global sea routes and two international airports. In 1922, Liverpool City Council reported on the options to improve traffic facilities across the River Mersey, recommending a tunnel on the basis that a bridge would prove a susceptible target in case of war. Construction of the Queensway Tunnel commenced in December 1925 and King George V opened it in July 1934. At the time, it ranked financially as the biggest municipal enterprise ever undertaken in the UK and was considered one of the largest underwater crossings in its time. The tunnel is 13.4m in diameter, and carries four lanes of traffic for just over 3km. During the 1950s the need for a second crossing became evident and in January 1966, work commenced on a second crossing. The Kingsway Tunnel, opened by Queen Elizabeth II in June 1971, is a 3,200m long twin tube tunnel with two traffic lanes of 3.65m wide in each.
The Mersey Tunnels, unlike any other estuarial crossing in the UK, are in the heart of a large conurbation. Patronage of the tunnels is mainly local and the majority of journeys are work-orientated. The Queensway Tunnel operates on a tidal flow system with morning peak traffic flow predominantly into the city, and evening peak flows out of the city. Closures of one of the Kingsway tunnels are possible with operation of the other tunnel in bi-directional traffic flow.
Kingsway tunnels
The tunnels are circular in cross section and lined with a composite reinforced concrete steel-faced segmental lining, with an internal diameter of 9.63m. Welded steel strips cover the lining joints between the segments and the steel facing was painted to provide the final finish for the road tunnel. An 11.2m diameter Robbins TBM excavated the majority of the tunnels through the underlying sandstone rock formation. Where the tunnels pass under the River Mersey, rock cover above the crown varies between 7m-15m. Cut and cover sections were constructed through superficial soils and Boulder Clay to form the approaches. The twin tunnels are located approximately 27m apart between centres and at approximately the same level. Located in the South Tunnel is an oversized section of tunnel at mid-river that forms a 35m long emergency lay-by. This was constructed by enlarging the tunnel locally and lining it with an elliptical shaped cast iron lining, with internal dimensions of 12.37m wide and 10.76m high.
The original configuration of the Mersey Kingsway Tunnels incorporated links between the two road tunnels at the ventilation shafts, approximately 460m from the tunnel portals, with an additional link below deck level at the mid-river emergency lay-by. The passages located at the ventilation shafts are identified as emergency cross passages and comprise a single door opening at walkway level with an indirect and narrow route to the adjacent road tunnel. Access to the passages is via vertical ladder onto the walkway.
Need for additional cross passages
Following fatal incidents in the Channel Tunnel, Mont Blanc Tunnel and Tauern Tunnel, concerns were raised about how to improve the capacity for emergency evacuation in case of an incident in one of the Mersey Kingsway Tunnels. The configuration of the original links between the tunnels were not entirely suitable as an escape provision. Discussions between Mersey Tunnels and Merseyside Fire Brigade identified the need for additional links between the road tunnels to satisfy their evacuation strategy in case of an incident within one of the road tunnels. It was considered that provision of three additional cross passages within the central section of the tunnels, located between the two existing ventilation shafts satisfied the objectives of the evacuation strategy.
Early in 2000 Mersey Tunnels commissioned Mott MacDonald to carry out a feasibility study for the construction of new cross passages between the existing twin road tunnels and advice for the refurbishment of the secondary lining in the south tunnel lay-by. The lay-by had suffered for a number of years from leaks and badly corroded secondary lining panels.
Geotechnical analysis
As part of the feasibility study, a geotechnical desk study was carried out to assist in determining the location of the new cross passages. This analysis included a review of published material and data from the original ground investigation along with construction records from the Kingsway Tunnels.
The existing road tunnels were constructed through the Triassic Bunter Sandstone (now referred to as the Sherwood Sandstone). The bedding structure of the sandstone has a north-to-south strike with an easterly dip of between 2°-10°. It was observed that false bedding can be much steeper, and thin ‘seams’ of silt can be present where coarse-grained rock can break away. The sandstone is quite fissile along the bedding due to the micaceous partings.
Before the construction of both existing road tunnels, 3.35m diameter pilot tunnels were driven using drill and blast techniques along the centre line of the main tunnels to give an indication of the geology. The pilot tunnels were generally unlined giving an indication of the unsupported rock strength. The pilot tunnels were also used to carry out injection ground treatment before advance of the main tunnel drives, although according to the records available the rock did not respond satisfactorily to the treatment. The major problems encountered during the construction of the main tunnel drives were caused by heavy water inflows making working conditions difficult and by localised zones of loose ground.
The most reliable guide to the expected ground conditions were found in records of the actual construction of the existing tunnels, obtained from a thesis undertaken on the behaviour of the rock during construction. Fault zones were highlighted as an area to avoid. However, water was not included in the assessment due to lack of information. The greatest water inflows were anticipated to be located in fracture and fault zones. During this assessment, it also became clear that the intensity of fracturing and fault zones were markedly different from one road tunnel to the other, despite their close proximity.
Cross passage design
The design developed between Mersey Tunnels and Mott MacDonald included the construction of three new cross passages, located between the existing tunnel ventilation shafts. Equally spaced passages along the tunnel are ideal but the need to avoid zones of geotechnical difficulty, and jet fan and emergency panel locations determined the final location and spacing of the new cross passages. Consequently, the average spacing between the three new cross passages is 325m. The spacing of the new cross passages is significantly greater than that required by the UK Highways Agency and that recommended for US road tunnels. Due to the reasons listed above and the fact that the tunnel is privately operated the greater passage spacing was acceptable. Consideration was given to the construction of new cross passages between the tunnel portals and the ventilation shafts. This was discounted due to the close proximity of the portals and visibility problems as the tunnel alignments curve in these locations.
Design of the cross passages focussed on the following important areas to determine spatial requirements:
Wheelchair/Mobility Impaired Access: The walkway is currently about 1m above roadway level. In order to enable access to the cross passages in an emergency the new cross passages are at road level.
Dimension of Cross Passage: Whilst no specific regulations exist on the minimum dimension requirements for personnel access, a minimum width of 1.8m was provided with a clear height of 2m, which allows for up to three adults side by side, or a wheel chair and an adult alongside. This results in the 3.35m diameter cross passage lining and a 2.4m wide, two-ring opening in the permanent lining of the main tunnels. Double doors were provided at the entrance to each cross passage.
Excavation method: Traditionally, hand excavation techniques involving miners using hand held compressed air tools would have been employed. However the recently issued UK Health & Safety Executive guidelines to prevent hand arm vibration syndrome (‘whitefinger’) effectively means the use of these tools should be minimised. Therefore, it was anticipated at design stage that a small excavator/ breaker would be used to perform the bulk of the excavation. Although the excavation was considered to be self-supporting during construction, careful monitoring of the ground was undertaken on a regular basis and rock support measures held available to be installed if necessary. Temporary rock dowels were installed through the tunnel lining above the entrance to the cross passages to support the lining and loose ground in the vicinity of the openings (Figure 2).
Procurement strategy
Following on from the feasibility study, Mersey Tunnels and Mott MacDonald developed a procurement strategy during December 2000 with preparation of the contract documents in May 2001. Due to planning constraints, Mersey Tunnels were not able to appoint a contractor early on in the project, therefore a Design-Bid-Build approach was adopted. With respect to the form of contract for the works, the feasibility study suggested that some form of Target Cost or Cost Reimbursable type contract along with Partnering be employed.
A Target Cost contract was agreed upon, providing a means of managing the risks between both the client and the contractor, while allowing the flexibility to overcome unforeseen problems and offer performance-based incentives to the contractor.
The most critical risks, as far as Mersey Tunnels were concerned, were the effects of construction on the public perception, operation of the Kingsway Tunnels, and protection of the toll revenue stream. For these reasons, the appointed contractor had to demonstrate familiarity with working in restricted hours and night work environment and had to demonstrate experience of similar projects. In order to achieve these requirements, Mersey Tunnels adopted a two envelope tendering system where price was not the sole deciding factor in the evaluation of the tenders. That is, tenders were assessed and ranked technically before opening the financial envelope.
Detail design of the cross passages and lay-by refurbishment works and preparation of the tender documents was carried out in May 2001 by Mott MacDonald, with responsibility for all permanent works designs retained by them. Tenders were issued in July 2001 with return of tenders in mid-September 2001. Following evaluation of the tenders, the contract was awarded to AMCO Donelon in October 2001 at a target cost of US$3.4M. It was anticipated that the successful contractor would work in a spirit of mutual trust and co-operation with Mersey Tunnels, the project manager and designer. A Partnering workshop in late November 2001 resulted in all parties signing up to a partnership charter, including the AMCO Donelon’s principal sub-contractor and designer.
Site Works
Construction commenced on 7 January 2002, after a period of planning and design review. The contract programme set by Mersey Tunnels included for an ‘out-of-tunnel’ completion date of 31 July 2002 and overall completion date on 11 August 2002.
The works included the construction of the three 3.35m i.d. segmental lined cross passages, the replacement of secondary lining panels in the mid-river emergency lay-by with vitreous enamel steel panels and the refurbishment of existing Dado panels located along the walkways. Where necessary, sealing and caulking of the existing cast iron segmental lining in the lay-by would be carried out following removal of the cladding.
Constraints imposed during the construction works included: traffic management, requiring close liaison with Mersey Tunnels Police (the works being carried out during night time possessions available in only one tunnel at a time); the presence of tunnel services and requirement for service diversions; control of silt in the invert of the Kingsway Tunnels to minimise disruption to Mersey Tunnels sump pumps; and excavation methodology (limitation on the use of hand excavation through the use of mechanical excavators).
Before the excavation of the cross passages AMCO Donelon undertook extensive preparatory works. These included the diversion of a large number of services that ran along the length of the tunnels behind the Dado panels, and the removal of sections of the tunnel walkway. Blockwork bunds, constructed in the invert of the existing road tunnels at each cross passage opening, trapped silt from the groundwater during tunnelling. Temporary works designed by AMCO Donelon to support the existing road tunnel lining during construction included rock dowels above the openings and fabricated welded steel bottom sills to prevent movement of the existing segments. In addition, a remote access movement measuring (electro-level) system was installed to monitor background movements and potential lining movements.
To control groundwater inflow and excavation stability, a pattern of grouting and drainage holes drilled from the south tunnel to the back of the north tunnel lining were installed, in and around the circumference of each cross passage opening. Grouting was carried out using a cement grout mix with drilling and grouting carried out in stages of approximately 6m along the length of each grout hole. Spiling bars above the crown of each cross passage aided in reinforcing the rock. To minimize disturbance of the existing lining, stitch drilling and diamond sawing of the opening was carried out at each cross passage location.
Excavation of cross passage (CP) 1, 2 and 3 commenced in April 2002 at CP1 with the remaining two following in quick succession. Tunnel excavation and ring building was initially carried out from the south tunnel. After approximately 11 weeks of excavation by robotic breakers and, in the case of CP1 and CP3, a combination of robotic breaker and roadheader, breakthrough of the north tunnel lining occurred in CP 1 on 23 June 2002 and the remaining two cross passages by 30 June 2002. Erection of the concrete segmental linings was carried out concurrent with excavation.
On completion of excavation of each of the cross passages AMCO proceeded with construction of the cast-in-place concrete portal structures to join the segmental lining of the cross passages to the existing lining of the main tunnels. This work was carried out with tunnel closures in the south tunnel initially, crossing over to the north tunnel on 29 July 2002.
During the contract Mersey Tunnels identified the need for two new under deck invert access passages between the two road tunnels, to be located at each end of the Kingsway Tunnel between the portals and the ventilation shafts. These passages were to assist Mersey Tunnels in its maintenance operations as the current configuration only allowed access below the deck level at the mid-river section and at each portal. Mott MacDonald carried out the design of these new passages and the work was added to AMCO Donelon’s contract and completed within the original programme.
Lay-by refurbishment
As with the main road tunnels, vertical asbestos backed Dado panels are installed at the back of the walkway to a height of 2.1m. Above the top of the Dado panels, over the soffit of the tunnel, galvanised steel sheet secondary lining panels were originally installed, which also had an asbestos backing board. The traffic face of the panels were coated with a PVC Plastisol ‘paint’.
During the planning and design stage of the project there was evidence from the condition of the traffic face of the soffit secondary lining of 15 areas where water had leaked through the cast iron tunnel lining onto the back of the panels. In these areas, besides water staining, there was evidence that the lining panels were corroding from the back outwards and the Plastisol coating blistering and delaminating. Where the removal of the Dado panels allowed access to the bottom of the soffit panels it was evident that there was some deterioration in the condition of the asbestos backing sheeting, which had become friable.
As a result of the poor condition of the soffit panels, and deterioration of their asbestos backing sheets, it was recommended that the contract include removal of all the secondary lining from the tunnel, to be replaced with non-asbestos backed soffit panels. The Dado panels were to have their asbestos backing replaced with a safe cement based material combined with an overall refurbishment, before re-installation.
As works in the lay-by proceeded, it was apparent that the extent of damage to the secondary lining was greater than first perceived, with corrosion of the supporting steelwork having occurred. Mersey Tunnels agreed to implement complete refurbishment of the lay-by area, including re-design of the secondary lining support system in stainless steel and replacement of the original secondary lining system with a new ceramic coated steel sheet lining system. Cleaning and re-priming of the original segmental lining in the lay-by area was also carried out to increase life expectancy. This additional work necessitated a further increase to the target cost.
Conclusions
The majority of site works were complete by the end of August 2002. During the contract, working relationships between all parties were excellent, and certainly within the spirit of Partnering. Regular site progress meetings attended by representatives from the client, project manager, contractor and designer quickly identified potential problems and design issues, and implemented measures and actions to address both.
Through the chosen contract, it was possible to readily adjust the target price to accommodate unforeseen problems and additional works inherent in this type of refurbishment project, while maintaining cost control for the client. Although works did run over the original programme, it was recognised by Mersey Tunnels that this was necessary given the increased scope of work, achieved with minimal disturbance to the operation of the tunnels. Final cost of the project is expected to be in the order of US$4.5M.
The completed work provides additional emergency facilities for Mersey Tunnels, and improved under deck access for their maintenance staff. Amendments to the originally planned refurbishment works of the of the mid-river lay-by will result in reduced maintenance and increased life expectancy for this structure.
Related Files
Figure 3 – Cross section of the Mersey Kingsway Tunnels, showing the new cross passage layout
Figure 1 – Location of the Mersey Kingsway & Queensway Tunnels
